Complexins: cytosolic proteins that regulate SNAP receptor function

RAB3 and synaptotagmin: the yin and yang of synaptic membrane fusion

Synaptic vesicle exocytosis occurs in consecutive steps: docking, which specifically attaches vesicles to the active zone; priming, which makes the vesicles competent for Ca(2+)-triggered release and may involve a partial fusion reaction; and the final Ca(2+)-regulated step that completes fusion. Recent evidence suggests that the critical regulation of the last step in the reaction is mediated by two proteins with opposite actions: synaptotagmin, a Ca(2+)-binding protein that is essential for Ca(2+)-triggered release and probably serves as the Ca(2+)-sensor in fusion, and rab3, which limits the number of vesicles that can be fused as a function of Ca2+ in order to allow a temporally limited, repeatable signal.

Analysis of regulated exocytosis in adrenal chromaffin cells: insights into NSF/SNAP/SNARE function

Many of the proteins that function in regulated exocytosis have now been identified. Several proteins form part of a conserved core machinery that acts in many intracellular vesicular fusion steps and their essential roles confirmed by molecular genetic analysis. In addition, studies with adrenal chromaffin and PC12 cells have demonstrated the function of various proteins in regulated exocytosis and have permitted dissection of the stages of exocytosis in which they act. N-Ethylmaleimide-sensitive fusion protein (NSF) and soluble NSF attachment proteins (SNAPs) are key proteins in exocytosis. Examination of their function has indicated that they have a predocking role most likely as molecular chaperones to prepare the docking/fusion machinery. The exact site and time of action in exocytosis of many of the other identified proteins are unknown. A major emphasis for the future will be analysis of the molecular physiology of regulated exocytosis to permit the assignment of functions to identified proteins in particular stages of the regulated exocytotic pathway.

Synapsins I and II are ATP-binding proteins with differential Ca2+ regulation

Synapsins I and II are abundant phosphoproteins that are localized to synaptic vesicles and have essential functions in regulating synaptic vesicle exocytosis. Synapsins contain a single evolutionarily conserved, large central domain, the C-domain, that accounts for the majority of their sequences. Unexpectedly, the crystal structure of the C-domain from synapsin I revealed that it is structurally closely related to several ATPases despite the absence of sequence similarities (Esser, L., Wang, C.-R., Hosaka, M., Smagula, C. S., Südhof, T. C., and Deisenhofer, J. (1998) EMBO J., in press). We now show that the C-domains of both synapsin I and synapsin II constitute high affinity ATP-binding modules. The two C-domains exhibit similar ATP affinities but are differentially regulated; ATP binding to synapsin I is Ca(2+)-dependent whereas ATP binding to synapsin II is Ca(2+)-independent. In synapsin I, the Ca2+ requirement for ATP binding is mediated by a single, evolutionarily conserved glutamate residue (Glu373) at a position where synapsin II contains a lysine residue. Exchange of Glu373 for lysine converts synapsin I from a Ca(2+)-dependent protein into a Ca(2+)-independent ATP-binding protein. Our studies suggest that synapsins I and II function on synaptic vesicles as ATP-binding proteins that are differentially regulated by Ca2+.

ROP, the Drosophila Sec1 homolog, interacts with syntaxin and regulates neurotransmitter release in a dosage-dependent manner

The Sec1 family of proteins is thought to function in both non-neuronal and neuronal secretion, although the precise role of this protein family has not been defined. Here, we study the function of ROP, the Drosophila Sec1 homolog, in neurotransmitter release. Electrophysiological analyses of transgenic lines overexpressing ROP and syntaxin, a presynaptic membrane protein, indicate that ROP interacts with syntaxin in vivo. Characterization of four point mutations in ROP shows that they fall into two phenotypic classes. Two mutations cause a dramatic reduction in both evoked and spontaneous neurotransmitter release. In contrast, the other two mutations reveal an increase in evoked neurotransmission. Our data further show that neurotransmission is highly sensitive to the levels of ROP function. Studies on heterozygote animals indicate that half the amount of wild-type ROP results in a dramatic decrease in evoked and spontaneous exocytosis. Taken together, these results suggest that ROP interacts with syntaxin in vivo and is a rate-limiting regulator of exocytosis that performs both positive and inhibitory functions in neurotransmission.

