Roles of SCaBP8 in salt stress response

The tissue-preferential distributed calcium sensors, SOS3 and SCaBP8, play important roles in SOS pathway to cope with saline conditions. Both SOS3 and SCaBP8 interact with and activate SOS2. However the regulatory mechanism for SOS2 activation and membrane recruitment by SCaBP8 differs from SOS3. SCaBP8 is phosphorylated by SOS2 at plasma membrane (PM) under salt stress. This phosphorylation anchors the SCaBP8-SOS2 complex on plasma membrane and activates PM Na⁺/H⁺ anti-porter, such as SOS1. Here, we describe that SOS2 has high binding affinity and catalytic efficiency to SCaBP8, suggesting that phosphorylation of SCaBP8 by SOS2 perhaps occurs rapidly in salt condition. SCaBP8 is also phosphorylated by PKS5 (SOS2-like Protein Kinase5) which negatively regulates PM H⁺-ATPase activity and functions in plant alkaline tolerance, providing a clue to roles of SCaBP8 in both salt and alkaline tolerance. SOS2 interacts with SOS3 and SCaBP8 with its FISL motif at C-terminus. However, luciferase activity complement assay indicates that SOS2 N-terminal is also essential for interacting with these proteins in plant.Key words: calcium signal, kinase activity, luciferase complementDue to their sessile nature, plants have developed elaborate strategies to deal with a number of environmental challenges. One overwhelming constraint is high salinity in the soil, which inhibits plant growth and decreases the agricultural productivity. Efflux and/or sequestering of sodium ion to apoplastic space/vacuolar are well-known cellular mechanisms that plants protect them from saline stress.¹ Recently identified SOS (salt overly sensitive) pathway plays critical roles in maintaining ion homeostasis in response to high salinity.² Two calcium sensors, SOS3 and SCaBP8 (SOS3-like calcium binding protein8), perceive cytosolic calcium signature triggered by salt, interact with and activate a Thr/Ser protein kinase, SOS2 and recruit it to the plasma membrane. Then, the formed SOS3-SOS2 or SCaBP8-SOS2 complex activates a PM Na⁺/H⁺ anti-porter, SOS1.^2–4 Moreover, SOS2 also regulates vascular Na⁺/H⁺ antiporter activity.⁵ Previously, we reported that SCaBP8 and SOS3 function distinctly in activation of SOS2.³ For instance, N-terminal myristoylation of SOS3 plays an important role in salt tolerance.⁶ However, there is no consensus myristoylated motif in SCaBP8. Instead, an N-terminal hydrophobic domain is sufficient to facilitate the association of SCaBP8 to plasma membrane.³ In addition, SCaBP8 is phosphorylated by SOS2 under salt stress and this phosphorylation stabilizes the interaction of SOS2 and SCaBP8.⁴ In this report, we describe that SCaBP8 possibly is rapidly phosphorylated by SOS2 under salt stress and also phosphorylated by another stress responsible protein kinase, implying additional roles of SCaBP8 in stress responses.     

A Conserved Na+ Binding Site of the Sodium-coupled Neutral Amino Acid Transporter 2 (SNAT2)

The SLC38 family of solute transporters mediates the coupled transport of amino acids and Na⁺ into or out of cells. The structural basis for this coupled transport process is not known. Here, a profile-based sequence analysis approach was used, predicting a distant relationship with the SLC5/6 transporter families. Homology models using the LeuT_Aa and Mhp1 transporters of known structure as templates were established, predicting the location of a conserved Na⁺ binding site in the center of membrane helices 1 and 8. This homology model was tested experimentally in the SLC38 member SNAT2 by analyzing the effect of a mutation to Thr-384, which is predicted to be part of this Na⁺ binding site. The results show that the T384A mutation not only inhibits the anion leak current, which requires Na⁺ binding to SNAT2, but also dramatically lowers the Na⁺ affinity of the transporter. This result is consistent with a previous analysis of the N82A mutant transporter, which has a similar effect on anion leak current and Na⁺ binding and which is also expected to form part of the Na⁺ binding site. In contrast, random mutations to other sites in the transporter had little or no effect on Na⁺ affinity. Our results are consistent with a cation binding site formed by transmembrane helices 1 and 8 that is conserved among the SLC38 transporters as well as among many other bacterial and plant transporter families of unknown structure, which are homologous to SLC38.The sodium-coupled neutral amino acid transporter, SNAT2,² belongs to the SLC38 gene family of solute carrier proteins (1). Together with SNAT1 and -4 (2), it is believed to mediate Na⁺-dependent amino acid transport activity that was classically assigned to System A transporters (3–8). In addition to SNAT1 and -2, the SLC38 family has four other known members, two of which predominantly mediate glutamine transport (SNAT3 and -5, System N (9–11)). SNAT2 is widely expressed in mammalian tissue (1, 7), but it may play a particularly critical role in the brain (12), where it may help shuttle glutamine from astrocytes to neurons via the glutamate-glutamine cycle (1). This process is essential for recycling the neurotransmitter glutamate (13). However, the exact contribution of SNAT2 to the glutamate-glutamine cycle is still controversially discussed (14).Despite this physiological importance, surprisingly little is known about the functional properties and the structural basis of amino acid transport by the SLC38 proteins. Although hydropathy analysis predicts 11 transmembrane helices (TMs), with an intracellular N terminus and an extracellular C terminus (1), it is not clear whether the transporters belong to a large superfamily of transporters, of which members have been characterized structurally through x-ray crystallography. At present, sequence homology has only been established with transporters of the mammalian SLC32 and SLC36 families as well as with the more distantly related plant auxin carriers and the bacterial amino acid-polyamine-organocation (APC) family (15, 16). High resolution crystal structures are not available for any of the transporters from these families, although low resolution projection structures were recently reported for the APC family members AdiC (17) and SteT (18). However, these structures do not allow the assignment of transmembrane helices. Thus, it remains unknown whether the SLC38 fold is similar to established transport protein folds, although homology to the major facilitator superfamily seems unlikely.We have recently identified a conserved amino acid residue in SNAT2, Asn-82, which is involved in controlling the Na⁺ affinity of the transporter (19). Interestingly, Asn-82 is localized in the predicted TM1 of SNAT2. This first transmembrane helix was recently found to contribute ligands to a Na⁺ binding site in several bacterial transporters, which are related to the SLC5 (sodium glucose symporter) and SLC6 (sodium- and chloride-dependent neurotransmitter transporter) family members (20–22), which also comprises bacterial members (23, 24). Although sequence similarity with SLC5 and -6 is not detectable, SLC38 may be a member of a possibly very large superfamily with the same general fold, which also contains many amino acid transport proteins.Here, we used a homology modeling approach based on profile-based sequence alignment (25, 26). A search against sequences deposited in the Protein Data Bank (PDB (27)) revealed that the transporters with the highest likelihood to share an analogous fold are a leucine transporter from Aquifex aeolicus, LeuT_Aa, and a homologous hydantoin transporter from Microbacterium liquefaciens, Mhp1. We established a homology model based on these structures, which predicts Asn-82 to be part of a Na⁺ binding site. Furthermore, another conserved hydrophilic amino acid residue in TM8, Thr-384, was predicted to be near this cation binding site. When Thr-384 was mutated to alanine, a dramatic loss of the affinity of SNAT2 for Na⁺ was observed, whereas mutations to other sites that were spatially removed from the predicted Na⁺ binding site had little or no effect on Na⁺ affinity. We hypothesize that the SLC38 family is a member of a large superfamily of cation/organic substrate transporters which includes the mammalian SLC5 and -6 proteins and which has a conserved cation binding site formed by TMs 1 and 8.     

