The synaptic vesicle protein, cysteine-string protein, is associated with the plasma membrane in 3T3-L1 adipocytes and interacts with syntaxin 4

Tissue distribution of cysteine string protein/DNAJC5 in C. elegans analysed by CRISPR/Cas9-mediated tagging of endogenous DNJ-14

Cysteine string protein (CSP) is a member of the DnaJ/Hsp40 family of molecular chaperones. CSP is enriched in neurons, where it mainly localises to synaptic vesicles. Mutations in CSP-encoding genes in flies, worms, mice and humans result in neuronal dysfunction, neurodegeneration and reduced lifespan. Most attention has therefore focused on CSP's neuronal functions, although CSP is also expressed in non-neuronal cells. Here, we used genome editing to fluorescently tag the Caenorhabditis elegans CSP orthologue, dnj-14, to identify which tissues preferentially express CSP and hence may contribute to the observed mutant phenotypes. Replacement of dnj-14 with wrmScarlet caused a strong chemotaxis defect, as seen with other dnj-14 null mutants. In contrast, inserting the reporter in-frame to create a DNJ-14-wrmScarlet fusion protein had no effect on chemotaxis, indicating that C-terminal tagging does not impair DNJ-14 function. WrmScarlet fluorescence appeared most obvious in the intestine, head/pharynx, spermathecae and vulva/uterus in the reporter strains, suggesting that DNJ-14 is preferentially expressed in these tissues. Crossing the DNJ-14-wrmScarlet strain with GFP marker strains confirmed the intestinal and pharyngeal expression, but only a partial overlap with neuronal GFP was observed. DNJ-14-wrmScarlet fluorescence in the intestine was increased in response to starvation, which may be relevant to mammalian CSPα's role in microautophagy. DNJ-14's enrichment in worm reproductive tissues (spermathecae and vulva/uterus) parallels the testis-specific expression of CSPβ and CSPγ isoforms in mammals. Furthermore, CSPα messenger RNA is highly expressed in the human proximal digestive tract, suggesting that CSP may have a conserved, but overlooked, function within the gastrointestinal system.

Proximity labelling reveals effects of disease-causing mutation on the DNAJC5/cysteine string protein α interactome

Cysteine string protein α (CSPα), also known as DNAJC5, is a member of the DnaJ/Hsp40 family of co-chaperones. The name derives from a cysteine-rich domain, palmitoylation of which enables localization to intracellular membranes, notably neuronal synaptic vesicles. Mutations in the DNAJC5 gene that encodes CSPα cause autosomal dominant, adult-onset neuronal ceroid lipofuscinosis (ANCL), a rare neurodegenerative disease. As null mutations in CSP-encoding genes in flies, worms and mice similarly result in neurodegeneration, CSP is evidently an evolutionarily conserved neuroprotective protein. However, the client proteins that CSP chaperones to prevent neurodegeneration remain unclear. Traditional methods for identifying protein-protein interactions such as yeast 2-hybrid and affinity purification approaches are poorly suited to CSP, due to its requirement for membrane anchoring and its tendency to aggregate after cell lysis. Therefore, we employed proximity labelling, which enables identification of interacting proteins in situ in living cells via biotinylation. Neuroendocrine PC12 cell lines stably expressing wild type or L115R ANCL mutant CSP constructs fused to miniTurbo were generated; then the biotinylated proteomes were analysed by liquid chromatographymass spectrometry (LCMS) and validated by western blotting. This confirmed several known CSP-interacting proteins, such as Hsc70 and SNAP-25, but also revealed novel binding proteins, including STXBP1/Munc18-1. Interestingly, some protein interactions (such as Hsc70) were unaffected by the L115R mutation, whereas others (including SNAP-25 and STXBP1/Munc18-1) were inhibited. These results define the CSP interactome in a neuronal model cell line and reveal interactions that are affected by ANCL mutation and hence may contribute to the neurodegeneration seen in patients.

Safeguarding Lysosomal Homeostasis by DNAJC5/CSPα-Mediated Unconventional Protein Secretion and Endosomal Microautophagy

Neuronal ceroid lipofuscinosis (NCL) is a collection of genetically inherited neurological disorders characterized by vision loss, seizure, brain death, and premature lethality. At the cellular level, a key pathologic hallmark of NCL is the build-up of autofluorescent storage materials (AFSM) in lysosomes of both neurons and non-neuronal cells. Molecular dissection of the genetic lesions underlying NCLs has shed significant insights into how disruption of lysosomal homeostasis may lead to lipofuscin accumulation and NCLs. Intriguingly, recent studies on DNAJC5/CSPα, a membrane associated HSC70 co-chaperone, have unexpectedly linked lipofuscin accumulation to two intimately coupled protein quality control processes at endolysosomes. This review discusses how deregulation of unconventional protein secretion and endosomal microautophagy (eMI) contributes to lipofuscin accumulation and neurodegeneration.