Role of Rab GTPases in membrane traffic

Small GTPases of the Rab subfamily have been known to be key regulators of intracellular membrane traffic since the late 1980s. Today this protein group amounts to more than 40 members in mammalian cells which localize to distinct membrane compartments and exert functions in different trafficking steps on the biosynthetic and endocytic pathways. Recent studies indicate that cycles of GTP binding and hydrolysis by the Rab proteins are linked to the recruitment of specific effector molecules on cellular membranes, which in turn impact on membrane docking/fusion processes. Different Rabs may, nevertheless, have slightly different principles of action. Studies performed in yeast suggest that connections between the Rabs and the SNARE machinery play a central role in membrane docking/fusion. Further elucidation of this linkage is required in order to fully understand the functional mechanisms of Rab GTPases in membrane traffic.

Mints, Munc18-interacting proteins in synaptic vesicle exocytosis

Munc18-1 is a neuronal protein that interacts with syntaxin 1 and is required for synaptic vesicle exocytosis. We have now identified two Munc18-1-interacting proteins called Mint1 and Mint2 that may mediate the function of Munc18-1. Mint proteins are detectable only in brain and are composed of an N-terminal sequence that binds Munc18-1, a middle phosphotyrosine-binding domain, and two C-terminal PDZ domains thought to attach proteins to the plasma membrane. In brain, Mint proteins are part of a multimeric complex containing Munc18-1 and syntaxin that likely functions as an intermediate in synaptic vesicle docking/fusion. The phosphotyrosine-binding domain specifically binds to phosphatidylinositol phosphates known to be produced during vesicle exocytosis (Hay, J. C., Fisette, P. L., Jenkins, G. H., Fukami, K., Takonawa, T., Anderson, R. A., and Martin, T. F. J. (1995) Nature 374, 173-177). Our data suggest a model whereby local production of phosphatidylinositol phosphates may trigger the binding of vesicles to the active zone via the Mint.Munc18-1 complex in conjunction with syntaxin 1.

Molecular cloning of synaphins/complexins, cytosolic proteins involved in transmitter release, in the electric organ of an electric ray (Narke japonica)

Synaphins/complexins are cytosolic proteins associated with the docking/fusion complex crucial to transmitter release. The electric organ of the electric ray Narke japonica contained at least two kinds of synaphins as revealed by immunoblotting. cDNAs for three synaphins were cloned from a cDNA library prepared from the electric lobe where cell bodies of electromotor nerves innervating the electric organ exist. The proteins encoded by these cDNAs were named Nj-synaphins 1a, 1b and 2 on the basis of their high homologies (83-93%) to mammalian synaphins 1 and 2. Nj-Synaphins were immunoprecipitated by an anti-syntaxin monoclonal antibody, together with syntaxin, SNAP-25 and VAMP (synaptobrevin), suggesting the presence of a docking/fusion complex similar to that in the mammalian brain.

Protein phosphorylation events in exocytosis and endocytosis

1. Exocytosis and endocytosis are the continuous outward and inward vesicular transports in a cell that occur constantly for intra- and inter-cellular communications. These events are accomplished with the release and uptake of chemical messages fundamental in a variety of cellular functions, such as neurotransmitter release, hormone secretion and receptor internalization. 2. Although the mechanisms underlying these events have not been fully established, it is widely accepted that they are largely mediated and controlled by a number of effector proteins. These proteins can operate individually and in concert to produce specialized machineries in the sequential steps of exocytotic and endocytic transports. 3. Protein phosphorylation, the most common covalent modification of proteins in cells, has been implicated as playing an important role in the regulation of exocytosis and endocytosis. Many proteins involved in these processes have been identified to be phosphorylated under certain conditions. 4. For instance, synapsin I, myristoylated alanine-rich C kinase substrate and dynamin I undergo dynamic phosphorylation and dephosphorylation cycles during exocytosis and endocytosis, implying that protein phosphorylation regulates the functions of these proteins and, thus, exocytosis and endocytosis.