Mutation of Asparagine 76 in the Center of Glutamine Transporter SNAT3 Modulates Substrate-induced Conductances and Na+ Binding

The glutamine transporter SLC38A3 (SNAT3) plays an important role in the release of glutamine from brain astrocytes and the uptake of glutamine into hepatocytes. It is related to the vesicular GABA (γ-aminobutyric acid) transporter and the SLC36 family of proton-amino acid cotransporters. The transporter carries out electroneutral Na⁺-glutamine cotransport-H⁺ antiport. In addition, substrate-induced uncoupled cation currents are observed. Mutation of asparagine 76 to glutamine or histidine in predicted transmembrane helix 1 abolished all substrate-induced currents. Mutation of asparagine 76 to aspartate rendered the transporter Na⁺-independent and resulted in a gain of a large substrate-induced chloride conductance in the absence of Na⁺. Thus, a single residue is critical for coupled and uncoupled ion flows in the glutamine transporter SNAT3. Homology modeling of SNAT3 along the structure of the related benzyl-hydantoin permease from Microbacterium liquefaciens reveals that Asn-76 is likely to be located in the center of the membrane close to the translocation pore and forms part of the predicted Na⁺ -binding site.The amino acid and auxin permease superfamily comprises a wide variety of transport proteins. In mammals, three distinct solute carrier families (SLC) belong to this superfamily, namely SLC32, SLC36, and SLC38 (1). Despite belonging to the same superfamily, the three solute carrier families have different transport mechanisms. The SLC32 family has only one member, the vesicular inhibitory amino acid transporter, which supposedly carries out a H⁺-GABA (γ-aminobutyric acid) antiport (2). The SLC36 family comprises four members, two of which have been characterized in more detail. These are the proton amino acid cotransporters 1 and 2 (PAT1 and 2) that carry out glycine and proline uptake in kidney and intestine and are mutated in iminoglycinuria (3, 4). The SLC38 family is comprised of 11 members, 5 of which have been characterized in more detail (5). Two different transport mechanisms are found within this family, namely the Na⁺-amino acid cotransporters SNAT1, SNAT2, and SNAT4 and the Na⁺-amino acid cotransporters-H⁺-antiporters SNAT3 and SNAT5. Transporters of the superfamily play a key role in inhibitory and excitatory neurotransmission, metabolite absorption, and liver metabolism. Despite their important roles in mammalian physiology, relatively little is known about the structure and function of these transporters.The activity of ion-coupled membrane transporters is frequently associated with currents which de- or hyperpolarize the cell membrane. These currents may be due to electrogenic transport stoichiometry and/or to a non-stoichiometric ion conductance (6). Transport-associated ion conductances have been identified in a number of transporters but have been particularly well studied in several Na⁺-coupled neurotransmitter transporters (7–11). Transport-associated conductances have also been observed in electroneutral transporters that do not carry out net charge movement (8, 12–15). The glutamine transporter SNAT3, for instance, has a transport mechanism in which glutamine uptake is coupled to the cotransport of 1Na⁺ and the antiport of 1H⁺ and, hence, is unaffected by changes of the membrane potential (13, 16). Despite the electroneutral transport mechanism, substrate uptake is accompanied by inward currents, which are carried by cations below pH 7 and by protons at alkaline pH. In addition, a substrate-independent cation conductance and a Na⁺/H⁺ exchange activity has been observed (17). Non-stoichiometric currents can be mediated by the same ions that are involved in the coupled transport process, such as in the case of SNAT3, but may also be carried by different ions. Stoichiometric glutamate transport, for instance, involves Na⁺, H⁺, and K⁺ ions, whereas the glutamate transport-associated conductance is carried by chloride (18).A crucial question concerning transporter-associated ion conductances is whether the conducting pore coincides with the translocation pathway of the substrate and whether both use the same critical residues. In the case of the glutamate transporters, evidence has been presented suggesting that different residues are critical for the anion conductance than for substrate transport (19, 20) but that they all line the same pathway (21). Here we show that asparagine 76 of SNAT3 is critical for substrate-induced ion conductance and affects binding of the cosubstrate Na⁺. In addition we show that this residue is likely to be localized in the translocation pore in the center of the membrane.     

Regulation of Epithelial Na+ Transport by Soluble Adenylyl Cyclase in Kidney Collecting Duct Cells

Alkalosis impairs the natriuretic response to diuretics, but the underlying mechanisms are unclear. The soluble adenylyl cyclase (sAC) is a chemosensor that mediates bicarbonate-dependent elevation of cAMP in intracellular microdomains. We hypothesized that sAC may be an important regulator of Na⁺ transport in the kidney. Confocal images of rat kidney revealed specific immunolocalization of sAC in collecting duct cells, and immunoblots confirmed sAC expression in mouse cortical collecting duct (mpkCCD_c14) cells. These cells exhibit aldosterone-stimulated transepithelial Na⁺ currents that depend on both the apical epithelial Na⁺ channel (ENaC) and basolateral Na⁺,K⁺-ATPase. RNA interference-mediated 60-70% knockdown of sAC expression comparably inhibited basal transepithelial short circuit currents (I_sc) in mpkCCD_c14 cells. Moreover, the sAC inhibitors KH7 and 2-hydroxyestradiol reduced I_sc in these cells by 50-60% within 30 min. 8-Bromoadenosine-3′,5′-cyclic-monophosphate substantially rescued the KH7 inhibition of transepithelial Na⁺ current. Aldosterone doubled ENaC-dependent I_sc over 4 h, an effect that was abolished in the presence of KH7. The sAC contribution to I_sc was unaffected with apical membrane nystatin-mediated permeabilization, whereas the sAC-dependent Na⁺ current was fully inhibited by basolateral ouabain treatment, suggesting that the Na⁺,K⁺-ATPase, rather than ENaC, is the relevant transporter target of sAC. Indeed, neither overexpression of sAC nor treatment with KH7 modulated ENaC currents in Xenopus oocytes. ATPase and biotinylation assays in mpkCCD_c14 cells demonstrated that sAC inhibition decreases catalytic activity rather than surface expression of the Na⁺,K⁺-ATPase. In summary, these results suggest that sAC regulates both basal and agonist-stimulated Na⁺ reabsorption in the kidney collecting duct, acting to enhance Na⁺,K⁺-ATPase activity.Maintenance of intracellular pH depends in part on the extracellular to intracellular Na⁺ gradient, and elevation of intracellular [Na⁺] can lead to acidification of the cytoplasm. It has been shown that acidification of the cytoplasm of cells from frog skin and toad bladder by increased partial pressure of CO₂ reduces Na⁺ transport and permeability (1, 2). Conversely, the rise in plasma bicarbonate caused by metabolic alkalosis with chronic diuretic use has been shown to increase net renal Na⁺ reabsorption independently of volume status, electrolyte depletion, and/or increased aldosterone secretion (3, 4). However, the underlying mechanisms involved in these phenomena remain unclear.The soluble adenylyl cyclase (sAC)² is a chemosensor that mediates the elevation of cAMP in intracellular microdomains (5-7). Unlike transmembrane adenylyl cyclases (tmACs), sAC is insensitive to regulation by forskolin or heterotrimeric G proteins (8) and is directly activated by elevations of intracellular calcium (9, 10) and/or bicarbonate ions (11). Thus, sAC mediates localized intracellular increases in cAMP in response to variations in bicarbonate levels or its closely related parameters, partial pressure of CO₂ and pH. Mammalian sAC is more similar to bicarbonate-regulated cyanobacterial adenylyl cyclases than to other mammalian nucleotidyl cyclases, which may indicate that there is a unifying mechanism for the regulation of cAMP signaling by bicarbonate across biological systems. Although sAC appears to be encoded by a single gene, there is significant isoform diversity for this ubiquitously expressed enzyme (11, 12) generated by alternative splicing (reviewed in Ref. 13). sAC has been shown to regulate the subcellular localization and/or activity of membrane transport proteins such as the vacuolar H⁺-ATPase (V-ATPase) and cystic fibrosis transmembrane conductance regulator in epithelial cells (14, 15). Functional activity of sAC has been reported in the kidney (16), and sAC has been localized to epithelial cells in the distal nephron (14, 17).Given that natriuresis is decreased during metabolic alkalosis, when bicarbonate is elevated, and Na⁺ reabsorption is impaired by high partial pressure of CO₂, we hypothesized that bicarbonate-regulated sAC may play a key role in the regulation of transepithelial Na⁺ transport in the distal nephron. Reabsorption of Na⁺ in the kidney and other epithelial tissues is mediated by the parallel operation of apical ENaC and basolateral Na⁺,K⁺-ATPase, and both transport proteins can be stimulated by cAMP via the cAMP-dependent protein kinase (PKA) (18, 53). The aims of this study were to investigate the role of sAC in the regulation of transepithelial Na⁺ transport in the kidney through the use of specific sAC inhibitors and electrophysiological measurements. We found that sAC inhibition blocks transepithelial Na⁺ reabsorption in polarized mpkCCD_c14 cells under both basal and hormone-stimulated conditions. Selective membrane permeabilization studies revealed that although ENaC activity appears to be unaffected by sAC inhibition, flux through the Na⁺,K⁺-ATPase is sensitive to sAC modulation. Inhibiting sAC decreases ATPase activity without affecting plasma membrane expression of the pump; thus, tonic sAC activity appears to be required for Na⁺ reabsorption in kidney collecting duct.     