Characterizing Post-Translational Modifications and Their Effects on Protein Conformation Using NMR Spectroscopy

The diversity of the cellular proteome substantially exceeds the number of genes coded by the DNA of an organism because one or more residues in a majority of eukaryotic proteins are post-translationally modified (PTM) by the covalent conjugation of specific chemical groups. We now know that PTMs alter protein conformation and function in ways that are not entirely understood at the molecular level. NMR spectroscopy has been particularly successful as an analytical tool in elucidating the themes underlying the structural role of PTMs. In this Perspective, we focus on the NMR-based characterization of three abundant PTMs: phosphorylation, acetylation, and glycosylation. We detail NMR methods that have found success in detecting these modifications at a site-specific level. We also highlight NMR studies that have mapped the conformational changes ensuing from these PTMs as well as evaluated their relation to function. The NMR toolbox is expanding rapidly with experiments available to probe not only the average structure of biomolecules but also how this structure changes with time on time scales ranging from picoseconds to seconds. The atomic resolution insights into the biomolecular structure, dynamics, and mechanism accessible from NMR spectroscopy ensure that NMR will continue to be at the forefront of research in the structural biology of PTMs.

GPCR regulation of secretion

Modulation of neurotransmitter exocytosis by activated Gi/o coupled G-protein coupled receptors (GPCRs) is a universal regulatory mechanism used both to avoid overstimulation and to influence circuitry. One of the known modulation mechanisms is the interaction between Gβγ and the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNAREs). There are 5 Gβ and 12 Gγ subunits, but specific Gβγs activated by a given GPCR and the specificity to effectors, such as SNARE, in vivo are not known. Although less studied, Gβγ binding to the exocytic fusion machinery (i.e. SNARE) provides a more direct regulatory mechanism for neurotransmitter release. Here, we review some recent insights in the architecture of the synaptic terminal, modulation of synaptic transmission, and implications of G protein modulation of synaptic transmission in diseases. Numerous presynaptic proteins are involved in the architecture of synaptic terminals, particularly the active zone, and their importance in the regulation of exocytosis is still not completely understood. Further understanding of the Gβγ-SNARE interaction and the architecture and mechanisms of exocytosis may lead to the discovery of novel therapeutic targets to help patients with various disorders such as hypertension, attention-deficit/hyperactivity disorder, post-traumatic stress disorder, and acute/chronic pain.

DNAJ Proteins in neurodegeneration: essential and protective factors

Maintenance of protein homeostasis is vitally important in post-mitotic cells, particularly neurons. Neurodegenerative diseases such as polyglutamine expansion disorders-like Huntington's disease or spinocerebellar ataxia (SCA), Alzheimer's disease, fronto-temporal dementia (FTD), amyotrophic lateral sclerosis (ALS) and Parkinson's disease-are often characterized by the presence of inclusions of aggregated protein. Neurons contain complex protein networks dedicated to protein quality control and maintaining protein homeostasis, or proteostasis. Molecular chaperones are a class of proteins with prominent roles in maintaining proteostasis, which act to bind and shield hydrophobic regions of nascent or misfolded proteins while allowing correct folding, conformational changes and enabling quality control. There are many different families of molecular chaperones with multiple functions in proteostasis. The DNAJ family of molecular chaperones is the largest chaperone family and is defined by the J-domain, which regulates the function of HSP70 chaperones. DNAJ proteins can also have multiple other protein domains such as ubiquitin-interacting motifs or clathrin-binding domains leading to diverse and specific roles in the cell, including targeting client proteins for degradation via the proteasome, chaperone-mediated autophagy and uncoating clathrin-coated vesicles. DNAJ proteins can also contain ER-signal peptides or mitochondrial leader sequences, targeting them to specific organelles in the cell. In this review, we discuss the multiple roles of DNAJ proteins and in particular focus on the role of DNAJ proteins in protecting against neurodegenerative diseases caused by misfolded proteins. We also discuss the role of DNAJ proteins as direct causes of inherited neurodegeneration via mutations in DNAJ family genes.This article is part of the theme issue 'Heat shock proteins as modulators and therapeutic targets of chronic disease: an integrated perspective'.

