The role of G proteins in transmembrane signalling.

In contrast to water-soluble fuels such as glucose or ketone bodies, the use of lipids as an energy source for tissues has required the development of complex structures for their transport through the aqueous plasma. In the case of endogenously synthesized triacylglycerol this is achieved by the assembly and secretion of hepatic VLDL which provides the necessary stability in an aqueous medium. An essential component of this assembly process is apo B. Dietary changes which require an increase in hepatic VLDL secretion appear to be accompanied by increases in the availability of functional apo B. Interesting questions relate to: (a) the intracellular site(s) of triacylglycerol association with apo B, and (b) the mechanism(s) by which the availability of functional apo B at this site responds to metabolic and hormonal signals which reflect dietary status and, thus, the need to secrete triacylglycerol. As regards the latter, although in some cases changes in apo B synthesis occur in response to VLDL secretion hepatic apo B mRNA levels appear to be quite stable in vitro. Intracellular switching of apo B between the secretory and degradative pathways may be important in controlling VLDL assembly and post-translational modifications of the apoprotein may also play a role by influencing its ability to bind to triacylglycerol. Transport is not the only problem associated with the utilization of a concentrated energy source such as triacylglycerol and the complex problems of waste product disposal and recycling have to be dealt with. In the case of triacylglycerol, potentially toxic waste products include atherogenic remnants and LDL. The overall problem, then, in the long-term, involves the development of a 'safe' means of utilizing triacylglycerol and this requirement accounts for much of the complexity of plasma lipoprotein metabolism. In this area, the rat could teach the human a few tricks. One of these appears to be the utilization of hepatic apo B48 rather than apo B100 for VLDL assembly in response to increases in the extrahepatic utilization of hepatically synthesized triacylglycerol. Under these conditions, the remnants of hepatic triacylglycerol utilization by peripheral tissues are cleared from the plasma much more readily via a process which seems to involve the cycling of more triacylglycerol back to the liver than that which occurs in humans. The means by which this is achieved, though, are obscure and may involve a chylomicron remnant receptor, the nature of which, itself, remains controversial.(ABSTRACT TRUNCATED AT 400 WORDS)     

A dimer of the Toll-like receptor 4 cytoplasmic domain provides a specific scaffold for the recruitment of signalling adaptor proteins

The Toll-like receptor 4 (TLR4) is a class I transmembrane receptor expressed on the surface of immune system cells. TLR4 is activated by exposure to lipopolysaccharides derived from the outer membrane of Gram negative bacteria and forms part of the innate immune response in mammals. Like other class 1 receptors, TLR4 is activated by ligand induced dimerization, and recent studies suggest that this causes concerted conformational changes in the receptor leading to self association of the cytoplasmic Toll/Interleukin 1 receptor (TIR) signalling domain. This homodimerization event is proposed to provide a new scaffold that is able to bind downstream signalling adaptor proteins. TLR4 uses two different sets of adaptors; TRAM and TRIF, and Mal and MyD88. These adaptor pairs couple two distinct signalling pathways leading to the activation of interferon response factor 3 (IRF-3) and nuclear factor kappaB (NFkappaB) respectively. In this paper we have generated a structural model of the TLR4 TIR dimer and used molecular docking to probe for potential sites of interaction between the receptor homodimer and the adaptor molecules. Remarkably, both the Mal and TRAM adaptors are strongly predicted to bind at two symmetry-related sites at the homodimer interface. This model of TLR4 activation is supported by extensive functional studies involving site directed mutagenesis, inhibition by cell permeable peptides and stable protein phosphorylation of receptor and adaptor TIR domains. Our results also suggest a molecular mechanism for two recent findings, the caspase 1 dependence of Mal signalling and the protective effects conferred by the Mal polymorphism Ser180Leu.     

Coupling of viral membrane proteins to phosphatidylinositide signalling system

C6 rat glioma cells persistently infected with subacute sclerosing panencephalitis virus (C6/SSPE) were treated with measles antiserum and purified anti-measles IgG. This stimulated phosphoinositide breakdown and an increase in inositol phosphates. In uninfected C6 cells, however, only fetal calf serum (FCS), but not measles antiserum could induce inositol polyphosphate production.     

