Effects of foot-and-mouth disease virus nonstructural proteins on the structure and function of the early secretory pathway: 2BC but not 3A blocks endoplasmic reticulum-to-Golgi transport

Infection of cells by picornaviruses leads to the generation of intracellular membrane vesicles. The expression of poliovirus (PV) 3A protein causes swelling of the endoplasmic reticulum (ER) and inhibition of protein trafficking between the ER and the Golgi apparatus. Here, we report that the nonstructural proteins of a second picornavirus, foot-and-mouth disease virus (FMDV), also perturb the secretory pathway. FMDV proteins 3A, 2B, 2C, and 2BC expressed alone in cells were recovered from crude membrane fractions, indicating membrane association. Immunofluorescence microscopy showed that 3A was located in a reticular structure and 2B was located in the ER, while 2C was located in both the ER and the bright punctate structures within the Golgi apparatus. 2BC gave punctate cytoplasmic staining and also caused accumulation of ER proteins in large vesicular structures located around the nuclei. The effect of the FMDV proteins on the trafficking of the vesicular stomatitis virus glycoprotein (G protein) from the ER to the cell surface was determined. Unlike its PV counterpart, the 3A protein of FMDV did not prevent trafficking of the G protein to the cell surface. Instead, surface expression of the G protein was blocked by 2BC, with retention of the G protein in a modified ER compartment staining for 2BC. The results suggest that the nonstructural proteins of different picornaviruses may vary in their ability to perturb the secretory pathway. Since FMDV 2BC can block the delivery of proteins to the cell surface, it may, as shown for PV 3A, play a role in immune evasion and contribute to the persistent infections observed in ruminants.     

Inhibition of cellular protein secretion by norwalk virus nonstructural protein p22 requires a mimic of an endoplasmic reticulum export signal

Protein trafficking between the endoplasmic reticulum (ER) and Golgi apparatus is central to cellular homeostasis. ER export signals are utilized by a subset of proteins to rapidly exit the ER by direct uptake into COPII vesicles for transport to the Golgi. Norwalk virus nonstructural protein p22 contains a YXΦESDG motif that mimics a di-acidic ER export signal in both sequence and function. However, unlike normal ER export signals, the ER export signal mimic of p22 is necessary for apparent inhibition of normal COPII vesicle trafficking, which leads to Golgi disassembly and antagonism of Golgi-dependent cellular protein secretion. This is the first reported function for p22. Disassembly of the Golgi apparatus was also observed in cells replicating Norwalk virus, which may contribute to pathogenesis by interfering with cellular processes that are dependent on an intact secretory pathway. These results indicate that the ER export signal mimic is critical to the antagonistic function of p22, shown herein to be a novel antagonist of ER/Golgi trafficking. This unique and well-conserved human norovirus motif is therefore an appealing target for antiviral drug development.     

The coxsackievirus 2B protein increases efflux of ions from the endoplasmic reticulum and Golgi, thereby inhibiting protein trafficking through the Golgi

Coxsackievirus infection leads to a rapid reduction of the filling state of the endoplasmic reticulum (ER) and Golgi Ca2+ stores. The coxsackievirus 2B protein, a small membrane protein that localizes to the Golgi and to a lesser extent to the ER, has been proposed to play an important role in this effect by forming membrane-integral pores, thereby increasing the efflux of Ca2+ from the stores. Here, evidence is presented that supports this idea and that excludes the possibility that 2B reduces the uptake of Ca2+ into the stores. Measurement of intra-organelle-free Ca2+ in permeabilized cells revealed that the ability of 2B to reduce the Ca2+ filling state of the stores was preserved at steady ATP. Biochemical analysis in a cell-free system further showed that 2B had no adverse effect on the activity of the sarco/endoplasmic reticulum calcium ATPase, the Ca2+-ATPase that transports Ca2+ from the cytosol into the stores. To investigate whether 2B specifically affects Ca2+ homeostasis or other ion gradients, we measured the lumenal Golgi pH. Expression of 2B resulted in an increased Golgi pH, indicative for the efflux of H+ from the Golgi lumen. Together, these data support a model that 2B increases the efflux of ions from the ER and Golgi by forming membrane-integral pores. We have demonstrated that a major consequence of this activity is the inhibition of protein trafficking through the Golgi complex.     