Syntaxin 4 mediates endosome recycling for lytic granule exocytosis in cytotoxic T-lymphocytes

Adaptive and innate immunity utilize the perforin-killing pathway to eliminate virus-infected or cancer cells. Cytotoxic T-lymphocytes (CTLs) and natural killer cells mediate this process by releasing toxic proteins at the contact area with target cells known as immunological synapse (IS). Formation of a stable IS and exocytosis of toxic proteins requires persistent fusion of Rab11a recycling endosomes with the plasma membrane (PM) that may assure the delivery of key effector proteins. Despite the importance of the recycling endosomal compartment, the membrane fusion proteins that control this process at the IS remain elusive. Here, by performing knockdown experiments we found that syntaxin 4 (STX4) is necessary for cytotoxic activity and CD107a degranulation against target cells in a similar fashion to syntaxin 11, which is involved in lytic granule (LG) exocytosis and immunodeficiency when it is mutated. Using total internal reflection fluorescent microscopy we identified that STX4 mediates fusion of EGFP-Rab11a vesicles at the IS. Immunoprecipitation experiments in lysates of activated CTLs indicate that endogenous STX4 may drive this fusion step by interacting with cognate proteins: Munc18-3/SNAP23/VAMP7 and/or VAMP8. These results reveal the role of STX4 in mediating fusion of Rab11a endosomes upstream of lytic granules (LGs) exocytosis and further demonstrate the importance of this pathway in controlling CTL-mediated cytotoxicity.

Neurons Export Extracellular Vesicles Enriched in Cysteine String Protein and Misfolded Protein Cargo

The fidelity of synaptic transmission depends on the integrity of the protein machinery at the synapse. Unfolded synaptic proteins undergo refolding or degradation in order to maintain synaptic proteostasis and preserve synaptic function, and buildup of unfolded/toxic proteins leads to neuronal dysfunction. Many molecular chaperones contribute to proteostasis, but one in particular, cysteine string protein (CSPα), is critical for proteostasis at the synapse. In this study we report that exported vesicles from neurons contain CSPα. Extracellular vesicles (EV’s) have been implicated in a wide range of functions. However, the functional significance of neural EV’s remains to be established. Here we demonstrate that co-expression of CSPα with the disease-associated proteins, polyglutamine expanded protein 72Q huntingtin^ex°ⁿ¹ or superoxide dismutase-1 (SOD-1^G93A) leads to the cellular export of both 72Q huntingtin^ex°ⁿ¹ and SOD-1^G93A via EV’s. In contrast, the inactive CSPα_HPD-AAA mutant does not facilitate elimination of misfolded proteins. Furthermore, CSPα-mediated export of 72Q huntingtin^ex°ⁿ¹ is reduced by the polyphenol, resveratrol. Our results indicate that by assisting local lysosome/proteasome processes, CSPα-mediated removal of toxic proteins via EVs plays a central role in synaptic proteostasis and CSPα thus represents a potential therapeutic target for neurodegenerative diseases.

Phosphorylation of Cysteine String Protein Triggers a Major Conformational Switch

Cysteine string protein (CSP) is a member of the DnaJ/Hsp40 chaperone family that localizes to neuronal synaptic vesicles. Impaired CSP function leads to neurodegeneration in humans and model organisms as a result of misfolding of client proteins involved in neurotransmission. Mammalian CSP is phosphorylated in vivo on Ser10, and this modulates its protein interactions and effects on neurotransmitter release. However, there are no data on the structural consequences of CSP phosphorylation to explain these functional effects. We show that Ser10 phosphorylation causes an order-to-disorder transition that disrupts CSP's extreme N-terminal α helix. This triggers the concomitant formation of a hairpin loop stabilized by ionic interactions between phosphoSer10 and the highly conserved J-domain residue, Lys58. These phosphorylation-induced effects result in significant changes to CSP conformation and surface charge distribution. The phospho-switch revealed here provides structural insight into how Ser10 phosphorylation modulates CSP function and also has potential implications for other DnaJ phosphoproteins.

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First structure of a phosphorylated DnaJ/Hsp40 protein

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Phosphorylation destabilizes CSP's N-terminal α helix

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Newly disordered, phosphorylated N-terminal loop binds to the J domain

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Phosphorylation causes significant changes to CSP conformation and surface charge

Cysteine string protein (CSP) and its role in preventing neurodegeneration

Cysteine string protein (CSP) is a member of the DnaJ/Hsp40 family of co-chaperones that localises to neuronal synaptic vesicles. Its name derives from the possession of a string of 12–15 cysteine residues, palmitoylation of which is required for targeting to post-Golgi membranes. The DnaJ domain of CSP enables it to bind client proteins and recruit Hsc70 chaperones, thereby contributing to the maintenance of protein folding in the presynaptic compartment. Mutation of CSP in flies, worms and mice reduces lifespan and causes synaptic dysfunction and neurodegeneration. Furthermore, recent studies have revealed that the neurodegenerative disease, adult onset neuronal ceroid lipofuscinosis, is caused by mutations in the human CSPα-encoding DNAJC5 gene. Accumulating evidence suggests that the major mechanism by which CSP prevents neurodegeneration is by maintaining the conformation of SNAP-25, thereby facilitating its entry into the membrane-fusing SNARE complex. In this review, we focus on the role of CSP in preventing neurodegeneration and discuss how recent studies of this universal neuroprotective chaperone are being translated into potential novel therapeutics for neurodegenerative diseases.