Conserved Glutamate Residues Glu-343 and Glu-519 Provide Mechanistic Insights into Cation/Nucleoside Cotransport by Human Concentrative Nucleoside Transporter hCNT3

Human concentrative nucleoside transporter 3 (hCNT3) utilizes electrochemical gradients of both Na⁺ and H⁺ to accumulate pyrimidine and purine nucleosides within cells. We have employed radioisotope flux and electrophysiological techniques in combination with site-directed mutagenesis and heterologous expression in Xenopus oocytes to identify two conserved pore-lining glutamate residues (Glu-343 and Glu-519) with essential roles in hCNT3 Na⁺/nucleoside and H⁺/nucleoside cotransport. Mutation of Glu-343 and Glu-519 to aspartate, glutamine, and cysteine severely compromised hCNT3 transport function, and changes included altered nucleoside and cation activation kinetics (all mutants), loss or impairment of H⁺ dependence (all mutants), shift in Na⁺:nucleoside stoichiometry from 2:1 to 1:1 (E519C), complete loss of catalytic activity (E519Q) and, similar to the corresponding mutant in Na⁺-specific hCNT1, uncoupled Na⁺ currents (E343Q). Consistent with close-proximity integration of cation/solute-binding sites within a common cation/permeant translocation pore, mutation of Glu-343 and Glu-519 also altered hCNT3 nucleoside transport selectivity. Both residues were accessible to the external medium and inhibited by p-chloromercuribenzene sulfonate when converted to cysteine.Physiologic nucleosides and the majority of synthetic nucleoside analogs with antineoplastic and/or antiviral activity are hydrophilic molecules that require specialized plasma membrane nucleoside transporter (NT)³ proteins for transport into or out of cells (1–4). NT-mediated transport is required for nucleoside metabolism by salvage pathways and is a critical determinant of the pharmacologic actions of nucleoside drugs (3–6). By regulating adenosine availability to purinoreceptors, NTs also modulate a diverse array of physiological processes, including neurotransmission, immune responses, platelet aggregation, renal function, and coronary vasodilation (4, 6, 7). Two structurally unrelated NT families of integral membrane proteins exist in human and other mammalian cells and tissues as follows: the SLC28 concentrative nucleoside transporter (CNT) family and the SLC29 equilibrative nucleoside transporter (ENT) family (3, 4, 6, 8, 9). ENTs are normally present in most, possibly all, cell types (4, 6, 8). CNTs, in contrast, are found predominantly in intestinal and renal epithelia and other specialized cell types, where they have important roles in absorption, secretion, distribution, and elimination of nucleosides and nucleoside drugs (1–3, 5, 6, 9).The CNT protein family in humans is represented by three members, hCNT1, hCNT2, and hCNT3. Belonging to a CNT subfamily phylogenetically distinct from hCNT1/2, hCNT3 utilizes electrochemical gradients of both Na⁺ and H⁺ to accumulate a broad range of pyrimidine and purine nucleosides and nucleoside drugs within cells (10, 11). hCNT1 and hCNT2, in contrast, are Na⁺-specific and transport pyrimidine and purine nucleosides, respectively (11–13). Together, hCNT1–3 account for the three major concentrative nucleoside transport processes of human and other mammalian cells. Nonmammalian members of the CNT protein family that have been characterized functionally include hfCNT, a second member of the CNT3 subfamily from the ancient marine prevertebrate the Pacific hagfish Eptatretus stouti (14), CeCNT3 from Caenorhabditis elegans (15), CaCNT from Candida albicans (16), and the bacterial nucleoside transporter NupC from Escherichia coli (17). hfCNT is Na⁺- but not H⁺-coupled, whereas CeCNT3, CaCNT, and NupC are exclusively H⁺-coupled. Na⁺:nucleoside coupling stoichiometries are 1:1 for hCNT1 and hCNT2 and 2:1 for hCNT3 and hfCNT3 (11, 14). H⁺:nucleoside coupling ratios for hCNT3 and CaCNT are 1:1 (11, 16).Although much progress has been made in molecular studies of ENT proteins (4, 6, 8), studies of structurally and functionally important regions and residues within the CNT protein family are still at an early stage. Topological investigations suggest that hCNT1–3 and other eukaryote CNT family members have a 13 (or possibly 15)-transmembrane helix (TM) architecture, and multiple alignments reveal strong sequence similarities within the C-terminal half of the proteins (18). Prokaryotic CNTs lack the first three TMs of their eukaryotic counterparts, and functional expression of N-terminally truncated human and rat CNT1 in Xenopus oocytes has established that these three TMs are not required for Na⁺-dependent uridine transport activity (18). Consistent with this finding, chimeric studies involving hCNT1 and hfCNT (14) and hCNT1 and hCNT3 (19) have demonstrated that residues involved in Na⁺- and H⁺-coupling reside in the C-terminal half of the protein. Present in this region of the transporter, but of unknown function, is a highly conserved (G/A)XKX₃NEFVA(Y/M/F) motif common to all eukaryote and prokaryote CNTs.By virtue of their negative charge and consequent ability to interact directly with coupling cations and/or participate in cation-induced and other protein conformational transitions, glutamate and aspartate residues play key functional and structural roles in a broad spectrum of mammalian and bacterial cation-coupled transporters (20–30). Little, however, is known about their role in CNTs. This study builds upon a recent mutagenesis study of conserved glutamate and aspartate residues in hCNT1 (31) to undertake a parallel in depth investigation of corresponding residues in hCNT3. By employing the multifunctional capability of hCNT3 as a template for these studies, this study provides novel mechanistic insights into the molecular mechanism(s) of CNT-mediated cation/nucleoside cotransport, including the role of the (G/A)XKX₃NEFVA(Y/M/F) motif.     

A Calcium-Independent Activation of the Arabidopsis SOS2-Like Protein Kinase24 by Its Interacting SOS3-Like Calcium Binding Protein1