Enterovirus protein 2B po(u)res out the calcium: a viral strategy to survive?

Enteroviruses modify several cellular functions to ensure efficient replication. However, some of these alterations can trigger a defensive apoptotic host-cell program. To prevent premature abortion of their productive cycle, enteroviruses have developed anti-apoptotic countermeasures. Here, we discuss recent evidence that the enterovirus 2B protein exerts an anti-apoptotic activity that is related to its ability to form pores in endoplasmic reticulum (ER) and Golgi membranes, thereby reducing their Ca(2+) content and perturbing ER-mitochondrial Ca(2+) signaling.     

Involvement of the penta-EF-hand protein Pef1p in the Ca2+-dependent regulation of COPII subunit assembly in Saccharomyces cerevisiae

Although it is well established that the coat protein complex II (COPII) mediates the transport of proteins and lipids from the endoplasmic reticulum (ER) to the Golgi apparatus, the regulation of the vesicular transport event and the mechanisms that act to counterbalance the vesicle flow between the ER and Golgi are poorly understood. In this study, we present data indicating that the penta-EF-hand Ca(2+)-binding protein Pef1p directly interacts with the COPII coat subunit Sec31p and regulates COPII assembly in Saccharomyces cerevisiae. ALG-2, a mammalian homolog of Pef1p, has been shown to interact with Sec31A in a Ca(2+)-dependent manner and to have a role in stabilizing the association of the Sec13/31 complex with the membrane. However, Pef1p displayed reversed Ca(2+) dependence for Sec13/31p association; only the Ca(2+)-free form of Pef1p bound to the Sec13/31p complex. In addition, the influence on COPII coat assembly also appeared to be reversed; Pef1p binding acted as a kinetic inhibitor to delay Sec13/31p recruitment. Our results provide further evidence for a linkage between Ca(2+)-dependent signaling and ER-to-Golgi trafficking, but its mechanism of action in yeast seems to be different from the mechanism reported for its mammalian homolog ALG-2.     

Determinants for membrane association and permeabilization of the coxsackievirus 2B protein and the identification of the Golgi complex as the target organelle

The 2B protein of enterovirus is responsible for the alterations in the permeability of secretory membranes and the plasma membrane in infected cells. The structural requirements for the membrane association and the subcellular localization of this essential virus protein, however, have not been defined. Here, we provide evidence that the 2B protein is an integral membrane protein in vivo that is predominantly localized at the Golgi complex upon individual expression. Addition of organelle-specific targeting signals to the 2B protein revealed that the Golgi localization is an absolute prerequisite for the ability of the protein to modify plasma membrane permeability. Expression of deletion mutants and heterologous proteins containing specific domains of the 2B protein demonstrated that each of the two hydrophobic regions could mediate membrane binding individually. However, the presence of both hydrophobic regions was required for the correct membrane association, efficient Golgi targeting, and the membrane-permeabilizing activity of the 2B protein, suggesting that the two hydrophobic regions are cooperatively involved in the formation of a membrane-integral complex. The formation of membrane-integral pores by the 2B protein in the Golgi complex and the possible mechanism by which a Golgi-localized virus protein modifies plasma membrane permeability are discussed.     

The mammalian Sec6/8 complex interacts with Ca(2+) signaling complexes and regulates their activity