The Golgi S-acylation machinery comprises zDHHC enzymes with major differences in substrate affinity and S-acylation activity

S-acylation, the attachment of fatty acids onto cysteine residues, regulates protein trafficking and function and is mediated by a family of zDHHC enzymes. The S-acylation of peripheral membrane proteins has been proposed to occur at the Golgi, catalyzed by an S-acylation machinery that displays little substrate specificity. To advance understanding of how S-acylation of peripheral membrane proteins is handled by Golgi zDHHC enzymes, we investigated interactions between a subset of four Golgi zDHHC enzymes and two S-acylated proteins-synaptosomal-associated protein 25 (SNAP25) and cysteine-string protein (CSP). Our results uncover major differences in substrate recognition and S-acylation by these zDHHC enzymes. The ankyrin-repeat domains of zDHHC17 and zDHHC13 mediated strong and selective interactions with SNAP25/CSP, whereas binding of zDHHC3 and zDHHC7 to these proteins was barely detectable. Despite this, zDHHC3/zDHHC7 could S-acylate SNAP25/CSP more efficiently than zDHHC17, whereas zDHHC13 lacked S-acylation activity toward these proteins. Overall the results of this study support a model in which dynamic intracellular localization of peripheral membrane proteins is achieved by highly selective recruitment by a subset of zDHHC enzymes at the Golgi, combined with highly efficient S-acylation by other Golgi zDHHC enzymes.

Cellular regulation of glucose uptake by glucose transporter GLUT4

GLUT4 is regulated by its intracellular localization. In the absence of insulin, GLUT4 is efficiently retained intracellularly within storage compartments in muscle and fat cells. Upon insulin stimulation (and contraction in muscle), GLUT4 translocates from these compartments to the cell surface where it transports glucose from the extracellular milieu into the cell. Its implication in insulin-regulated glucose uptake makes GLUT4 not only a key player in normal glucose homeostasis but also an important element in insulin resistance and type 2 diabetes. Nevertheless, how GLUT4 is retained intracellularly and how insulin acts on this retention mechanism is largely unclear. In this review, the current knowledge regarding the various molecular processes that govern GLUT4 physiology is discussed as well as the questions that remain.

Interrogation of brain miRNA and mRNA expression profiles reveals a molecular regulatory network that is perturbed by mutant huntingtin

Emerging evidence indicates that microRNAs (miRNAs) may play an important role in the pathogenesis of Huntington's disease (HD). To identify the individual miRNAs that are altered in HD and may therefore regulate a gene network underlying mutant huntingtin-induced neuronal dysfunction in HD, we performed miRNA array analysis combined with mRNA profiling in the cerebral cortex from N171-82Q HD mice. Expression profiles of miRNAs as well as mRNAs in HD mouse cerebral cortex were analyzed and confirmed at different stages of disease progression; the most significant changes of miRNAs in the cerebral cortex were also detected in the striatum of HD mice. Our results revealed a significant alteration of miR-200 family members, miR-200a, and miR-200c in the cerebral cortex and the striatum, at the early stage of disease progression in N171-82Q HD mice. We used a coordinated approach to integrate miRNA and mRNA profiling, and applied bioinformatics to predict a target gene network potentially regulated by these significantly altered miRNAs that might be involved in HD disease progression. Interestingly, miR-200a and miR-200c are predicted to target genes regulating synaptic function, neurodevelopment, and neuronal survival. Our results suggest that altered expression of miR-200a and miR-200c may interrupt the production of proteins involved in neuronal plasticity and survival, and further investigation of the involvement of perturbed miRNA expression in HD pathogenesis is warranted, and may lead to reveal novel approaches for HD therapy.

Cysteine-string protein's neuroprotective role

Cysteine-string protein (CSP), a member of the DnaJ/Hsp40 family of cochaperones, is critical for maintaining neurotransmitter release and preventing neurodegeneration. CSP likely forms a chaperone complex on synaptic vesicles together with the 70-kDa heat shock cognate (Hsc70) and the small glutamine-rich tetratricopeptide repeat (TPR)-containing protein (SGT) that may control or protect the assembly and activity of SNARE proteins and various other protein substrates. Here, the author summarizes studies that elucidated CSP's neuroprotective role.