The salt stress-induced SALT-OVERLY-SENSITIVE (SOS) pathway in Arabidopsis (Arabidopsis thaliana) involves the perception of a calcium signal by the SOS3 and SOS3-like CALCIUM-BINDING PROTEIN8 (SCaBP8) calcium sensors, which then interact with and activate the SOS2 protein kinase, forming a complex at the plasma membrane that activates the SOS1 Na⁺/H⁺ exchanger. It has recently been reported that phosphorylation of SCaBP proteins by SOS2-like protein kinases (PKSs) stabilizes the interaction between the two proteins as part of a regulatory mechanism that was thought to be common to all SCaBP and PKS proteins. Here, we report the calcium-independent activation of PKS24 by SCaBP1 and show that activation is dependent on interaction of PKS24 with the C-terminal tail of SCaBP1. However, unlike what has been found for other PKS-SCaBP pairs, multiple amino acids in SCaBP1 are phosphorylated by PKS24, and this phosphorylation is dependent on the interaction of the proteins through the PKS24 FISL motif and on the efficient activation of PKS24 by the C-terminal tail of SCaBP1. In addition, we show that Thr-211 and Thr-212, which are not common phosphorylation sites in the conserved PFPF motif found in most SCaBP proteins, are important for this activation. Finally, we also found that SCaBP1-regulated PKS24 kinase activity is important for inactivating the Arabidopsis plasma membrane proton-translocating adenosine triphosphatase. Together, these results suggest the existence of a novel SCaBP-PKS regulatory mechanism in plants.Calcium is a ubiquitous second messenger that plays an important role in the regulation of plant growth and development. Many different types of calcium-binding proteins have been identified in plants (Harper et al., 2004), including the SALT-OVERLY-SENSITIVE3 (SOS3)-LIKE CALCIUM BINDING PROTEINS (SCaBPs; Liu and Zhu, 1998; Gong et al., 2004). Because the calcium-binding domain of these proteins shares sequence similarity with the yeast calcineurin B subunit, they have also been called CALCINEURIN B-LIKE PROTEINS (CBLs; Kudla et al., 1999; Luan et al., 2002). The founding member of this gene family, SOS3, was identified in a genetic screen from a salt-sensitive Arabidopsis (Arabidopsis thaliana) mutant (Liu and Zhu, 1998). SCaBP/CBL proteins interact with the SOS2-LIKE PROTEIN KINASES (PKSs)/CBL-INTERACTING PROTEIN KINASES (CIPKs; Shi et al., 1999; Halfter et al., 2000; Guo et al., 2001). The genetic linkage between these two families was established after identification of SOS2 from a genetic screen similar to the one that identified the sos3 mutant (Liu et al., 2000). SOS3 interacts with SOS2 in vivo and in vitro and activates SOS2 in a calcium-dependent manner in vitro (Halfter et al., 2000). The SOS3-SOS2 complex further activates SOS1, a plasma membrane (PM) Na⁺/H⁺ antiporter, by directly phosphorylating the SOS1 C terminus (Shi et al., 2000; Qiu et al., 2002; Quintero et al., 2002, 2011; Yu et al., 2010).In addition to the calcium-dependent activation of PKSs by SCaBP calcium sensors, two other regulatory mechanisms have been identified for these protein families. First, PKSs have a conserved 21-amino acid peptide (FISL motif) in their regulatory domain that is necessary for efficient interaction with the SCaBP calcium sensors (Guo et al., 2001; Albrecht et al., 2001; Gong et al., 2004). The PKS regulatory domain interacts with its kinase domain via the FISL motif to repress PKS activity; interaction of SCaBP with the PKS FISL motif releases the kinase domain inhibition allowing for kinase activity (Guo et al., 2001; Gong et al., 2004). Second, the PKSs phosphorylate a Ser residue in the conserved C-terminal PFPF motif of the SCaBP proteins. This phosphorylation enhances the interaction between the two proteins and fully activates the complex (Lin et al., 2009; Du et al., 2011; Hashimoto et al., 2012).In this study, we identified a novel PKS activation mechanism involving the calcium-independent activation of PKS24 by SCaBP1 and show that it requires binding of SCaBP1 to the FISL motif of PKS24 and the involvement of two Thr residues in the SCaBP1 C-terminal tail.     

Secretory Carrier Membrane Protein 2 Regulates Cell-surface Targeting of Brain-enriched Na+/H+ Exchanger NHE5

NHE5 is a brain-enriched Na⁺/H⁺ exchanger that dynamically shuttles between the plasma membrane and recycling endosomes, serving as a mechanism that acutely controls the local pH environment. In the current study we show that secretory carrier membrane proteins (SCAMPs), a group of tetraspanning integral membrane proteins that reside in multiple secretory and endocytic organelles, bind to NHE5 and co-localize predominantly in the recycling endosomes. In vitro protein-protein interaction assays revealed that NHE5 directly binds to the N- and C-terminal cytosolic extensions of SCAMP2. Heterologous expression of SCAMP2 but not SCAMP5 increased cell-surface abundance as well as transporter activity of NHE5 across the plasma membrane. Expression of a deletion mutant lacking the SCAMP2-specific N-terminal cytosolic domain, and a mini-gene encoding the N-terminal extension, reduced the transporter activity. Although both Arf6 and Rab11 positively regulate NHE5 cell-surface targeting and NHE5 activity across the plasma membrane, SCAMP2-mediated surface targeting of NHE5 was reversed by dominant-negative Arf6 but not by dominant-negative Rab11. Together, these results suggest that SCAMP2 regulates NHE5 transit through recycling endosomes and promotes its surface targeting in an Arf6-dependent manner.Neurons and glial cells in the central and peripheral nervous systems are especially sensitive to perturbations of pH (1). Many voltage- and ligand-gated ion channels that control membrane excitability are sensitive to changes in cellular pH (1-3). Neurotransmitter release and uptake are also influenced by cellular and organellar pH (4, 5). Moreover, the intra- and extracellular pH of both neurons and glia are modulated in a highly transient and localized manner by neuronal activity (6, 7). Thus, neurons and glia require sophisticated mechanisms to finely tune ion and pH homeostasis to maintain their normal functions.Na⁺/H⁺ exchangers (NHEs)³ were originally identified as a class of plasma membrane-bound ion transporters that exchange extracellular Na⁺ for intracellular H⁺, and thereby regulate cellular pH and volume. Since the discovery of NHE1 as the first mammalian NHE (8), eight additional isoforms (NHE2-9) that share 25-70% amino acid identity have been isolated in mammals (9, 10). NHE1-5 commonly exhibit transporter activity across the plasma membrane, whereas NHE6-9 are mostly found in organelle membranes and are believed to regulate organellar pH in most cell types at steady state (11). More recently, NHE10 was identified in human and mouse osteoclasts (12, 13). However, the cDNA encoding NHE10 shares only a low degree of sequence similarity with other known members of the NHE gene family, raising the possibility that this sodium-proton exchanger may belong to a separate gene family distantly related to NHE1-9 (see Ref. 9).NHE gene family members contain 12 putative transmembrane domains at the N terminus followed by a C-terminal cytosolic extension that plays a role in regulation of the transporter activity by protein-protein interactions and phosphorylation. NHEs have been shown to regulate the pH environment of synaptic nerve terminals and to regulate the release of neurotransmitters from multiple neuronal populations (14-16). The importance of NHEs in brain function is further exemplified by the findings that spontaneous or directed mutations of the ubiquitously expressed NHE1 gene lead to the progression of epileptic seizures, ataxia, and increased mortality in mice (17, 18). The progression of the disease phenotype is associated with loss of specific neuron populations and increased neuronal excitability. However, NHE1-null mice appear to develop normally until 2 weeks after birth when symptoms begin to appear. Therefore, other mechanisms may compensate for the loss of NHE1 during early development and play a protective role in the surviving neurons after the onset of the disease phenotype.NHE5 was identified as a unique member of the NHE gene family whose mRNA is expressed almost exclusively in the brain (19, 20), although more recent studies have suggested that NHE5 might be functional in other cell types such as sperm (21, 22) and osteosarcoma cells (23). Curiously, mutations found in several forms of congenital neurological disorders such as spinocerebellar ataxia type 4 (24-26) and autosomal dominant cerebellar ataxia (27-29) have been mapped to chromosome 16q22.1, a region containing NHE5. However, much remains unknown as to the molecular regulation of NHE5 and its role in brain function.Very few if any proteins work in isolation. Therefore identification and characterization of binding proteins often reveal novel functions and regulation mechanisms of the protein of interest. To begin to elucidate the biological role of NHE5, we have started to explore NHE5-binding proteins. Previously, β-arrestins, multifunctional scaffold proteins that play a key role in desensitization of G-protein-coupled receptors, were shown to directly bind to NHE5 and promote its endocytosis (30). This study demonstrated that NHE5 trafficking between endosomes and the plasma membrane is regulated by protein-protein interactions with scaffold proteins. More recently, we demonstrated that receptor for activated C-kinase 1 (RACK1), a scaffold protein that links signaling molecules such as activated protein kinase C, integrins, and Src kinase (31), directly interacts with and activates NHE5 via integrin-dependent and independent pathways (32). These results further indicate that NHE5 is partly associated with focal adhesions and that its targeting to the specialized microdomain of the plasma membrane may be regulated by various signaling pathways.Secretory carrier membrane proteins (SCAMPs) are a family of evolutionarily conserved tetra-spanning integral membrane proteins. SCAMPs are found in multiple organelles such as the Golgi apparatus, trans-Golgi network, recycling endosomes, synaptic vesicles, and the plasma membrane (33, 34) and have been shown to play a role in exocytosis (35-38) and endocytosis (39). Currently, five isoforms of SCAMP have been identified in mammals. The extended N terminus of SCAMP1-3 contain multiple Asn-Pro-Phe (NPF) repeats, which may allow these isoforms to participate in clathrin coat assembly and vesicle budding by binding to Eps15 homology (EH)-domain proteins (40, 41). Further, SCAMP2 was shown recently to bind to the small GTPase Arf6 (38), which is believed to participate in traffic between the recycling endosomes and the cell surface (42, 43). More recent studies have suggested that SCAMPs bind to organellar membrane type NHE7 (44) and the serotonin transporter SERT (45) and facilitate targeting of these integral membrane proteins to specific intracellular compartments. We show in the current study that SCAMP2 binds to NHE5, facilitates the cell-surface targeting of NHE5, and elevates Na⁺/H⁺ exchange activity at the plasma membrane, whereas expression of a SCAMP2 deletion mutant lacking the N-terminal domain containing the NPF repeats suppresses the effect. Further we show that this activity of SCAMP2 requires an active form of a small GTPase Arf6, but not Rab11. We propose a model in which SCAMPs bind to NHE5 in the endosomal compartment and control its cell-surface abundance via an Arf6-dependent pathway.     