The adaptor protein Gads (Grb2-related adaptor downstream of Shc) is implicated in coupling hemopoietic progenitor kinase-1 to the activated TCR

The hemopoietic-specific Gads (Grb2-related adaptor downstream of Shc) adaptor protein possesses amino- and carboxyl-terminal Src homology 3 (SH3) domains flanking a central SH2 domain and a unique region rich in glutamine and proline residues. Gads functions to couple the activated TCR to distal signaling events through its interactions with the leukocyte-specific signaling proteins SLP-76 (SH2 domain-containing leukocyte protein of 76 kDa) and LAT (linker for activated T cells). Expression library screening for additional Gads-interacting molecules identified the hemopoietic progenitor kinase-1 (HPK1), and we investigated the HPK1-Gads interaction within the DO11.10 murine T cell hybridoma system. Our results demonstrate that HPK1 inducibly associates with Gads and becomes tyrosine phosphorylated following TCR activation. HPK1 kinase activity is up-regulated in response to activation of the TCR and requires the presence of its proline-rich motifs. Mapping experiments have revealed that the carboxyl-terminal SH3 domain of Gads and the fourth proline-rich region of HPK1 are essential for their interaction. Deletion of the fourth proline-rich region of HPK1 or expression of a Gads SH2 mutant in T cells inhibits TCR-induced HPK1 tyrosine phosphorylation. Together, these data suggest that HPK1 is involved in signaling downstream from the TCR, and that SH2/SH3 domain-containing adaptor proteins, such as Gads, may function to recruit HPK1 to the activated TCR complex.     

Why cytoplasmic signalling proteins should be recruited to cell membranes

It has been suggested that localization of signal-transduction proteins close to the cell membrane causes an increase in their rate of encounter after activation. We maintain that such an increase in the first-encounter rate is too small to be responsible for truly enhanced signal transduction. Instead, the function of membrane localization is to increase the number (or average lifetime) of complexes between cognate signal transduction proteins and hence increase the extent of activation of downstream processes. This is achieved by concentrating the proteins in the small volume of the area just below the plasma membrane. The signal-transduction chain is viewed simply as operating at low default intensity because one of its components is present at a low concentration. The steady signalling level of the chain is enhanced 1000-fold by increasing the concentration of that component. This occurs upon 'piggyback' binding to a membrane protein, such as the activated receptor, initiating the signal-transduction chain. For the effect to occur, the protein translocated to the membrane cannot be free but has to remain organized by being piggyback bound to a receptor, membrane lipid(s) or scaffold. We discuss an important structural constraint imposed by this mechanism on signal transduction proteins that might also account for the presence of adaptor proteins.     

HDL3 binds to glycosylphosphatidylinositol-anchored proteins to activate signalling pathways

Previous studies have indicated that in HepG₂ cells HDL₃-signalling involves glycosylphosphatidylinositol (GPI) anchored proteins. HDL₃-binding to HepG₂ cells was found to be enhanced by cellular preincubation with PI-PLC inhibitors and sensitive to a cellular preincubation with exogenous PI-PLC, suggesting that HDL₃ binds directly on GPI-anchored proteins to initiate signaling. Moreover HDL₃-binding was found to be partly inhibited by antibodies against the HDL-binding protein (Ab_HBP).HDL₃, when binding to HepG₂ cells, promoted the release in the culture medium of a 110 kDa protein that binds Ab_HBP, while a cellular preincubation with antibodies against the inositol-phosphoglycan (IPG) moiety of GPI-anchor (Ab_IPG), used to block lipolytic cleavage of the GPI-anchor, inhibits HDL₃-induced release of the 110 kDa protein in the culture medium.In [³H]-PC prelabeled HepG₂ cells, Ab_HBP were found to stimulate PC-hydrolysis and DAG generation within 5 min as did HDL₃ stimulation. Cellular preincubation with Ab_IPG was found to inhibit only the HDL₃-signal and not the Ab_HBP-signal, while a prior cellular pretreatment with PI-PLC from Bacillus cereus was found to inhibit the HDL₃-and Ab_HBP-signal. Moreover cellular preincubation with Ab_HBP for 1 h at 37°C was found to inhibit HDL₃-signalling pathways.Our results suggest that in HepG2 cells a 110 kDa protein, which could be HBP, can be anchored to the membrane via GPI, and can function in HDL₃-signalling pathways as binding sites.     