CSPalpha: the neuroprotective J protein

Cysteine string protein (CSPalpha, also called DnaJC5) is unique among J proteins. Similar to other J proteins, CSPalpha interacts with and activates the ATPase of Hsc70s (heat shock proteins of 70 kDa), thereby harnessing the ATPase activity for conformational work on client proteins. In contrast to other J proteins, CSPalpha is anchored to synaptic vesicles, as well as to exocrine, endocrine and neuroendocrine secretory granules, and has been shown to have an essential anti-neurodegenerative role. CSPalpha-null organisms exhibit progressive neurodegeneration, behavioural deficits, and premature death, most likely due to the progressive misfolding of one or more client proteins. Here we highlight recent advances in our understanding of the critical role that CSPalpha plays in governing exocytotic secretory functions.

Functional role of J domain of cysteine string protein in Ca2+-dependent secretion from acinar cells

The heat shock protein 70 family members Hsc70 and Hsp70 are known to play a protective role against the onset of experimental pancreatitis, yet their molecular function in acini is unclear. Cysteine string protein (CSP-alpha) is a zymogen granule (ZG) membrane protein characterized by an NH(2)-terminal "J domain" and a central palmitoylated string of cysteine residues. The J domain functions as a cochaperone by modulating the activity of Hsc70/Hsp70 family members. A role for CSP-alpha in regulating digestive enzyme exocytosis from pancreas was investigated by introducing CSP-alpha truncations into isolated acini following their permeabilization with Perfringolysin O. Incubation of acini with CSP-alpha(1-82), containing the J domain, significantly augmented Ca(2+)-stimulated amylase secretion. Effects of CSP-alpha(1-82) were concentration dependent, with a maximum 80% increase occurring at 200 microg/ml of protein. Although CSP-alpha(1-82) had no effects on basal secretion measured in the presence of < or =10 nM free Ca(2+), it did significantly augment GTP-gammaS-induced secretion under basal Ca(2+) conditions by approximately 25%. Mutation of the J domain to abolish its cochaperone activity failed to augment Ca(2+)-stimulated secretion, implicating the CSP-alpha/Hsc70 cochaperone system as a regulatory component of the secretory pathway. CSP-alpha physically associates with vesicle-associated membrane protein 8 (VAMP 8) on ZGs, and the CSP-alpha-VAMP 8 interaction was dependent on amino acids 83-112 of CSP-alpha. Immunofluorescence analysis of acinar lobules or purified ZGs confirmed the CSP-alpha colocalization with VAMP 8. These data establish a role for CSP-alpha in regulating digestive enzyme secretion and suggest that CSP-alpha and Hsc70 modulate specific soluble N-ethylmaleimide-sensitive attachment receptor interactions necessary for exocytosis.

Palmitoylation and membrane interactions of the neuroprotective chaperone cysteine-string protein

Cysteine-string protein (CSP) is an extensively palmitoylated DnaJ-family chaperone, which exerts an important neuroprotective function. Palmitoylation is required for the intracellular sorting and function of CSP, and thus it is important to understand how this essential modification of CSP is regulated. Recent work identified 23 putative palmitoyl transferases containing a conserved DHHC domain in mammalian cells, and here we show that palmitoylation of CSP is enhanced specifically by co-expression of the Golgi-localized palmitoyl transferases DHHC3, DHHC7, DHHC15, or DHHC17. Indeed, these DHHC proteins promote stable membrane attachment of CSP, which is otherwise cytosolic. An inverse correlation was identified between membrane affinity of unpalmitoylated CSP mutants and subsequent palmitoylation: mutants with an increased membrane affinity localize to the endoplasmic reticulum (ER) and are physically separated from the Golgi-localized DHHC proteins. Palmitoylation of an ER-localized mutant could be rescued by brefeldin A treatment, which promotes the mixing of ER and Golgi membranes. Interestingly though, the palmitoylated mutant remained at the ER following brefeldin A washout and did not traffic to more distal membrane compartments. We propose that CSP has a weak membrane affinity that allows the protein to locate its partner Golgi-localized DHHC proteins directly by membrane "sampling." Mutations that enhance membrane association prevent sampling and lead to accumulation of CSP on cellular membranes such as the ER. The coupling of CSP palmitoylation to Golgi membranes may thus be an important requirement for subsequent sorting.

Lipid rafts association of synaptotagmin I on synaptic vesicles

We confirmed the raft association of synaptotagmin I (syt I) in synaptic vesicles by sucrose density gradient centrifugation, cholesterol depletion, and temperature dependence, and Ca2+ was found to positively regulate this association. Furthermore, using syt I mutants we found that the transmembrane domain (TMD) of syt I plays an important role in localizing syt I into the lipid rafts of synaptic vesicles, and the raft association of the TMD can be regulated by its phosphorylation status.