Why have chloroplasts developed a unique motility system?

Organelle movement in plants is dependent on actin filaments with most of the organelles being transported along the actin cables by class XI myosins. Although chloroplast movement is also actin filament-dependent, a potential role of myosin motors in this process is poorly understood. Interestingly, chloroplasts can move in any direction and change the direction within short time periods, suggesting that chloroplasts use the newly formed actin filaments rather than preexisting actin cables. Furthermore, the data on myosin gene knockouts and knockdowns in Arabidopsis and tobacco do not support myosins'' XI role in chloroplast movement. Our recent studies revealed that chloroplast movement and positioning are mediated by the short actin filaments localized at chloroplast periphery (cp-actin filaments) rather than cytoplasmic actin cables. The accumulation of cp-actin filaments depends on kinesin-like proteins, KAC1 and KAC2, as well as on a chloroplast outer membrane protein CHUP1. We propose that plants evolved a myosin XI-independent mechanism of the actin-based chloroplast movement that is distinct from the mechanism used by other organelles.Key words: actin, Arabidopsis, blue light, kinesin, myosin, organelle movement, phototropinOrganelle movement and positioning are pivotal aspects of the intracellular dynamics in most eukaryotes. Although plants are sessile organisms, their organelles are quickly repositioned in response to fluctuating environmental conditions and certain endogenous signals. By and large, plant organelle movements and positioning are dependent on actin filaments, although microtubules play certain accessory roles in organelle dynamics.^1,2 Actin inhibitors effectively retard the movements of mitochondria,^3–6 peroxisomes,^5,7–11 Golgi stacks,^12,13 endoplasmic reticulum (ER),^14,15 and nuclei.^16–18 These organelles are co-aligned and associated with actin filaments.^{5,7,8,10–12,15,18} Recent progress in this field started to reveal the molecular motility system responsible for the organelle transport in plants.¹⁹Chloroplast movement is among the most fascinating models of organelle movement in plants because it is precisely controlled by ambient light conditions.^20,21 Weak light induces chloroplast accumulation response so that chloroplasts can capture photosynthetic light efficiently (Fig. 1A). Strong light induces chloroplast avoidance response to escape from photodamage (Fig. 1B).²² The blue light-induced chloroplast movement is mediated by the blue light receptor phototropin (phot). In some cryptogam plants, the red light-induced chloroplast movement is regulated by a chimeric phytochrome/phototropin photoreceptor neochrome.^23–25 In a model plant Arabidopsis, phot1 and phot2 function redundantly to regulate the accumulation response,²⁶ whereas phot2 alone is essential for the avoidance response.^27,28 Several additional factors regulating chloroplast movement were identified by analyses of Arabidopsis mutants deficient in chloroplast photorelocation.^29–32 In particular, identification of CHUP1 (chloroplast unusual positioning 1) revealed the connection between chloroplasts and actin filaments at the molecular level.²⁹ CHUP1 is a chloroplast outer membrane protein capable of interacting with F-actin, G-actin and profilin in vitro.^29,33,34 The chup1 mutant plants are defective in both the chloroplast movement and chloroplast anchorage to the plasma membrane,^22,29,33 suggesting that CHUP1 plays an important role in linking chloroplasts to the plasma membrane through the actin filaments. However, how chloroplasts move using the actin filaments and whether chloroplast movement utilizes the actin-based motility system similar to other organelle movements remained to be determined.Open in a separate windowFigure 1Schematic distribution patterns of chloroplasts in a palisade cell under different light conditions, weak (A) and strong (B) lights. Shown as a side view of mid-part of the cell and a top view with three different levels (i.e., top, middle and bottom of the cell). The cell was irradiated from the leaf surface shown as arrows. Weak light induces chloroplast accumulation response (A) and strong light induces the avoidance response (B).Here, we review the recent findings pointing to existence of a novel actin-based mechanisms for chloroplast movement and discuss the differences between the mechanism responsible for movement of chloroplasts and other organelles.     

Cell Migration from Baby to Mother

Fetal cells migrate into the mother during pregnancy. Fetomaternal transfer probably occurs in all pregnancies and in humans the fetal cells can persist for decades. Microchimeric fetal cells are found in various maternal tissues and organs including blood, bone marrow, skin and liver. In mice, fetal cells have also been found in the brain. The fetal cells also appear to target sites of injury. Fetomaternal microchimerism may have important implications for the immune status of women, influencing autoimmunity and tolerance to transplants. Further understanding of the ability of fetal cells to cross both the placental and blood-brain barriers, to migrate into diverse tissues, and to differentiate into multiple cell types may also advance strategies for intravenous transplantation of stem cells for cytotherapeutic repair. Here we discuss hypotheses for how fetal cells cross the placental and blood-brain barriers and the persistence and distribution of fetal cells in the mother.Key Words: fetomaternal microchimerism, stem cells, progenitor cells, placental barrier, blood-brain barrier, adhesion, migrationMicrochimerism is the presence of a small population of genetically distinct and separately derived cells within an individual. This commonly occurs following transfusion or transplantation.^1–3 Microchimerism can also occur between mother and fetus. Small numbers of cells traffic across the placenta during pregnancy. This exchange occurs both from the fetus to the mother (fetomaternal)^4–7 and from the mother to the fetus.^8–10 Similar exchange may also occur between monochorionic twins in utero.^11–13 There is increasing evidence that fetomaternal microchimerism persists lifelong in many child-bearing women.^7,14 The significance of fetomaternal microchimerism remains unclear. It could be that fetomaternal microchimerism is an epiphenomenon of pregnancy. Alternatively, it could be a mechanism by which the fetus ensures maternal fitness in order to enhance its own chances of survival. In either case, the occurrence of pregnancy-acquired microchimerism in women may have implications for graft survival and autoimmunity. More detailed understanding of the biology of microchimeric fetal cells may also advance progress towards cytotherapeutic repair via intravenous transplantation of stem or progenitor cells.Trophoblasts were the first zygote-derived cell type found to cross into the mother. In 1893, Schmorl reported the appearance of trophoblasts in the maternal pulmonary vasculature.¹⁵ Later, trophoblasts were also observed in the maternal circulation.^16–20 Subsequently various other fetal cell types derived from fetal blood were also found in the maternal circulation.^21,22 These fetal cell types included lymphocytes,²³ erythroblasts or nucleated red blood cells,^24,25 haematopoietic progenitors^7,26,27 and putative mesenchymal progenitors.^14,28 While it has been suggested that small numbers of fetal cells traffic across the placenta in every human pregnancy,^29–31 trophoblast release does not appear to occur in all pregnancies.³² Likewise, in mice, fetal cells have also been reported in maternal blood.^33,34 In the mouse, fetomaternal transfer also appears to occur during all pregnancies.³⁵     

Quantification of Na+,K+ pumps and their transport rate in skeletal muscle: Functional significance