Structural and functional diversity of adaptor proteins involved in tyrosine kinase signalling

Adaptors are proteins of multi-modular structure without enzymatic activity. Their capacity to organise large, temporary protein complexes by linking proteins together in a regulated and selective fashion makes them of outstanding importance in the establishment and maintenance of specificity and efficiency in all known signal transduction pathways. This review focuses on the structural and functional characterisation of adaptors involved in tyrosine kinase (TK) signalling. TK-linked adaptors can be distinguished by their domain composition and binding specificities. However, such structural classifications have proven inadequate as indicators of functional roles. A better way to understand the logic of signalling networks might be to look at functional aspects of adaptor proteins such as signalling specificity, negative versus positive contribution to signal propagation, or their position in the signalling hierarchy. All of these functions are dynamic, suggesting that adaptors have important regulatory roles rather than acting only as stable linkers in signal transduction.     

G proteins control diverse pathways of transmembrane signaling

Hormones, neurotransmitters, and autacoids interact with specific receptors and thereby trigger a series of molecular events that ultimately produce their biological effects. These receptors, localized in the plasma membrane, carry binding sites for ligands as diverse as peptides (e.g., glucagon, neuropeptides), lipids (e.g., prostaglandins), nucleosides and nucleotides (e.g., adenosine), and amines (e.g., catecholamines, serotonin). These receptors do not interest directly with their respective downstream effector (i.e., an ion channel and/or an enzyme that synthesizes a second messenger); rather, they control one or several target systems via the activation of an intermediary guanine nucleotide-binding regulatory protein or G protein. G proteins serve as signal transducers, linking extracellularly oriented receptors to membrane-bound effectors. Traffic in these pathways is regulated by a GTP (on)-GDP (off) switch, which is regulated by the receptor. The combination of classical biochemistry and recombinant DNA technology has resulted in the discovery of many members of the G protein family. These approaches, complemented in particular by electrophysiological experiments, have also identified several effectors that are regulated by G proteins. We can safely assume that current lists of G proteins and the functions that they control are incomplete.     

Directional gating of synaptic plasticity by GPCRs and their distinct downstream signalling pathways