Dual role of the cysteine-string domain in membrane binding and palmitoylation-dependent sorting of the molecular chaperone cysteine-string protein

S-palmitoylation occurs on intracellular membranes and, therefore, membrane anchoring of proteins must precede palmitate transfer. However, a number of palmitoylated proteins lack any obvious membrane targeting motifs and it is unclear how this class of proteins become membrane associated before palmitoylation. Cysteine-string protein (CSP), which is extensively palmitoylated on a "string" of 14 cysteine residues, is an example of such a protein. In this study, we have investigated the mechanisms that govern initial membrane targeting, palmitoylation, and membrane trafficking of CSP. We identified a hydrophobic 31 amino acid domain, which includes the cysteine-string, as a membrane-targeting motif that associates predominantly with endoplasmic reticulum (ER) membranes. Cysteine residues in this domain are not merely sites for the addition of palmitate groups, but play an essential role in membrane recognition before palmitoylation. Membrane association of the cysteine-string domain is not sufficient to trigger palmitoylation, which requires additional downstream residues that may regulate the membrane orientation of the cysteine-string domain. CSP palmitoylation-deficient mutants remain "trapped" in the ER, suggesting that palmitoylation may regulate ER exit and correct intracellular sorting of CSP. These results reveal a dual function of the cysteine-string domain: initial membrane binding and palmitoylation-dependent sorting.

The cysteine string protein multimeric complex

Cysteine string protein (CSPalpha) is a member of the cellular folding machinery that is located on regulated secretory vesicles. We have previously shown that CSPalpha in association with Hsc70 (70kDa heat shock cognate protein) and SGT (small glutamine-rich tetratricopeptide repeat domain protein) is a guanine nucleotide exchange factor (GEF) for G(alphas). Association of this CSPalpha complex with N-type calcium channels, a channel key in coupling calcium influx with synaptic vesicle exocytosis, triggers tonic G protein inhibition of the channels. Syntaxin 1A, a plasma membrane SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) critical for neurotransmission, coimmunoprecipitates with the CSPalpha/G protein/N-type calcium channel complex, however the significance of syntaxin 1A as a component of this complex remains unknown. In this report, we establish that syntaxin 1A interacts with CSPalpha, Hsc70 as well as the synaptic protein interaction (synprint) region of N-type channels. We demonstrate that huntingtin(exon1), a putative biologically active fragment of huntingtin, displaces both syntaxin 1A and CSPalpha from N-type channels. Identification of the protein components of the CSPalpha/GEF system is essential in establishing its precise role in synaptic transmission.

Evidence for regulation of ER/Golgi SNARE complex formation by hsc70 chaperones

SNARE proteins control intracellular membrane fusion through formation of membrane-bridging helix bundles of amphipathic SNARE motifs. Repetitive cycles of membrane fusion likely involve repetitive folding/unfolding of the SNARE motif helical structure. Despite these conformational demands, little is known about conformational regulation of SNAREs by other proteins. Here we demonstrate that hsc70 chaperones stimulate in vitro SNARE complex formation among the ER/Golgi SNAREs syntaxin 5, membrin, rbetl and sec22b, under conditions in which assembly is normally inhibited. Thus, molecular chaperones can render the SNARE motif more competent for assembly. Partially purified hsc70 fractions from brain cytosol had higher specific activities than fully purified hsc70, suggesting the involvement of unidentified cofactors. Using chemical crosslinking of cells followed by immunoprecipitation, we found that hsc70 was associated with ER/Golgi SNAREs in vivo. Consistent with a modulatory role for hsc70 in transport, we found that excess hsc70 specifically inhibited ER-to-Golgi transport in permeabilized cells.

Protein kinase B/Akt is a novel cysteine string protein kinase that regulates exocytosis release kinetics and quantal size

Protein kinase B/Akt has been implicated in the insulin-dependent exocytosis of GLUT4-containing vesicles, and, more recently, insulin secretion. To determine if Akt also regulates insulin-independent exocytosis, we used adrenal chromaffin cells, a popular neuronal model. Akt1 was the predominant isoform expressed in chromaffin cells, although lower levels of Akt2 and Akt3 were also found. Secretory stimuli in both intact and permeabilized cells induced Akt phosphorylation on serine 473, and the time course of Ca2+-induced Akt phosphorylation was similar to that of exocytosis in permeabilized cells. To determine if Akt modulated exocytosis, we transfected chromaffin cells with Akt constructs and monitored catecholamine release by amperometry. Wild-type Akt had no effect on the overall number of exocytotic events, but slowed the kinetics of catecholamine release from individual vesicles, resulting in an increased quantal size. This effect was due to phosphorylation by Akt, because it was not seen in cells transfected with kinase-dead mutant Akt. As overexpression of cysteine string protein (CSP) results in a similar alteration in release kinetics and quantal size, we determined if CSP was an Akt substrate. In vitro 32P-phosphorylation studies revealed that Akt phosphorylates CSP on serine 10. Using phospho-Ser10-specific antisera, we found that both transfected and endogenous cellular CSP is phosphorylated by Akt on this residue. Taken together, these findings reveal a novel role for Akt phosphorylation in regulating the late stages of exocytosis and suggest that this is achieved via the phosphorylation of CSP on serine 10.