During excitation, muscle cells gain Na⁺ and lose K⁺, leading to a rise in extracellular K⁺ ([K⁺]_o), depolarization, and loss of excitability. Recent studies support the idea that these events are important causes of muscle fatigue and that full use of the Na⁺,K⁺-ATPase (also known as the Na⁺,K⁺ pump) is often essential for adequate clearance of extracellular K⁺. As a result of their electrogenic action, Na⁺,K⁺ pumps also help reverse depolarization arising during excitation, hyperkalemia, and anoxia, or from cell damage resulting from exercise, rhabdomyolysis, or muscle diseases. The ability to evaluate Na⁺,K⁺-pump function and the capacity of the Na⁺,K⁺ pumps to fill these needs require quantification of the total content of Na⁺,K⁺ pumps in skeletal muscle. Inhibition of Na⁺,K⁺-pump activity, or a decrease in their content, reduces muscle contractility. Conversely, stimulation of the Na⁺,K⁺-pump transport rate or increasing the content of Na⁺,K⁺ pumps enhances muscle excitability and contractility. Measurements of [³H]ouabain binding to skeletal muscle in vivo or in vitro have enabled the reproducible quantification of the total content of Na⁺,K⁺ pumps in molar units in various animal species, and in both healthy people and individuals with various diseases. In contrast, measurements of 3-O-methylfluorescein phosphatase activity associated with the Na⁺,K⁺-ATPase may show inconsistent results. Measurements of Na⁺ and K⁺ fluxes in intact isolated muscles show that, after Na⁺ loading or intense excitation, all the Na⁺,K⁺ pumps are functional, allowing calculation of the maximum Na⁺,K⁺-pumping capacity, expressed in molar units/g muscle/min. The activity and content of Na⁺,K⁺ pumps are regulated by exercise, inactivity, K⁺ deficiency, fasting, age, and several hormones and pharmaceuticals. Studies on the α-subunit isoforms of the Na⁺,K⁺-ATPase have detected a relative increase in their number in response to exercise and the glucocorticoid dexamethasone but have not involved their quantification in molar units. Determination of ATPase activity in homogenates and plasma membranes obtained from muscle has shown ouabain-suppressible stimulatory effects of Na⁺ and K⁺.

Introduction: Transport and content of Na⁺ and K⁺ in skeletal muscle

The Na⁺,K⁺-ATPase (also known as the Na⁺,K⁺ pump) is the major translator of metabolic energy in the form of ATP to electrical and chemical gradients for the two most common ions in the body. These gradients enable the generation of action potentials, which are essential for muscle cell function. Evaluation of the physiological and clinical significance of the Na⁺,K⁺ pumps requires measuring the transmembrane fluxes of Na⁺ and K⁺ in intact muscles or cultured muscle cells. The simplest approach involves incubating intact muscles isolated from small animals in temperature-controlled and oxygenated buffers with electrolyte and glucose concentration comparable to that normally present in blood plasma. Initial studies used cut hemi- or quarter-diaphragm muscles from rats, mice, or guinea pigs for incubation because these muscles were considered thin enough to allow adequate oxygenation under these conditions (Gemmill, 1940). However, such preparations have numerous cut muscle ends, allowing large passive movements of Na⁺ and K⁺ and free access of Ca²⁺ to the cell interior. This unavoidably boosts the energy required for active transport of Na⁺, K⁺, and Ca²⁺, and leads to impaired cell survival. Thus, in cut muscles, the components of O₂ consumption and ⁴²K uptake attributable to the Na⁺,K⁺ pump (i.e., the fraction suppressible by the cardiac glycoside ouabain, which binds to and inhibits the Na⁺,K⁺-ATPase) have been severely overestimated. (The ouabain-suppressible components of O₂ consumption or ⁴²K uptake are measured in isolated muscles incubated without or with ouabain and calculated as the difference.) Such overestimation led to the assumption that in skeletal muscle, the Na⁺,K⁺ pumps mediate a large fraction of total energy turnover, suggesting that a major part of the thermogenic action of thyroid hormone is caused by an increased rate of active Na⁺,K⁺ transport (Asano et al., 1976). In contrast, in intact resting muscle preparations, only 2–10% of the total energy turnover is used for active Na⁺,K⁺ transport (Creese, 1968; for details, see Clausen et al., 1991). Even during maximum contractile work in human muscles, only a small fraction (2%) (Medbø and Sejersted, 1990) of total energy release is used for the Na⁺,K⁺ pumps. Thus, in skeletal muscle, the thermogenic action of the Na⁺,K⁺ pumps is modest.For the analysis of Na⁺,K⁺ transport in skeletal muscle, isolated intact limb muscles are used, primarily mammalian soleus, extensor digitorum longus (EDL), extensor digitorum brevis, or epitrochlearis muscles. These preparations can survive during incubation for many hours and can undergo repeated excitation. More recently, the isolated rat sternohyoid muscle, which also offers thin dimensions, cellular integrity, and simple handling, has been introduced (Mu et al., 2011).Measurement of muscle Na⁺ and K⁺ content requires extraction. In the past, this was done by digesting the muscle preparation in nitric acid, a sometimes risky procedure. More recently, this approach has been superseded by homogenization of the tissue in 0.3 M trichloroacetic acid, followed by centrifugation to sediment the proteins (Kohn and Clausen, 1971). The clear supernatant may then be diluted for flame photometric determination of Na⁺ and K⁺ or counting of the isotopes ²²Na or ⁴²K. Because ⁴²K has a short half-life (12.5 h) and is of limited availability, the K⁺ analogue ⁸⁶Rb is often used as a tracer for K⁺. For the quantification of Na⁺,K⁺ pump–mediated (i.e., ouabain-suppressible) transport of K⁺, ⁸⁶Rb gives the same results as ⁴²K (Clausen et al., 1987; Dørup and Clausen, 1994). For other flux measurements, however, the results obtained with ⁸⁶Rb differ appreciably from those obtained with ⁴²K. For example, the fractional loss of ⁸⁶Rb from intact resting rat soleus muscles is only 45% of that measured using ⁴²K. Moreover, two agents shown to stimulate the Na⁺,K⁺ pumps in isolated rat soleus muscle, salbutamol (Clausen and Flatman, 1977) and rat calcitonin gene–related peptide (CGRP) (Andersen and Clausen, 1993), both induce a highly significant stimulation of ⁸⁶Rb efflux from the same muscle (Dørup and Clausen, 1994). In contrast, these same agents induced a rapid but transient (20-min duration) inhibition of the fractional loss of ⁴²K, indicating that a large fraction of the ⁴²K lost from the cells is reaccumulated as a result of stimulation of the Na⁺,K⁺ pumps (Andersen and Clausen, 1993; Dørup and Clausen, 1994). Finally, in skeletal muscle, bumetanide, an inhibitor of the NaK2Cl cotransporter, produces no inhibition of ⁴²K uptake but clear-cut inhibition of ⁸⁶Rb uptake (see ​and22 in Dørup and Clausen, 1994). Thus, results obtained with ⁸⁶Rb must be verified with ⁴²K or flame photometric measurements of changes in intracellular Na⁺ and K⁺ content. The activity of the Na⁺,K⁺ pump depends on ATP supplied by glycolysis or oxidative phosphorylation. Excitability of isolated rat soleus or EDL muscles can be maintained in the presence of the electron transport inhibitor cyanide or during anoxia (Murphy and Clausen, 2007; Fredsted et al., 2012), whereas contractions are markedly suppressed by 2-deoxyglucose, which interferes with the production of glycolytic ATP. Thus, in keeping with the studies of Glitsch (2001), these data suggest that glycolytic ATP is a primary source of energy for the Na⁺,K⁺ pumps (see also Okamoto et al., 2001). More focused studies showed that Na⁺,K⁺-pump function is not adequately supported when cytoplasmic ATP is at the normal resting concentration of ∼8 mM, but that the addition of as little as 1 mM phosphoenol pyruvate produces a marked increase in Na⁺,K⁺-pump function that is supported by endogenous pyruvate kinase bound within the t-tubular triad (Dutka and Lamb, 2007). In anoxic rat EDL muscles, contractility could be restored by stimulating the Na⁺,K⁺ pumps with the β₂ agonists salbutamol or terbutaline, effects that were abolished by ouabain or 2-deoxyglucose (Fredsted et al., 2012). Glycolytic ATP furnishes energy for contractile activity under the critical condition of anoxia. Because the Na⁺,K⁺ pumps are essential for the maintenance of excitability and contractions, it would not be surprising if they were also kept going on glycolytic ATP.