EMBO J 31 4, 805–816 (2012); published online December202011Synaptic plasticity, the activity-dependent modification of synaptic strength, plays a fundamental role in learning and memory as well as in developmental maturation of neuronal circuitry. However, how synaptic plasticity is induced and regulated remains poorly understood. In this issue of The EMBO Journal, Yang and colleagues present sets of exciting data, suggesting that G-protein-coupled receptors (GPCRs) selectively execute distinct signalling pathways to differentially regulate induction thresholds of hippocampal long-term potentiation (LTP) and long-term depression (LTD), thereby governing the direction of synaptic plasticity. These results shed significant light on our current understanding of how bidirectional synaptic plasticity is regulated.Synaptic plasticity has been demonstrated at synapses in various brain regions; the most well-characterized forms are LTP and LTD at hippocampal CA1 glutamatergic synapses (Collingridge et al, 2004). In experimental models, LTP and LTD can be, respectively, induced by high-frequency stimulation (HFS) and low-frequency stimulation (LFS) via activation of the N-methyl-D-aspartic acid (NMDA) subtype ionotropic glutamate receptor (NMDAR). However, how HFS and LFS activate NMDARs and thereby lead to synaptic plasticity remains poorly understood and highly controversial. It is even more unclear how the bidirectional synaptic plasticity is produced and regulated in response to physiological or pathological changes.Functional NMDARs consist primarily of two GluN1 subunits and two GluN2 subunits, with GluN2A and GluN2B subunits being the most common NMDAR subunits found in the cortical and hippocampal regions of the adult brain (Cull-Candy et al, 2001). GluN2A and GluN2B subunits may confer distinct gating and pharmacological properties to NMDARs and couple them to distinct intracellular signalling machineries (Cull-Candy et al, 2001). Moreover, the ratio of these two subpopulations of NMDARs at the glutamatergic synapse is dynamically regulated in an activity-dependent manner (Bellone and Nicoll, 2007; Cho et al, 2009; Xu et al, 2009). Although controversial, GluN2A- and GluN2B-containing NMDARs have been suggested to have differential roles in regulating the direction of synaptic plasticity (Collingridge et al, 2004; Morishita et al, 2007). Among the factors shown to regulate NMDAR function, Src family tyrosine kinases may be the best characterized, with both Src and Fyn able to upregulate NMDAR function, and thus LTP induction (Salter and Kalia, 2004). However, if these kinases modulate NMDAR function in a NMDAR subunit-specific manner remains unknown. To explore this concept, Yang et al (2012) investigated the potential subunit-specific regulation of NMDARs by Src and Fyn using whole-cell patch clamp recording of NMDAR-mediated currents from acutely dissociated CA1 hippocampal neurons or from rat hippocampal slices. They found that intracellular perfusion of recombinant Src or Fyn increased the NMDAR-mediated currents. By applying subunit-preferential antagonists of GluN2A- or GluN2B-containing NMDARs, or by using neurons obtained from GluN2A knockout mice, they discovered that Src and Fyn differentially enhanced currents gated through GluN2A- and GluN2B-containing NMDARs, respectively.Can physiological or pathological factors differentially activate Src or Fyn, thereby exerting subunit-specific regulation of NMDAR function? To answer this question, Yang et al focused their investigation on the role of GPCRs, specifically pituitary adenylate cyclase activating peptide receptor (PAC1R) and dopamine D1 receptor (D1R), both of which have recently been shown to potentiate NMDARs through Src family kinases (Macdonald et al, 2005; Hu et al, 2010). Indeed, they found that activation of PAC1R specifically increased GluN2A-NMDAR-mediated currents without affecting currents gated through GluN2B-NMDARs, and this potentiation was prevented by the Src-specific inhibitory peptide Src(40–58) (Salter and Kalia, 2004). To rule out the contribution of Fyn, the authors developed a novel-specific Fyn inhibitory peptide Fyn(39–57), and demonstrated that it had little effect on PAC1R potentiation. In contrast, activation of D1R potentiated GluN2B- (but not GluN2A-) NMDAR-mediated currents, and this potentiation was specifically eliminated by Fyn(39–57), but not by Src(40–58). The authors further demonstrated that stimulation of PAC1Rs resulted in a selective activation of Src kinase and consequent tyrosine phosphorylation of the GluN2A subunit, whereas activation of D1Rs led to a specific increase in Fyn-mediated tyrosine phosphorylation of the GluN2B subunit. To provide convincing evidence that these subunit-differential modulations are indeed the result of tyrosine phosphorylation of the respective NMDAR subunits, the authors then performed electrophysiological experiments using neurons from two knockin mouse lines GluN2A(Y1325F) and GluN2B(Y1472F), in which the tyrosine phosphorylation residues in native GluN2A and GluN2B subunits were, respectively, replaced with non-phosphorylatable phenylalanine residues. As expected, the authors found that PAC1R-mediated potentiation of NMDA currents was lost in neurons from GluN2A(Y1325F) mice (but maintained in neurons from GluN2B(Y1472F) mice), while D1R-mediated enhancement of NMDA currents was only observed in neurons from GluN2A(Y1325F) mice. Together, as illustrated in Figure 1, the authors have made a very convincing case that PAC1R and D1R, respectively, enhance function of GluN2A- and GluN2B-containing NMDARs by differentially activating Src- and Fyn-mediated phosphorylation of respective NMDAR subunits.Open in a separate windowFigure 1GPCRs regulate the direction of synaptic plasticity via activating distinct signalling pathways. Synaptic NMDA receptors, both GluN2A- and GluN2B-containing, play key roles in the induction of various