Cysteine-string proteins: a cycle of acylation and deacylation?

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.

Drosophila huntingtin-interacting protein 14 is a presynaptic protein required for photoreceptor synaptic transmission and expression of the palmitoylated proteins synaptosome-associated protein 25 and cysteine string protein

Palmitoylation affects the trafficking, stability, aggregation, and/or functional activity of a substantial number of neuronal proteins. We identified mutations in dHIP14, the Drosophila homolog of the human palmitoyl transferase, Huntingtin-interacting protein 14 (HIP14). HIP14 was previously reported to localize primarily to Golgi and to palmitoylate the neuronal proteins synaptosome-associated protein 25 (SNAP-25), PSD-95 (postsynaptic density-95), GAD65, Synaptotagmin, and Huntingtin in mammalian neurons. We find dHIP14 to be an essential maternal effect gene required for photoreceptor synaptic transmission and for proper in vivo expression of the palmitoylated presynaptic proteins SNAP-25 and cysteine string protein. In non-neuronal cells in the fly, dHIP14 protein is found in Golgi. However, in fly neurons, we find dHIP14 primarily in presynaptic terminals, something we also observe with HIP14. In mammalian neurons, we also find a significant fraction of HIP14 colocalizing with a synaptic vesicle marker. Based on localization of the palmitoyl transferase HIP14 within the presynaptic nerve terminal, we propose palmitoylation as a possible mechanism that may be operating to rapidly regulate synaptic efficacy.

Cysteine string protein interacts with and modulates the maturation of the cystic fibrosis transmembrane conductance regulator

The cystic fibrosis transmembrane conductance regulator (CFTR) is a cAMP-regulated chloride channel whose phosphorylation regulates both channel gating and its trafficking at the plasma membrane. Cysteine string proteins (Csps) are J-domain-containing, membrane-associated proteins that have been functionally implicated in regulated exocytosis. Therefore, we evaluated the possibility that Csp is involved in regulated CFTR trafficking. We found Csp expressed in mammalian epithelial cell lines, several of which express CFTR. In Calu-3 airway cells, immunofluorescence colocalized Csp with calnexin in the endoplasmic reticulum and with CFTR at the apical membrane domain. CFTR coprecipitated with Csp from Calu-3 cell lysates. Csp associated with both core-glycosylated immature and fully glycosylated mature CFTRs (bands B and C); however, in relation to the endogenous levels of the B and C bands expressed in Calu-3 cells, the Csp interaction with band B predominated. In vitro protein binding assays detected physical interactions of both mammalian Csp isoforms with the CFTR R-domain and the N terminus, having submicromolar affinities. In Xenopus oocytes expressing CFTR, Csp overexpression decreased the chloride current and membrane capacitance increases evoked by cAMP stimulation and decreased the levels of CFTR protein detected by immunoblot. In mammalian cells, the steady-state expression of CFTR band C was eliminated, and pulse-chase studies showed that Csp coexpression blocked the conversion of immature to mature CFTR and stabilized band B. These results demonstrate a primary role for Csp in CFTR protein maturation. The physical interaction of this Hsc70-binding protein with immature CFTR, its localization in the endoplasmic reticulum, and the decrease in production of mature CFTR observed during Csp overexpression reflect a role for Csp in CFTR biogenesis. The documented role of Csp in regulated exocytosis, its interaction with mature CFTR, and its coexpression with CFTR at the apical membrane domain of epithelial cells may reflect also a role for Csp in regulated CFTR trafficking at the plasma membrane.

Molecular chaperones and the regulation of neurotransmitter exocytosis11Abbreviations: SNARE, soluble NSF attachment protein (SNAP) receptor; NSF, N-ethylmaleimide-sensitive factor; Hsc70, 70-kDa heat-shock cognate protein; CSP, cysteine-string protein; VAMP vesicle-associated membrane protein; SNAP-25, synaptosome-associated protein 25 kDa; NEM, N-ethylmaleimide; AAA ATPases, ATPases Associated to a variety of Activities; and Hsp70, Hsp90, and Hsp60, 70-kDa, 90-kDa, and 60-kDa heat-shock protein, respectively