Table 1.

Acute stimulation of the Na⁺,K⁺ pumps in skeletal muscle

Identification and Characterization of the Na+/H+ Antiporter Nhas3 from the Thylakoid Membrane of Synechocystis sp. PCC 6803

Na⁺/H⁺ antiporters influence proton or sodium motive force across the membrane. Synechocystis sp. PCC 6803 has six genes encoding Na⁺/H⁺ antiporters, nhaS1–5 and sll0556. In this study, the function of NhaS3 was examined. NhaS3 was essential for growth of Synechocystis, and loss of nhaS3 was not complemented by expression of the Escherichia coli Na⁺/H⁺ antiporter NhaA. Membrane fractionation followed by immunoblotting as well as immunogold labeling revealed that NhaS3 was localized in the thylakoid membrane of Synechocystis. NhaS3 was shown to be functional over a pH range from pH 6.5 to 9.0 when expressed in E. coli. A reduction in the copy number of nhaS3 in the Synechocystis genome rendered the cells more sensitive to high Na⁺ concentrations. NhaS3 had no K⁺/H⁺ exchange activity itself but enhanced K⁺ uptake from the medium when expressed in an E. coli potassium uptake mutant. Expression of nhaS3 increased after shifting from low CO₂ to high CO₂ conditions. Expression of nhaS3 was also found to be controlled by the circadian rhythm. Gene expression peaked at the beginning of subjective night. This coincided with the time of the lowest rate of CO₂ consumption caused by the ceasing of O₂-evolving photosynthesis. This is the first report of a Na⁺/H⁺ antiporter localized in thylakoid membrane. Our results suggested a role of NhaS3 in the maintenance of ion homeostasis of H⁺, Na⁺, and K⁺ in supporting the conversion of photosynthetic products and in the supply of energy in the dark.Na⁺/H⁺ antiporters are integral membrane proteins that transport Na⁺ and H⁺ in opposite directions across the membrane and that occur in virtually all cell types. These transporters play an important role in the regulation of cytosolic pH and Na⁺ concentrations and influence proton or sodium motive force across the membrane (1, 2). In Escherichia coli, three Na⁺/H⁺ antiporters (NhaA, NhaB, and ChaA) have been described in detail. Of these, NhaA is the functionally best characterized transporter. The crystal structure of NhaA has been resolved (3). In addition, mutants of nhaA, nhaB, and chaA as well as the triple mutant have been generated (4). The triple mutant was shown to be hypersensitive to extracellular Na⁺. The genome of the cyanobacterium Synechocystis sp. PCC 6803 contains six genes encoding Na⁺/H⁺ antiporters (NhaS1–5 and sll0556). NhaS1 (slr1727) has also been designated SynNhaP (5, 6). Null mutants of nhaS1, nhaS2, nhaS4, and nhaS5 have been generated; however, a null mutant of nhaS3 could not be obtained, indicating that it is an essential gene (6–8). By heterologous expression in E. coli, Na⁺/H⁺ exchange activities could be shown for NhaS1–5 (5, 6). Inactivation of nhaS1 and nhaS2 results in retardation of growth of Synechocystis (5, 6). It has been reported that in these mutants the concentration of Na⁺ in cytosol and intrathylakoid space (lumen) increases and impairs the photosynthetic and/or respiratory activity of the cell (9, 10). Therefore the Na⁺ extrusion by Synechocystis Na⁺/H⁺ antiporters similar to E. coli NhaA, NhaB, and ChaA is essential for the adaptation to salinity stress.In contrast to the case in E. coli, Na⁺ is an essential element for the growth of some cyanobacteria (11, 12). Interestingly, the Na⁺/H⁺ antiporter homolog NhaS4 was identified as an uptake system for Na⁺ from the medium during a screen for mutations in Synechocystis that result in lack of growth at low Na⁺ concentrations (7). The requirement of a Na⁺ uptake antiporter for cell growth is consistent with the physiology of Synechocystis. Specifically, photoautotrophic bacteria like cyanobacteria share some components (plastoquinone, cytochrome b₆f, and c₆) of the thylakoid membrane for electron transport for both photophosphorylation and respiratory oxidative phosphorylation. Na⁺/H⁺ antiporters therefore may coordinate both H⁺ and Na⁺ gradients across the plasma and thylakoid membranes to adapt to daily environmental changes (11). It remains to be determined whether the six Na⁺/H⁺ antiporters are localized to the plasma membrane or to the thylakoid membrane in Synechocystis. Information on the membrane localization will also provide information on the physiological role in Synechocystis. In this study, we explored the membrane localization of NhaS3, the role of specific amino acid residues for its function, and the effect of CO₂ concentration and circadian rhythms on the expression pattern of nhaS3 to gain insight into the physiological role of NhaS3 in Synechocystis.     

Compatible solutes mitigate damaging effects of salt stress by reducing the impact of stress-induced reactive oxygen species

Under abiotic stress conditions, rapid increases in reactive oxygen species (ROS) levels occurs within plant cells. Although their role as a major signalling agent in plants is now acknowledged, elevated ROS levels can result in an impairment of membrane integrity. Similar to our previous findings on imposition of salt stress, application of the hydroxyl radical (OH^•) to Arabidopsis roots results in a massive efflux of K⁺ from epidermal cells. This is likely to cause significant damage to cell metabolism. Since K⁺ loss also occurs after salt application and salt stress leads to increased cellular ROS levels, we suggest that at least some of the detrimental effects of salinity is due to damage by its resulting ROS on K⁺ homeostasis. We also observed a comparative reduction in K⁺ efflux by compatible solutes after both oxidative and salt stress. Thus, we propose that under saline conditions, compatible solutes mitigate the oxidative stress damage to membrane transporters. Whether this amelioration is due to free-radical scavenging or by direct protection of transporter systems, warrants further investigation.Key words: compatible solutes, hydroxyl radical, potassium efflux, reactive oxygen species, salt stressReactive oxygen species (ROS) are continuously produced as by-products of various metabolic pathways.¹ Under unstressed steady-state conditions, cellular ROS levels are kept in check by the sophisticated antioxidant defence system.² However, under adverse environmental conditions, the balance between ROS production and its subsequent scavenging may be perturbed, leading to a rapid increase in ROS levels.³ Although significant progress has been made in defining ROS as a major signalling agent in plants,³ ROS can react with a large variety of biomolecules, causing lipid peroxidation and impairing membrane integrity.^4,5 One such abiotic stress is salt stress,⁶ with ROS generation occurring within minutes of salt application.⁷ Alleviation of oxidative damage may be, therefore, an important strategy of plant salt tolerance.⁸One of the earliest measurable responses to salt stress is a massive K⁺ efflux from plant roots.^9,10 Such K⁺ efflux is initiated within seconds of acute salt stress and may last for several hours^11,12 reducing the intracellular K⁺ pool^13,14 and significantly impairing cell metabolism. Consistent with the key role of K⁺ homeostasis in salt tolerance mechanisms¹⁵ a reduction of K⁺ efflux correlates with increased salt tolerance.^11,12We have previously reported that hydroxyl radical (OH^•) application to Arabidopsis roots also results in a rapid efflux of K⁺ from the epidermis.¹⁶ In this report, we find a similar K⁺ efflux response.¹⁷ As is the case for salt stress,⁹ we found that membrane depolarisation could be responsible for a substantial part of this efflux. However, an observed discrepancy between the membrane depolarisation and the pattern of K⁺ efflux indicates that voltage-dependence is not the only factor influencing K⁺ loss from the root cells after oxidative stress. Demidchik et al.¹⁶ demonstrated that stress-induced K⁺ efflux could be mediated by activation of K⁺ outward rectifying channels directly by OH^•. This direct effect on K⁺ transporters could also account for our observed delay before the peak efflux of K⁺ is measured, indicating that a certain amount of time is required before maximal direct damage by OH^• to transporters occurs. Because both K⁺ channel blockers and non-selective cation channel blockers reduce this efflux, it indicates non-specificity in OH^• attack. Furthermore, combinations of these channel blockers were effective in reducing K⁺ efflux implying that, at least in the short term, the damaging effects of OH^• is due to compromising the transporter systems as opposed to lipid peroxidation. Certainly, K⁺ channels harbour reactive groups, thus are expected to be sensitive to ROS.¹⁸We have previously shown that the exogenous application of low concentrations of a variety of compatible solutes reduces the salt-induced K⁺ efflux.^19,20 Plants, when confronted with a saline environment, respond with a significant elevation in their compatible solute levels. This ameliorates the detrimental effects of salinity.²¹ However, their original proposed role in cellular osmoregulation is under question: their concentration in transgenic plants overexpressing osmolyte biosynthetic genes is not significant for osmotic adjustment, despite showing improved salt tolerance.⁸ Furthermore, one hallmark of the detoxification effect is its lack of specificity, that is, transgenic plants have increased tolerance not only to high salt, but also to drought, cold and heat shock,^22,23 stresses that also result in ROS production.³ Certainly, ecotopic expression studies suggest that compatible solutes increase stress tolerance by protection of membranes and proteins against ROS.⁶We show that in this work that exogenous application of low concentrations of a range of compatible solutes significantly reduces OH^•-induced K⁺ efflux,¹⁷ a similar effect to that we reported after salt application to barley roots¹⁹ and also observed in Arabidopsis (Fig. 1). Interestingly, we found that not only known free-radical scavenging osmolytes,²⁴ but also glycine betaine, previously found to be non-effective in ROS scavenging,²⁴ were effective in reducing OH^•-induced K⁺ efflux. Indeed, glycine betaine showed a greater mitigation of OH^•-induced K⁺ efflux compared to that induced by 50 mM NaCl (Fig. 1). However, it is open to speculation as to whether this mitigation is via direct channel blocking, a direct protection of ion channel proteins or by some other protective mechanism.Open in a separate windowFigure 1Effects of exogenous supply of compatible solutes on net peak K⁺ efflux after application of either 1 mM Cu/a or 50 mM NaCl. Roots were preincubated for 1 h in 5 mM concentration of a number of compatible solutes prior to treatment, Mean ± SE (n = 6-8).In our further investigations we have found that salt-tolerant barley show a reduced ROS-induced K⁺ efflux compared to sensitive varieties.²⁵ This superior ability of salt-tolerant barley cultivars of preventing K⁺ loss further indicates a possible causal link between salt and oxidative stress tolerance. We propose that upon the imposition of salt stress, the instantaneously resulting membrane depolarisation9 results in activation of depolarisation activated K⁺ outward-rectifying channels, leading to the initial massive K⁺ efflux. Over the longer term, ROS levels within the plant cell increase,⁷ resulting in direct damage to K⁺ transporters and the longer-term sustained loss of K⁺ from the cell. Due to mitigation of both NaCl- and OH^•-induced K⁺ efflux by compatible solutes, we propose that one of their primary amelioratory effects is through reducing the damaging effects of salt-produced ROS on K⁺ transporter, and by this means, reducing the effects of stress damage. Whether this amelioration is achieved through free-radical scavenging or due to a direct protection of membrane transports warrants further investigation.     