forms of synaptic plasticity at the hippocampal CA1 glutamatergic synapse. Under the basal level of GluN2A and GluN2B ratio, stimulation with a train of pulses at frequencies from 1 to 100 Hz produces a frequency and plasticity (LTD–LTP) curve, with maximum LTD and LTP being, respectively, induced at 1 and 100 Hz. Activation of PAC1R with its agonist PACAP38 activates Src and thereby results in tyrosine phosphorylation and consequent functional upregulation of GluN2A-containing NMDARs, resulting in an increase in the ratio of functional GluN2A and GluN2B. The increased ratio in turn causes a left shift of frequency and plasticity curve, favouring LTP induction. In contrast, activation of D1R by the receptor agonist {"type":"entrez-protein","attrs":{"text":"SKF81297","term_id":"1156277425","term_text":"SKF81297"}}SKF81297 triggers Fyn-specific tyrosine phosphorylation and functional upregulation of GluN2B, causing a reduction of GluN2A and GluN2B ratio. This decreased ratio results in a right shift of the curve, favouring LTD induction. The ability of GPCRs to differentially activate distinct downstream signalling pathways involved in synaptic plasticity suggests the potential roles of GPCRs in governing the direction of synaptic plasticity.Given the coupling of NMDARs to the induction of synaptic plasticity, it is then reasonable to ask if activation of the two GPCRs can selectively affect the induction of LTP or LTD at CA1 synapses. Yang and colleagues investigated the effects of pharmacological activation of PAC1R and D1R on the induction of LTP and LTD by recording the field excitatory postsynaptic potentials from hippocampal slices. Consistent with differential roles of NMDAR subunits in governing directions of synaptic plasticity, the authors observed that activation of PAC1Rs reduces the induction threshold of LTP, while stimulation of D1Rs favours LTD induction (Figure 1). Facilitation of LTP by PAC1R and LTD by D1R were, respectively, prevented in the brain slices obtained from GluN2A(Y1325F) and GluN2B(Y1472F) knockin mice, supporting the differential involvements of Src-mediated GluN2A phosphorylation and Fyn-mediated GluN2B phosphorylation.Taken together, the authors'' results have demonstrated that activation of PAC1R and D1R can control the direction of synaptic plasticity at the hippocampal CA1 synapse by differentially regulating NMDAR_EPSCs in a subunit-specific fashion (Figure 1). Specifically, PAC1R enhances the function of GluN2A-containing NMDARs by increasing Src phosphorylation of GluN2A subunit at Y1325, whereas D1R upregulates GluN2B-containing NMDARs through increased Fyn phosphorylation of GluN2B at Y1472. Moreover, by regulating the ratio of functional GluN2A- and GluN2B-containing NMDARs, PAC1R and D1R in turn modulate the direction of synaptic plasticity, favouring the production of LTP and LTD, respectively.While consistent with the recently proposed hypothesis that GluN2A and GluN2B may have preferential roles in the induction of hippocampal CA1 LTP and LTD (Collingridge et al, 2004; but see also Morishita et al, 2007), the current study further emphasizes the importance of GluN2A/GluN2B ratios in regulating LTP and LTD thresholds: increased ratio favours LTP, while reduced ratio promotes LTD. However, this seems to contradict some recent studies where the reduction and increase in the GluN2A/GluN2B ratio appeared to, respectively, favour LTP (Cho et al, 2009; Xu et al, 2009) and LTD (Xu et al, 2009). Therefore, the direction of plasticity change is likely modulated not only by the GluN2A/GluN2B ratio, but also by additional factors such as experimental conditions, developmental stages, and brain regions.Under many experimental conditions, LTP and LTD are usually induced by HFS and LFS stimulating protocols, respectively, but it remains essentially unknown how LTP and LTD are physiologically or pathologically generated in animals. To this end, the identification of different GPCRs as the endogenous upstream regulators of NMDA receptor subpopulations, and hence regulators of synaptic plasticity, is the major novelty of Yang and colleagues'' work. Future studies are needed to investigate if and how PAC1R and/or D1R are critically involved in the production of LTP or LTD in animals under physiological or pathological conditions. Given the fact that Src family kinases may be required for LTP induced by HFS in hippocampal slices (Salter and Kalia, 2004), an equally intriguing question would be whether these GPCRs are actually required for LTP/LTD induced by HFS/LFS experimental paradigms. In line with this conjecture, it would be interesting to determine if ligands for various GPCRs co-exist in the glutamatergic presynaptic terminals and, if so, can be differentially co-released with glutamate in a frequency-dependent manner, thereby contributing to either HFS-induced LTP or LFS-induced LTD.The findings by Yang and colleagues establish an exciting mechanistic model by which GPCRs can govern the direction of synaptic plasticity by determining the contributions of GluN2A- and GluN2B-NMDARs through differential tyrosine phosphorylation of respective NMDA receptor subtypes. Additional studies further validating this model under physiological and pathological conditions will greatly improve our understanding of the molecular mechanisms underlying synaptic plasticity and cognitive brain functions. In addition, NMDARs, depending on their subunit composition and/or subcellular localization, may also have complex roles in mediating neuronal survival and death (Lai et al, 2011). Considering that neurotoxicity produced by over-activation of NMDARs is widely accepted to be a common mechanism for neuronal loss in a number of acute brain injuries and chronic neurodegenerative diseases, Yang and colleagues'' finding of the differential regulation of NMDAR subunits by different GPCRs could have wider implications beyond synaptic plasticity.     