SNAREs and NSF in targeted membrane fusion

A major current issue in vesicle trafficking is whether NSF (N-ethylmaleimide-sensitive factor) and alpha-SNAP (alpha-soluble NSF attachment protein) are required prior to SNARE (SNAP receptor) complex formation to allow vesicle docking, or after docking at a step close to membrane fusion. Recent studies of yeast vacuolar fusion indicated that the requirement for ATP, NSF and alpha-SNAP could be completely satisfied prior to SNARE docking complex assembly; however, the universality of a predocking role for these factors remains to be established. The vacuolar fusion system has also been used to directly demonstrate a requirement for SNARE proteins on both fusing membranes, verifying a central postulate of current fusion models.

Neurotransmitter release - four years of SNARE complexes

Exocytosis in neurons requires proteins known as SNAREs, membrane proteins that have now been implicated in many intracellular fusion events. SNAREs assemble into stable ternary complexes that are dissociated by the ATPase NSF (N-ethylmaleimide-sensitive factor), working together with SNAPs (soluble NSF attachment proteins). Recent results have shed new light on the mechanisms underlying SNARE (SNAP receptor) complex assembly and disassembly, and suggest changes in models that relate these reactions to vesicle docking and fusion.

DOC2 proteins in rat brain: complementary distribution and proposed function as vesicular adapter proteins in early stages of secretion

DOC2 proteins constitute a novel protein family that may function in secretion and contain a double C2 domain. We have cloned and characterized two DOC2 isoforms in rat brain and studied their interactions with other proteins implicated in secretion. DOC2A was virtually brain specific, DOC2B ubiquitous. Within brain, the isoforms were expressed nonuniformly and complementary within neurons, not astroglia, and copurified with synaptic vesicles. Affinity purification, yeast two-hybrid analysis, and coimmunoprecipitation revealed that DOC2 binds munc18, a protein also implicated in secretion. The first DOC2 C2 domain and most of munc18 are involved in direct interactions. Munc18 may regulate formation of 'core complexes' during vesicle docking, by interacting with syntaxin. We show that DOC2 and syntaxin compete for munc18. Other core complex components shifted the equilibrium between syntaxin-munc18 versus DOC2-munc18. These data suggest that DOC2 proteins are vesicular adapter proteins regulating munc18-syntaxin complexes and herewith synaptic vesicle docking.

Direct interaction of the rat unc-13 homologue Munc13-1 with the N terminus of syntaxin

unc-13 mutants in Caenorhabditis elegans are characterized by a severe deficit in neurotransmitter release. Their phenotype is similar to that of the C. elegans unc-18 mutation, which is thought to affect synaptic vesicle docking to the active zone. This suggests a crucial role for the unc-13 gene product in the mediation or regulation of synaptic vesicle exocytosis. Munc13-1 is one of three closely related rat homologues of unc-13. Based on the high degree of similarity between unc-13 and Munc13 proteins, it is thought that their essential function has been conserved from C. elegans to mammals. Munc13-1 is a brain-specific peripheral membrane protein with multiple regulatory domains that may mediate diacylglycerol, phospholipid, and calcium binding. In the present study, we demonstrate by three independent methods that the C terminus of Munc13-1 interacts directly with a putative coiled coil domain in the N-terminal part of syntaxin. Syntaxin is a component of the exocytotic synaptic core complex, a heterotrimeric protein complex with an essential role in transmitter release. Through this interaction, Munc13-1 binds to a subpopulation of the exocytotic core complex containing synaptobrevin, SNAP25 (synaptosomal-associated protein of 25 kDa), and syntaxin, but to no other tested syntaxin-interacting or core complex-interacting protein. The site of interaction in syntaxin is similar to the binding site for the unc-18 homologue Munc18, but different from that of all other known syntaxin interactors. These data indicate that unc-13-related proteins may indeed be involved in the mediation or regulation of synaptic vesicle exocytosis by modulating or regulating core complex formation. The similarity between the unc-13 and unc-18 phenotypes is paralleled by the coincidence of the binding sites for Munc13-1 and Munc18 in syntaxin. It is possible that the phenotype of unc-13 and unc-18 mutations is caused by the inability of the respective mutated gene products to bind to syntaxin.