A Trk/HKT-Type K+ Transporter from Trypanosoma brucei

The molecular mechanisms of K⁺ homeostasis are only poorly understood for protozoan parasites. Trypanosoma brucei subsp. parasites, the causative agents of human sleeping sickness and nagana, are strictly extracellular and need to actively concentrate K⁺ from their hosts’ body fluids. The T. brucei genome contains two putative K⁺ channel genes, yet the trypanosomes are insensitive to K⁺ antagonists and K⁺ channel-blocking agents, and they do not spontaneously depolarize in response to high extracellular K⁺ concentrations. However, the trypanosomes are extremely sensitive to K⁺ ionophores such as valinomycin. Surprisingly, T. brucei possesses a member of the Trk/HKT superfamily of monovalent cation permeases which so far had only been known from bacteria, archaea, fungi, and plants. The protein was named TbHKT1 and functions as a Na⁺-independent K⁺ transporter when expressed in Escherichia coli, Saccharomyces cerevisiae, or Xenopus laevis oocytes. In trypanosomes, TbHKT1 is expressed in both the mammalian bloodstream stage and the Tsetse fly midgut stage; however, RNA interference (RNAi)-mediated silencing of TbHKT1 expression did not produce a growth phenotype in either stage. The presence of HKT genes in trypanosomatids adds a further piece to the enigmatic phylogeny of the Trk/HKT superfamily of K⁺ transporters. Parsimonial analysis suggests that the transporters were present in the first eukaryotes but subsequently lost in several of the major eukaryotic lineages, in at least four independent events.Potassium (K⁺) is the most abundant cation in the cytosol of any cell and hence an essential macronutrient for life on earth. Concentrative K⁺ uptake across the plasma membrane is energized directly by ATPases and indirectly by the negative membrane potential or by coupling, via symport or antiport, to other transport processes such as H⁺ flux. The ancestry of K⁺ transporters renders them ideal subjects for phylogenetic comparisons. Indeed, the different kinds of known K⁺ transporters—pumps, channels, permeases, symporters, and antiporters—are all found in bacteria (43). Eukaryotes do not appear to have invented further mechanisms of K⁺ transport; on the contrary, some families of K⁺ transporters were lost over the course of eukaryote evolution, particularly among the metazoa (53).The Trk/HKT superfamily (TC transporter classification 2.A.38 [43]) consists of bacterial TrkH and KtrB, plant HKT, and fungal Trk transporters (15). These proteins share a topology with 8 transmembrane (TM) domains and, sandwiched between odd- and even-numbered TM domains, 4 shorter hydrophobic helices that resemble the P-loops of K⁺ channels (14, 27, 55). In the K⁺ channel, these pore-forming loops end in the filter residues glycine-tyrosine-glycine, which coordinate K⁺ by means of their backbones’ carbonyl oxygens (13). The P-loop-like helices of Trk/HKT transporters end in a single conserved glycine (48), and these glycines have been shown to determine K⁺ selectivity over Na⁺ of the transporters (34, 49). Thus, a Trk/HKT monomer with 8 TM domains and 4 P-loops is thought to have a similar pore architecture to a K⁺ channel tetramer with two TM domains and one P-loop per subunit. The Trk/HKT transporters are important for cellular K⁺ acquisition in microorganisms, since trk null mutant yeast or bacteria exhibit growth phenotypes on media containing low K⁺ concentrations (20, 46). The roles of the Trk/HKT transporters in plants are more diverse, including Na⁺ distribution (10, 33, 47), osmoregulation (32), and salt tolerance (39). So far, no HKT/Trk transporter has been described from the metazoa or protista.Trypanosoma brucei subsp. parasites comprise the causative agents of human and livestock trypanosomosis: sleeping sickness and nagana, respectively. The distribution of the parasites is restricted by that of their vector, the blood-sucking tsetse fly (Glossina spp.), to the so-called tsetse belt comprising 36 countries between the Sahara desert and the Kalahari (3, 8). African trypanosomes proliferate extracellularly in the blood, evading the mammalian immune response by antigenic variation. Untreated sleeping sickness is fatal. There is an urgent need for new and better drugs since the current ones, the arsenical melarsoprol in particular, suffer from severe side effects (31). In the mammalian bloodstream, the parasites encounter a rich and steady supply of nutrients, readily imported by specific permeases or endocytosed via receptors (7). Research on trypanosomal nutrient uptake has so far concentrated on transporters of organic substrates: nucleobases, nucleosides, sugars, and amino acids (4, 12, 26, 30, 35, 56). Little is known about how the parasites import inorganic nutrients. The malaria parasite Plasmodium falciparum possesses two putative K⁺ channel subunits with 6 TM domains and one P-loop (19, 52). Disruption of an orthologous gene in Plasmodium berghei strongly impaired the development of the malaria parasites in the mosquito (18). However, these putative channels have not yet been proven to be permeable to K⁺. The T. brucei genome (6) is annotated to contain two putative K⁺ channels; in addition, a putative ATPase has been identified resembling fungal Na⁺/K⁺ efflux ATPases (5, 45). None of these has been addressed experimentally. Here we present the identification and characterization of TbHKT1 (Tb10.70.2940), a Trk/HKT-type K⁺ transporter from Trypanosoma brucei and representative of a new clade of Trk/HKT genes from kinetoplastid parasites.