The localization of various Ca(2+) transport and signaling proteins in secretory cells is highly restricted, resulting in polarized agonist-stimulated Ca(2+) waves. In the present work, we examined the possible roles of the Sec6/8 complex or the exocyst in polarized Ca(2+) signaling in pancreatic acinar cells. Immunolocalization by confocal microscopy showed that the Sec6/8 complex is excluded from tight junctions and secretory granules in these cells. The Sec6/8 complex was found in at least two cellular compartments, part of the complex showed similar, but not identical, localization with the Golgi apparatus and part of the complex associated with Ca(2+) signaling proteins next to the plasma membrane at the apical pole. Accordingly, immunoprecipitation (IP) of Sec8 did not coimmunoprecipitate betaCOP, Golgi 58K protein, or mannosidase II, all Golgi-resident proteins. By contrast, IP of Sec8 coimmunoprecipitates Sec6, type 3 inositol 1,4,5-trisphosphate receptors (IP(3)R3), and the Gbetagamma subunit of G proteins from pancreatic acinar cell extracts. Furthermore, the anti-Sec8 antibodies coimmunoprecipitate actin, Sec6, the plasma membrane Ca(2+) pump, the G protein subunits Galphaq and Gbetagamma, the beta1 isoform of phospholipase C, and the ER resident IP(3)R1 from brain microsomal extracts. Antibodies against the various signaling and Ca(2+) transport proteins coimmunoprecipitate Sec8 and the other signaling proteins. Dissociation of actin filaments in the immunoprecipitate had no effect on the interaction between Sec6 and Sec8, but released the actin and dissociated the interaction between the Sec6/8 complex and Ca(2+) signaling proteins. Hence, the interaction between the Sec6/8 and Ca(2+) signaling complexes is likely mediated by the actin cytoskeleton. The anti-Sec6 and anti-Sec8 antibodies inhibited Ca(2+) signaling at a step upstream of Ca(2+) release by IP(3). Disruption of the actin cytoskeleton with latrunculin B in intact cells resulted in partial translocation of Sec6 and Sec8 from membranes to the cytosol and interfered with propagation of agonist-evoked Ca(2+) waves. Our results suggest that the Sec6/8 complex has multiple roles in secretory cells including governing the polarized expression of Ca(2+) signaling complexes and regulation of their activity.     

Progesterone impairs human ether-a-go-go-related gene (HERG) trafficking by disruption of intracellular cholesterol homeostasis

The prolongation of QT intervals in both mothers and fetuses during the later period of pregnancy implies that higher levels of progesterone may regulate the function of the human ether-a-go-go-related gene (HERG) potassium channel, a key ion channel responsible for controlling the length of QT intervals. Here, we studied the effect of progesterone on the expression, trafficking, and function of HERG channels and the underlying mechanism. Treatment with progesterone for 24 h decreased the abundance of the fully glycosylated form of the HERG channel in rat neonatal cardiac myocytes and HERG-HEK293 cells, a cell line stably expressing HERG channels. Progesterone also concentration-dependently decreased HERG current density, but had no effect on voltage-gated L-type Ca(2+) and K(+) channels. Immunofluorescence microscopy and Western blot analysis show that progesterone preferentially decreased HERG channel protein abundance in the plasma membrane, induced protein accumulation in the dilated endoplasmic reticulum (ER), and increased the protein expression of C/EBP homologous protein, a hallmark of ER stress. Application of 2-hydroxypropyl-β-cyclodextrin (a sterol-binding agent) or overexpression of Rab9 rescued the progesterone-induced HERG trafficking defect and ER stress. Disruption of intracellular cholesterol homeostasis with simvastatin, imipramine, or exogenous application of cholesterol mimicked the effect of progesterone on HERG channel trafficking. Progesterone may impair HERG channel folding in the ER and/or block its trafficking to the Golgi complex by disrupting intracellular cholesterol homeostasis. Our findings may reveal a novel molecular mechanism to explain the QT prolongation and high risk of developing arrhythmias during late pregnancy.     

Epstein-Barr virus latent membrane protein 1 increases calcium influx through store-operated channels in B lymphoid cells

Ca(2+) signaling plays an important role in B cell survival and activation and is dependent on Ca(2+) trapped in the endoplasmic reticulum (ER) and on extracellular Ca(2+). Epstein-Barr virus (EBV) can immortalize B cells and contributes to lymphomagenesis. Previously, we showed that the ER Ca(2+) content of Burkitt lymphoma cell lines was increased following infection with immortalization-competent virus expressing the full set of EBV latency genes (B95-8). In contrast, infection with an immortalization-deficient virus (P3HR-1) not expressing LMP-1 is without effect. LMP-1 protein expression was sufficient to increase the ER Ca(2+) content and to increase the cytosolic Ca(2+) concentration ([Ca(2+)](cyt)). In this follow-up study, we showed that the resting [Ca(2+)](cyt) of P3HR-1-infected cells was decreased, implying that EBV not only modified the ER homeostasis but also affected the cytosolic Ca(2+) homeostasis. Furthermore, even if the store-operated calcium entry (SOCE) of these cells was normal, the [Ca(2+)](cyt) increase after thapsigargin + CaCl(2) stimulation was blunted. In contrast, the resting [Ca(2+)](cyt) of B95-8 infected cells was not changed, even if their SOCE was increased significantly. When expressed alone, LMP-1 induced an increase of the SOCE amplitude and the expression of the protein allowing this influx, Orai1, showing the effect of EBV on SOCE of B cells are mediated by LMP-1. However, other hitherto unidentified EBV processes, unmasked in P3HR-1 infected cells, counteract this LMP-1-dependent increase of SOCE amplitude to impair a general and potentially toxic increase of [Ca(2+)](i). Thus, EBV infection modifies the cellular Ca(2+) homeostasis by acting on the ER and plasma membrane transporters.     