Coupling of antagonistic signalling pathways in modulation of neutrophil function

Modulation of neutrophil activation by catecholamines reflects a fine-tuning by coupling inhibitory and stimulatory receptor pathways. The catecholamine isoproterenol (ISO) binds to beta-adrenergic cell surface receptors and thereby inhibits cell responses such as O2- production stimulated by formyl peptides. However, ISO did not inhibit O2- generation activated by 1 microM ionophore A23187, the protein kinase C activators phorbol ester (PMA, 100 ng/ml) and oleoylacetylglycerol (OAG, 50 microM), and the G-protein activator NaF (40 mM). Furthermore, the overall kinetics of oxidant production in the presence of ISO were unchanged when cells were stimulated with PMA, OAG, A23187, and NaF. These results would imply that neither intracellular calcium, the activation of protein kinase C, nor the activation of G-protein are the primary target of the inhibitory pathway. Accordingly, pertussis toxin did not block PMA or NaF-stimulated superoxide generation. In contrast, formyl peptide-dependent GTPase activity is inhibited by ISO in sonicated cell preparations. Since ISO increases the cAMP concentration in the cell, the possibility is raised that a cAMP-dependent kinase inhibits signal transduction in part by blocking the interaction of this receptor with its G-protein.     

The role of 14-3-3 proteins in cell signalling pathways and virus infection

14-3-3 proteins are highly conserved in species ranging from yeast to mammals and regulate numerous signalling pathways via direct interactions with proteins carrying phosphorylated 14-3-3–binding motifs. Recent studies have shown that 14-3-3 proteins can also play a role in viral infections. This review summarizes the biological functions of 14-3-3 proteins in protein trafficking, cell-cycle control, apoptosis, autophagy and other cell signal transduction pathways, as well as the associated mechanisms. Recent findings regarding the role of 14-3-3 proteins in viral infection and innate immunity are also reviewed.     