Regulated neurotransmitter release depends on a precise sequence of events that lead to repeated cycles of exocytosis and endocytosis. These events are mediated by a series of molecular interactions among vesicular, plasma membrane, and cytosolic proteins. An emerging theme has been that molecular chaperones may guide the sequential restructuring of stable or transient protein complexes to promote a temporal and spatial regulation of the endo- and exocytotic machinery and to ensure a vectorial passage through the vesicle cycle. Chaperones, specialized for a few substrates, are ideally suited to participate in regulatory processes that require some molecular dexterity to rearrange conformational or oligomeric protein structures. This article emphasizes the significance of three molecular chaperone systems in regulated neurotransmitter release: the regulation of soluble NSF attachment protein receptor (SNARE) complexes by N-ethylmaleimide-sensitive factor (NSF) and the soluble NSF attachment protein (SNAP), the uncoating of clathrin-coated vesicles by the 70 kDa heat-shock cognate protein (Hsc70), and the regulation of SNARE complex-associated protein interactions by cysteine-string protein and Hsc70.

Intrinsic membrane association of Drosophila cysteine string proteins

Cysteine string proteins (csps) are highly conserved constituents of vertebrate and invertebrate secretory organelles. Biochemical and immunoprecipitation experiments implied that vertebrate csps were integral membrane proteins that were tethered to the outer leaflet of secretory vesicles via the fatty acyl residues of their extensively acylated cysteine string. Independently, work of others suggested that Drosophila csps were peripheral membrane proteins that were anchored to membranes by a mechanism that was independent of the cysteine string and its fatty acyl residues. We extended these investigation and found first that sodium carbonate treatment partially stripped both csps and the integral membrane protein, synaptotagmin, from Drosophila membranes. Concomitantly, carbonate released fatty acids into the medium, arguing that it has a mild, solubilizing effect on these membranes. Second, we observed that Drosophila csps behaved like integral membrane proteins in Triton X-114 partitioning experiments. Third, we found that when membrane-bound csps were deacylated, they remained membrane bound. Moreover, it appeared that hydrophobic interactions were necessary for this persistent membrane association of csps. Thus, neither reducing conditions, urea, nor chaotropic agents displaced deacylated csps from membranes. Only detergents were effective in solubilizing deacylated csps. Finally, by virtue of the inaccessibility of deacylated csps to thiol alkylation by the membrane-impermeant alkylating reagent, iodoacetic acid, we inferred that it was the cysteine string domain that mediated the membrane association of deacylated csps. Thus, we conclude that under physiological conditions csps are integral membrane proteins of secretory organelles, and that the cysteine string domain plays a vital role in the membrane association of these proteins.

Exocytosis in chromaffin cells of the adrenal medulla

The chromaffin cell has been used as a model to characterize releasable components present in secretory granules and to understand the cellular mechanisms involved in catecholamine release. Recent physiological and biochemical developments have revealed that molecular mechanisms implicated in granule trafficking are conserved in all eukaryotic species: a rise in intracellular calcium triggers regulated exocytosis, and highly conserved proteins are essential elements which interact with each other to form a molecular scaffolding, ensuring the docking of granules at the plasma membrane, and perhaps membrane fusion. However, the mechanisms regulating secretion are multiple and cell specific. They operate at different steps along the life of a granule, from the time of granule biosynthesis up to the last step of exocytosis. With regard to cell specificity, noradrenaline and adrenaline chromaffin cells display different receptor and signaling characteristics that may be important to exocytosis. Characterization of regulated exocytosis in chromaffin cells provides not only fundamental knowledge of neurosecretion but is of additional importance as these cells are used for therapeutic purposes.

The DnaJ-like cysteine string protein and exocytotic neurotransmitter release

The fast, tightly regulated release of neurotransmitters from presynaptic nerve terminals is effected by a complex molecular apparatus. The precise roles of the various proteins involved remain largely conjectural. Cysteine string proteins (CSPs) are novel synaptic vesicle components that have been conserved in evolution. They are characterized by an N-terminus 'J'-domain and a central, multiply palmitoylated string of cysteine residues. Vertebrate CSPs have been implicated in a functional interaction of synaptic vesicles with presynaptic Ca2+ channels. Genetic 'knockout' of CSPs in Drosophila results in a temperature-sensitive breakdown of elicited transmitter release. Here we try to integrate these observations into speculative functional models on the role of this new protein family in synaptic vesicle exocytosis.