Trafficking of TRPP2 by PACS proteins represents a novel mechanism of ion channel regulation

The trafficking of ion channels to the plasma membrane is tightly controlled to ensure the proper regulation of intracellular ion homeostasis and signal transduction. Mutations of polycystin-2, a member of the TRP family of cation channels, cause autosomal dominant polycystic kidney disease, a disorder characterized by renal cysts and progressive renal failure. Polycystin-2 functions as a calcium-permeable nonselective cation channel; however, it is disputed whether polycystin-2 resides and acts at the plasma membrane or endoplasmic reticulum (ER). We show that the subcellular localization and function of polycystin-2 are directed by phosphofurin acidic cluster sorting protein (PACS)-1 and PACS-2, two adaptor proteins that recognize an acidic cluster in the carboxy-terminal domain of polycystin-2. Binding to these adaptor proteins is regulated by the phosphorylation of polycystin-2 by the protein kinase casein kinase 2, required for the routing of polycystin-2 between ER, Golgi and plasma membrane compartments. Our paradigm that polycystin-2 is sorted to and active at both ER and plasma membrane reconciles the previously incongruent views of its localization and function. Furthermore, PACS proteins may represent a novel molecular mechanism for ion channel trafficking, directing acidic cluster-containing ion channels to distinct subcellular compartments.     

Syntaxin 17 cycles between the ER and ERGIC and is required to maintain the architecture of ERGIC and Golgi

Background information. Syntaxin 17 is a SNARE (soluble N‐ethylmaleimide‐sensitive‐factor‐attachment protein receptor) protein that predominantly localizes to the ER (endoplasmic reticulum) and to some extent in the ERGIC (ER—Golgi intermediate compartment). Syntaxin 17 has been suggested to function as a receptor at the ER membrane that mediates trafficking between the ER and post‐ER compartments. It has a unique 33 amino acid luminal tail whose function is not known. Here we have investigated the structural requirements for localization of syntaxin 17 to the ERGIC and its role in trafficking. Results. Deletion analysis showed that syntaxin 17 required its cytoplasmic domain to exit the ER and localize to the ERGIC. Mutation of a conserved tyrosine residue in the cytoplasmic domain resulted in reduced localization of syntaxin 17 in the ERGIC and ER‐exit sites, suggesting the presence of a tyrosine‐based ER export motif. Syntaxin 17 also required its C‐terminal tail to localize to the ERES (ER exit sites) and ERGIC. Knockdown of syntaxin 17 destabilized the ERGIC organization and also caused fragmentation of the Golgi complex. Syntaxin 17 showed direct interaction with transmembrane proteins p23 and p25 (cargo receptors that cycle between the ER and Golgi) with the help of its C‐terminal tail. Overexpression of syntaxin 17 redistributed β‐COP (β‐coatomer protein) which required its C‐terminal tail. Overexpression of syntaxin 17 also blocked the anterograde transport of VSVG (vesicular stomatitis virus G‐protein) in the ERGIC. Conclusions. We show that syntaxin 17 has a tyrosine‐based motif which is required for its incorporation into COPII (coatomer protein II) vesicles, exit from the ER and localization to the ERGIC. Our results suggest that syntaxin 17 cycles between the ER and ERGIC through classical trafficking pathways involving COPII and COPI (coatomer protein I) vesicles, which requires its unique C‐terminal tail. We also show that syntaxin 17 is essential for maintaining the architecture of ERGIC and Golgi.     