Pyridoxal phosphate as a probe of the cytoplasmic domains of transmembrane proteins: application to the nicotinic acetylcholine receptor

A novel procedure has been developed to specifically label the cytoplasmic domains of transmembrane proteins with the aldehyde pyridoxal 5-phosphate (PLP). Torpedo californica acetylcholine receptor (AcChR) vesicles were loaded with [3H]pyridoxine 5-phosphate ([3H]PNP) and pyridoxine-5-phosphate oxidase, followed by intravesicular enzymatic oxidation of [3H]PNP at 37 degrees C in the presence of externally added cytochrome c as a scavenger of possible leaking PLP product. The resulting Schiff's bases between PLP and AcChR amino groups were reduced with NaCNBH3, and the pyridoxylated proteins were analyzed by fluorography. The four receptor subunits were labeled whether the reaction was carried out on the internal surface or separately designed to mark the external one. On the other hand, the relative pyridoxylation of the subunits differed in both cases, reflecting differences in accessible lysyl residues in each side of the membrane. Proteinase K treatment of labeled AcChR vesicles generated a peptide of 13 kDa that could be detected with anti-PLP antibodies only when the pyridoxylation was carried out on the internal surface of the vesicles. Even though there are no large differences in the total lysine content among the subunits and there are two copies of the alpha-subunit, internal surface labeling by PLP was greatest for the highest molecular weight (delta) subunit, reinforcing the concept that the four receptor subunits are transmembranous and may protrude into the cytoplasmic face in a fashion [Strader, C. D., & Raftery, M. A. (1980) Proc. Natl. Acad. Sci. U.S.A. 77, 5807-5811] that is proportional to their subunit molecular weight.(ABSTRACT TRUNCATED AT 250 WORDS)     

The role of MAPK signalling pathways in the response to endoplasmic reticulum stress

Perturbations in endoplasmic reticulum (ER) homeostasis, including depletion of Ca^2 + or altered redox status, induce ER stress due to protein accumulation, misfolding and oxidation. This activates the unfolded protein response (UPR) to re-establish the balance between ER protein folding capacity and protein load, resulting in cell survival or, following chronic ER stress, promotes cell death. The mechanisms for the transition between adaptation to ER stress and ER stress-induced cell death are still being understood. However, the identification of numerous points of cross-talk between the UPR and mitogen-activated protein kinase (MAPK) signalling pathways may contribute to our understanding of the consequences of ER stress. Indeed, the MAPK signalling network is known to regulate cell cycle progression and cell survival or death responses following a variety of stresses. In this article, we review UPR signalling and the activation of MAPK signalling pathways in response to ER stress. In addition, we highlight components of the UPR that are modulated in response to MAPK signalling and the consequences of this cross-talk. We also describe several diseases, including cancer, type II diabetes and retinal degeneration, where activation of the UPR and MAPK signalling contribute to disease progression and highlight potential avenues for therapeutic intervention. This article is part of a Special Issue entitled: Calcium Signaling In Health and Disease. Guest Editors: Geert Bultynck, Jacques Haiech, Claus W. Heizmann, Joachim Krebs, and Marc Moreau.     

Protein fatty acylation: a novel mechanism for association of proteins with membranes and its role in transmembrane regulatory pathways

A wide range of proteins of cellular and viral origin have been shown to be modified covalently by long-chain fatty acids. Recent studies have revealed at least two distinct types of protein fatty acylation which involve different fatty acyltransferases. The abundant fatty acid, palmitate, is incorporated post-translationally through a thiol ester linkage into a variety of cell surface glycoproteins and non-glycosylated intracellular proteins. In contrast, the rare fatty acid, myristate, is incorporated co-translationally through an amide linkage into numerous intracellular proteins. Identification of proteins that contain covalent fatty acids has revealed that this modification is common to a broad array of proteins that play important roles in transmembrane regulatory pathways. For many of these proteins, the fatty acid moiety appears to play an important role in directing the polypeptide to the appropriate membrane and in mediating protein-protein interactions within the membrane. This review will summarize recent studies that define different pathways for protein fatty acylation and will consider the potential functions for this unique covalent modification of proteins.