The variable C-terminus of cysteine string proteins modulates exocytosis and protein-protein interactions

Cysteine string proteins (Csps) are vesicle proteins involved in neurotransmission and hormone exocytosis. They are composed of distinct domains: a variable N-terminus, a J-domain followed by a linker region, a cysteine-rich string, and a C-terminus which diverges among isoforms. Their precise function and interactions are not fully understood. Using insulin exocytosis as a model, we show that the linker region and the C-terminus, but not the variable N-terminus, regulate overall secretion. Moreover, endogenous Csp1 binds in a calcium-dependent manner to monomeric VAMP2, and this interaction requires the C-terminus of Csp. The interaction is isoform specific as recombinant Csp1 binds VAMP1 and VAMP7, but not VAMP3. Cross-linking in permeabilized clonal beta-cells revealed homodimerization of Csp which is stimulated by Ca(2+) and again modulated by the variant C-terminus. Our data suggest that both interactions of Csp occur during exocytosis and may explain the effect of the variant C-terminus of this chaperon protein on peptide hormone secretion.

The SNARE proteins SNAP-25 and SNAP-23 display different affinities for lipid rafts in PC12 cells. Regulation by distinct cysteine-rich domains

SNAP-25 and its ubiquitously expressed homologue, SNAP-23, are SNARE proteins that are essential for regulated exocytosis in diverse cell types. Recent work has shown that SNAP-25 and SNAP-23 are partly localized in sphingolipid/cholesterol-rich lipid raft domains of the plasma membrane and that the integrity of these domains is important for exocytosis. Here, we show that raft localization is mediated by a 36-amino-acid region of SNAP-25 that is also the minimal sequence required for membrane targeting; this domain contains 4 closely spaced cysteine residues that are sites for palmitoylation. Analysis of endogenous levels of SNAP-25 and SNAP-23 present in lipid rafts in PC12 cells revealed that SNAP-23 (54% raft-associated) was almost 3-fold more enriched in rafts when compared with SNAP-25 (20% raft-associated). We report that the increased raft association of SNAP-23 occurs due to the substitution of a highly conserved phenylalanine residue present in SNAP-25 with a cysteine residue. Intriguingly, although the extra cysteine in SNAP-23 enhances its raft association, the phenylalanine at the same position in SNAP-25 acts to repress the raft association of this protein. These different raft-targeting signals within SNAP-25 and SNAP-23 are likely important for fine-tuning the exocytic pathways in which these proteins operate.

Tying everything together: the multiple roles of cysteine string protein (CSP) in regulated exocytosis

In addition to the core vesicle fusion machinery, the SNARE proteins, a large number of regulatory proteins have been implicated in the process of Ca2+-dependent exocytosis. How these exocytotic proteins are properly targeted and how their myriad interactions are temporally and spatially coordinated is poorly understood. Cysteine string protein (CSP), a secretory vesicle membrane protein and a member of the dnaJ family of co-chaperones, may assist in performing this function. Through its interaction with the ubiquitous chaperone, Hsc70, it is thought that cysteine string protein targets chaperone complexes to the exocytotic machinery to facilitate the correct folding of polypeptides or to regulate the assembly of protein complexes. Since its discovery, there have been conflicting reports from different systems concerned with whether cysteine string protein exerts its effects on exocytosis either up- or down-stream of Ca2+-influx. In this review, we summarize recent experiments that associate cysteine string protein with the regulation of vesicle filling, vesicle docking, Ca2+-channels and the SNARE proteins themselves, hence supporting a role for cysteine string protein as a multifunctional secretory co-chaperone. In addition, we provide an update on the mammalian isoforms of cysteine string protein following the recent discovery of two novel cysteine string proteins.