Actin filaments play a critical role in vacuolar trafficking at the Golgi complex in plant cells

Actin filaments are thought to play an important role in intracellular trafficking in various eukaryotic cells. However, their involvement in intracellular trafficking in plant cells has not been clearly demonstrated. Here, we investigated the roles actin filaments play in intracellular trafficking in plant cells using latrunculin B (Lat B), an inhibitor of actin filament assembly, or actin mutants that disrupt actin filaments when overexpressed. Lat B and actin2 mutant overexpression inhibited the trafficking of two vacuolar reporter proteins, sporamin:green fluorescent protein (GFP) and Arabidopsis thaliana aleurain-like protein:GFP, to the central vacuole; instead, a punctate staining pattern was observed. Colocalization experiments with various marker proteins indicated that these punctate stains corresponded to the Golgi complex. The A. thaliana vacuolar sorting receptor VSR-At, which mainly localizes to the prevacuolar compartment, also accumulated at the Golgi complex in the presence of Lat B. However, Lat B had no effect on the endoplasmic reticulum (ER) to Golgi trafficking of sialyltransferase or retrograde Golgi to ER trafficking. Lat B also failed to influence the Golgi to plasma membrane trafficking of H+-ATPase:GFP or the secretion of invertase:GFP. Based on these observations, we propose that actin filaments play a critical role in the trafficking of proteins from the Golgi complex to the central vacuole.     

STAM Adaptor Proteins Interact with COPII Complexes and Function in ER-to-Golgi Trafficking

Signal-transducing adaptor molecules (STAMs) are involved in growth factor and cytokine signaling as well as receptor degradation, and they form complexes with a number of endocytic proteins, including Hrs and Eps15. In this study, we demonstrate that STAM proteins also localize prominently to early exocytic compartments and profoundly regulate Golgi morphology. Upon STAM overexpression in cells, the Golgi apparatus becomes extensively fragmented and dispersed, but when STAMs are depleted, the Golgi becomes highly condensed. Under both scenarios, vesicular stomatitis virus G protein–green fluorescent protein trafficking to the plasma membrane is markedly inhibited, and recovery of Golgi morphology after Brefeldin A treatment is substantially impaired in STAM-depleted cells. Furthermore, STAM proteins interact with coat protein II (COPII) proteins, probably at endoplasmic reticulum (ER) exit sites, and Sar1 activity is required to maintain the localization of STAMs at discrete sites. Thus, in addition to their roles in signaling and endocytosis, STAMs function prominently in ER-to-Golgi trafficking, most likely through direct interactions with the COPII complex.     

Disruption of Golgi morphology and trafficking in cells expressing mutant prenylated rab acceptor-1

Prenylated Rab acceptor (PRA1) is a protein that binds Rab GTPases and the v-SNARE VAMP2. The protein is localized to the Golgi complex and post-Golgi vesicles. To determine its functional role, we generated a number of point mutations and divided them into three classes based on cellular localization. Class A mutants were retained in the endoplasmic reticulum (ER) and exerted an inhibitory effect on transport of vesicular stomatitis virus envelope glycoprotein (VSVG) from the ER to Golgi as well as to the plasma membrane. Class B mutants exhibited a highly condensed Golgi complex and inhibited exit of anterograde cargo from this organelle. Class C mutants exhibited an intermediate phenotype with Golgi and ER localization along with extensive tubular structures emanating from the Golgi complex. There was a direct correlation between the cellular phenotype and binding to Rab and VAMP2. Class A and C mutants showed a significant decrease in Rab and VAMP2 binding, whereas an increase in binding was observed in the class B mutants. Thus, PRA1 is required for vesicle formation from the Golgi complex and might be involved in recruitment of Rab effectors and SNARE proteins during cargo sequestration.     

COG-7-deficient Human Fibroblasts Exhibit Altered Recycling of Golgi Proteins

Recently, we reported that two siblings presenting with the clinical syndrome congenital disorders of glycosylation (CDG) have mutations in the gene encoding Cog7p, a member of the conserved oligomeric Golgi (COG) complex. In this study, we analyzed the localization and trafficking of multiple Golgi proteins in patient fibroblasts under a variety of conditions. Although the immunofluorescent staining pattern of several Golgi proteins was indistinguishable from normal, the staining of endoplasmic reticulum (ER)-Golgi intermediate compartment (ERGIC)-53 and the vesicular-soluble N-ethylmaleimide-sensitive factor attachment protein receptors GS15 and GS28 was abnormal, and the steady-state level of GS15 was greatly decreased. Retrograde transport of multiple Golgi proteins to the ER in patient fibroblasts via brefeldin A-induced tubules was significantly slower than occurs in normal fibroblasts, whereas anterograde protein trafficking was much less affected. After prolonged treatment with brefeldin A, several Golgi proteins were detected in clusters that colocalize with the microtubule-organizing center in patient cells. All of these abnormalities were normalized in COG7-corrected patient fibroblasts. These results serve to better define the role of the COG complex in facilitating protein trafficking between the Golgi and ER and provide a diagnostic framework for the identification of CDG defects involving trafficking proteins.     

RNA-protein interactions in regulation of picornavirus RNA translation.

The translation of picornavirus RNA occurs by a cap-independent mechanism directed by a region of about 450 nucleotides from the 5' untranslated region, termed an internal ribosome entry site (IRES). Internal initiation of protein synthesis occurs without any requirement for viral proteins. Furthermore, it is maintained when host cell protein synthesis is almost abolished. By using in vitro translation systems, two distinct families of IRES elements which have very different predicted RNA secondary structures have been defined. The cardiovirus and aphthovirus elements function very efficiently in rabbit reticulocyte lysate, whereas the enterovirus and rhinovirus elements function poorly in this system. However, supplementation of this translation system with additional cellular proteins can stimulate translation directed by the enterovirus and rhinovirus RNAs and reduce production of aberrant initiation products. The characterization of cellular proteins interacting with the picornavirus IRES is a major focus of research. Many different protein species can be observed to interact with regions of the IRES by in vitro analyses, e.g., UV cross-linking. However, the function and significance of many of these interactions are not always known. For two proteins, La and the polypyrimidine tract-binding protein, evidence has been obtained for a functional role of their interaction with IRES elements.     

Aberrant cytoplasmic sequestration of eNOS in endothelial cells after monocrotaline, hypoxia, and senescence: live-cell caveolar and cytoplasmic NO imaging

We previously reported the disruption of caveolae/rafts, dysfunction of Golgi tethers, N-ethylmaleimide-sensitive factor-attachment protein (SNAP) receptor proteins (SNAREs), and SNAPs, and inhibition of anterograde trafficking in endothelial cells in culture and rat lung exposed to monocrotaline pyrrole (MCTP) as a prelude to the development of pulmonary hypertension. We have now investigated 1) whether this trafficking block affects subcellular localization and function of endothelial nitric oxide (NO) synthase (eNOS) and 2) whether Golgi blockade and eNOS sequestration are observed after hypoxia and senescence. Immunofluorescence data revealed that MCTP-induced "megalocytosis" of pulmonary arterial endothelial cells (PAEC) was accompanied by a loss of eNOS from the plasma membrane, with increased accumulation in the cytoplasm. This cytoplasmic eNOS was sequestered in heterogeneous compartments and partially colocalized with Golgi and endoplasmic reticulum (ER) markers, caveolin-1, NOSTRIN, and ER Tracker, but not Lyso Tracker. Hypoxia and senescence also produced enlarged PAEC, with dysfunctional Golgi and loss of eNOS from the plasma membrane, with sequestration in the cytoplasm. Live-cell imaging of caveolar and cytoplasmic NO with 4,5-diaminofluorescein diacetate (DAF-2DA) as probe showed a marked loss of caveolar NO after MCTP, hypoxia, and senescence. Although ionomycin stimulated DAF-2DA fluorescence in control PAEC, this ionophore decreased DAF-2DA fluorescence in MCTP-treated and senescent PAEC, suggesting localization of eNOS in an aberrant cytoplasmic compartment that was readily discharged by Ca(2+)-induced exocytosis. Thus monocrotaline, hypoxia, and senescence produce a Golgi blockade in PAEC, leading to sequestration of eNOS away from its functional caveolar location and providing a mechanism for the often-reported reduction in pulmonary arterial NO levels in experimental pulmonary hypertension, despite sustained eNOS protein levels.     