The vesicle- and target-SNARE proteins that mediate Glut4 vesicle fusion are localized in detergent-insoluble lipid rafts present on distinct intracellular membranes

Insulin stimulates the fusion of intracellular vesicles containing the glucose transporter Glut4 with the plasma membrane in adipocytes and muscle cells. Glut4 vesicle fusion is thought to be catalyzed by the interaction of the vesicle soluble N-ethyl-maleimide-sensitive fusion protein attachment protein receptor VAMP2 with the target soluble N-ethyl-maleimide-sensitive fusion protein attachment protein receptors SNAP-23 and syntaxin 4. Here, we use combined membrane fractionation, detergent solubility, and sucrose gradient flotation to demonstrate that the large majority (>70%) of SNAP-23 and a significant proportion of syntaxin 4 ( approximately 35%) are associated with plasma membrane lipid rafts in 3T3-L1 adipocytes. Furthermore, VAMP2 is shown to be concentrated in lipid rafts isolated from intracellular membranes. Insulin stimulation had no effect on the plasma membrane raft association of SNAP-23 or syntaxin 4 but promoted VAMP2 insertion into plasma membrane rafts. Immunofluorescence analysis revealed that SNAP-23 was clustered at the plasma membrane and almost completely segregated from the transferrin receptor. SNAP-23 distribution seemed to be distinct from caveolin-1, and clusters of SNAP-23 were dispersed after cholesterol extraction with methyl-beta-cyclodextrin, suggesting that the majority of SNAP-23 is associated with non-caveolar, cholesterol-rich lipid rafts. The results described implicate lipid rafts as important platforms for Glut4 vesicle fusion and suggest the hypothesis that such rafts may represent a spatial integration point of insulin signaling and membrane traffic.

Cysteine-string protein in inner hair cells of the organ of Corti: synaptic expression and upregulation at the onset of hearing

Cysteine-string protein is a vesicle-associated protein that plays a vital function in neurotransmitter release. We have studied its expression and regulation during cochlear maturation. Both the mRNA and the protein were found in primary auditory neurons and the sensory inner hair cells. More importantly, cysteine-string protein was localized on synaptic vesicles associated with the synaptic ribbon in inner hair cells and with presynaptic differentiations in lateral and medial olivocochlear terminals -- the cell bodies of which lie in the auditory brainstem. No cysteine-string protein was expressed by the sensory outer hair cells suggesting that the distinct functions of the two cochlear hair cell types imply different mechanisms of neurotransmitter release. In developmental studies in the rat, we observed that cysteine-string protein was present beneath the inner hair cells at birth and beneath outer hair cells by postnatal day 2 only. We found no expression in the inner hair cells before about postnatal day 12, which corresponds to the period during which the first cochlear action potentials could be recorded. In conclusion, the close association of cysteine-string protein with synaptic vesicles tethered to synaptic ribbons in inner hair cells and its synchronized expression with the appearance and maturation of the cochlear potentials strongly suggest that this protein plays a fundamental role in sound-evoked glutamate release by inner hair cells. This also suggests that this role may be common to ribbon synapses and conventional central nervous system synapses.

SNARE expression and distribution during 3T3-L1 adipocyte differentiation

Differentiation of 3T3-L1 cells into adipocytes presupposes the expression of the glucose transporter isoform GLUT4 and the acquisition of insulin-dependent GLUT4 translocation from intracellular storage vesicles to plasma membrane. This ability to translocate GLUT4 depends on the presence of a set of proteins of the SNARE category that are essential in the fusion step. The expression and levels of some of these SNARE proteins are altered during 3T3-L1 differentiation. Levels of the v-SNARE protein cellubrevin and of the t-SNARE protein syntaxin 4 were increased in this process in parallel to GLUT4. However, the levels of SNAP-23, another t-SNARE, were maintained during differentiation. Immunofluorescence images of SNAP-23 showed the initial distribution of this protein in a perinuclear region before differentiation and its redistribution towards plasma membrane in the adipocyte form. These results suggest a capital role in the expression levels and cellular distribution, during 3T3-L1 differentiation, of SNARE proteins involved in the late steps of GLUT4 translocation.

Phosphorylation of cysteine string protein by protein kinase A. Implications for the modulation of exocytosis

Cyclic AMP-dependent protein kinase (PKA) enhances regulated exocytosis in neurons and most other secretory cells. To explore the molecular basis of this effect, known exocytotic proteins were screened for PKA substrates. Both cysteine string protein (CSP) and soluble NSF attachment protein-alpha (alpha-SNAP) were phosphorylated by PKA in vitro, but immunoprecipitation of cellular alpha-SNAP failed to detect (32)P incorporation. In contrast, endogenous CSP was phosphorylated in synaptosomes, PC12 cells, and chromaffin cells. In-gel kinase assays confirmed PKA to be a cellular CSP kinase, with phosphorylation occurring on Ser(10). PKA phosphorylation of CSP reduced its binding to syntaxin by 10-fold but had little effect on its interaction with HSC70 or G-protein subunits. Furthermore, an in vivo role for Ser(10) phosphorylation at a late stage of exocytosis is suggested by analysis of chromaffin cells transfected with wild type or non-phosphorylatable mutant CSP. We propose that PKA phosphorylation of CSP could modulate the exocytotic machinery, by selectively altering its availability for protein-protein interactions.