Two distinctly localized p-type ATPases collaborate to maintain organelle homeostasis required for glycoprotein processing and quality control

Membrane transporter proteins are essential for the maintenance of cellular ion homeostasis. In the secretory pathway, the P-type ATPase family of transporters is found in every compartment and the plasma membrane. Here, we report the identification of COD1/SPF1 (control of HMG-CoA reductase degradation/SPF1) through genetic strategies intended to uncover genes involved in protein maturation and endoplasmic reticulum (ER)-associated degradation (ERAD), a quality control pathway that rids misfolded proteins. Cod1p is a putative ER P-type ATPase whose expression is regulated by the unfolded protein response, a stress-inducible pathway used to monitor and maintain ER homeostasis. COD1 mutants activate the unfolded protein response and are defective in a variety of functions apart from ERAD, which further support a homeostatic role. COD1 mutants display phenotypes similar to strains lacking Pmr1p, a Ca(2+)/Mn(2+) pump that resides in the medial-Golgi. Because of its localization, the previously reported role of PMR1 in ERAD was somewhat enigmatic. A clue to their respective roles came from observations that the two genes are not generally required for ERAD. We show that the specificity is rooted in a requirement for both genes in protein-linked oligosaccharide trimming, a requisite ER modification in the degradation of some misfolded glycoproteins. Furthermore, Cod1p, like Pmr1p, is also needed for the outer chain modification of carbohydrates in the Golgi apparatus despite its ER localization. In strains deleted of both genes, these activities are nearly abolished. The presence of either protein alone, however, can support partial function for both compartments. Taken together, our results reveal an interdependent relationship between two P-type ATPases to maintain homeostasis of the organelles where they reside.     

Inhibition of the secretory pathway by foot-and-mouth disease virus 2BC protein is reproduced by coexpression of 2B with 2C, and the site of inhibition is determined by the subcellular location of 2C

Infection of cells with picornaviruses can lead to a block in protein secretion. For poliovirus this is achieved by the 3A protein, and the consequent reduction in secretion of proinflammatory cytokines and surface expression of major histocompatibility complex class I proteins may inhibit host immune responses in vivo. Foot-and-mouth disease virus (FMDV), another picornavirus, can cause persistent infection of ruminants, suggesting it too may inhibit immune responses. Endoplasmic reticulum (ER)-to-Golgi apparatus transport of proteins is blocked by the FMDV 2BC protein. The observation that 2BC is processed to 2B and 2C during infection and that individual 2B and 2C proteins are unable to block secretion stimulated us to study the effects of 2BC processing on the secretory pathway. Even though 2BC was processed rapidly to 2B and 2C, protein transport to the plasma membrane was still blocked in FMDV-infected cells. The block could be reconstituted by coexpression of 2B and 2C, showing that processing of 2BC did not compromise the ability of FMDV to slow secretion. Under these conditions, 2C was located to the Golgi apparatus, and the block in transport also occurred in the Golgi apparatus. Interestingly, the block in transport could be redirected to the ER when 2B was coexpressed with a 2C protein fused to an ER retention element. Thus, for FMDV a block in secretion is dependent on both 2B and 2C, with the latter determining the site of the block.     

Glycosylation disorders of membrane trafficking

During evolution from prokaryotic to eukaryotic cells, compartmentalization of cellular functions has been achieved with a high degree of complexity. Notably, all secreted and transmembrane proteins travel through endoplasmic reticulum (ER) and Golgi apparatus, where they are synthesized, folded and subjected to covalent modifications, most particularly glycosylation. N-glycosylation begins in the ER with synthesis and transfer of glycan onto nascent protein and proceeds in Golgi apparatus where maturation occurs. This process not only requires the precise localization of glycosyltransferases, glycosidases and substrates but also an efficient, finely regulated and bidirectional vesicular trafficking among membrane-enclosed organelles. Basically, it is no surprise that alterations in membrane transport or related pathways can lead to glycosylation abnormalities. During the last few years, this has particularly been highlighted in genetic diseases called CDG (Congenital Disorders of Glycosylation). Alterations in mechanisms of vesicle formation due to COPII coat component SEC23B deficiency, or in vesicles tethering, caused by defects of the COG complex, but also impaired Golgi pH homeostasis due to ATP6V0A2 defects have been discovered in CDG patients. This mini review will summarize these fascinating discoveries.