Distinct properties of neuronal and astrocytic endopeptidase 3.4.24.16: a study on differentiation, subcellular distribution, and secretion processes

Endopeptidase 3.4.24.16 belongs to the zinc-containing metalloprotease family and likely participates in the physiological inactivation of neurotensin. The peptidase displays distinct features in pure primary cultured neurons and astrocytes. Neuronal maturation leads to a decrease in the proportion of endopeptidase 3.4.24.16-bearing neurons and to a concomitant increase in endopeptidase 3.4.24.16 activity and mRNA content. By contrast, there is no change with time in endopeptidase 3.4.24.16 activity or content in astrocytes. Primary cultured neurons exhibit both soluble and membrane-associated endopeptidase 3.4.24.16 activity. The latter behaves as an ectopeptidase on intact plated neurons and resists treatments with 0.2% digitonin and Na2CO3. Further evidence for an association of the enzyme with plasma membranes was provided by cryoprotection experiments and electron microscopic analysis. The membrane-associated form of endopeptidase 3.4.24.16 increased during neuronal differentiation and appears to be mainly responsible for the overall augmentation of endopeptidase 3.4.24.16 activity observed during neuronal maturation. Unlike neurons, astrocytes only contain soluble endopeptidase 3.4.24.16. Astrocytes secrete the enzyme through monensin, brefeldin A, and forskolin-independent mechanisms. This indicates that endopeptidase 3.4.24.16 is not released by classical regulated or constitutive secreting processes. However, secretion is blocked at 4 degrees C and by 8 bromo cAMP and is enhanced at 42 degrees C, two properties reminiscent of that of other secreted proteins lacking a classical signal peptide. By contrast, neurons appear unable to secrete endopeptidase 3.4.24.16.

A SNARE complex containing syntaxin 3 is present in ribbon synapses of the retina

In contrast to conventional synapses, which release neurotransmitter transiently, ribbon synapses formed by photoreceptors and bipolar cells of the retina release neurotransmitter continuously and modulate the rate in response to light. Both modes of release are mediated by synaptic vesicles but probably differ in the regulation of docking and fusion of synaptic vesicles with the plasma membrane. We have found that syntaxin 1, an essential component of the core fusion complex in conventional synapses, is absent from ribbon synapses of the retina, raising the possibility that these synapses contain a different type of syntaxin or syntaxin-like protein. By immunoprecipitating syntaxin 1-depleted retina and brain extracts with a SNAP-25 antibody and microsequencing the precipitated proteins, syntaxin 3 was detected in retina complexed with SNAP-25, synaptobrevin, and complexin. Using an anti-syntaxin 3 antiserum, syntaxin 3 was demonstrated to be present at high levels in retina compared to brain. Immunofluorescent staining of rat retina sections confirmed that syntaxin 3 is expressed by photoreceptor and bipolar cells in the retina. Thus, in the retina, expression of syntaxin 3 is correlated with ribbon synapses and may play a role in the tonic release of neurotransmitter.

Transport vesicle docking: SNAREs and associates

Proteins that function in transport vesicle docking are being identified at a rapid rate. So-called v- and t-SNAREs form the core of a vesicle docking complex. Additional accessory proteins are required to protect SNAREs from promiscuous binding and to deprotect SNAREs under conditions in which transport vesicle docking should occur. Because access to SNAREs must be regulated, other proteins must also contain specificity determinants to accomplish delivery of transport vesicles to their distinct and specific membrane targets.

Adrenal chromaffin cells contain functionally different SNAP-25 monomers and SNAP-25/syntaxin heterodimers

Syntaxin and SNAP-25 (synaptosome-associated protein of 25 kDa), associated with the neuronal plasmalemma, and synaptobrevin, a membrane protein of synaptic vesicles, are essential components of the exocytotic apparatus of synaptic vesicles. All three can be proteolytically cleaved by tetanus and/or botulinum neurotoxins. As a consequence of their cleavage, exocytosis of neurotransmitters is blocked. In adrenal chromaffin cells botulinum neurotoxin A only incompletely inhibits exocytosis. This incomplete inhibition of exocytosis is associated with only partial cleavage of SNAP-25 by the toxin, indicating that distinct pools of SNAP-25 may exist in chromaffin cells which differ in their sensitivities to botulinum neurotoxin A. In line with this result we localized SNAP-25 by immunogold electron microscopy not only to the plasmalemma but also to the chromaffin vesicle membrane. Moreover, in addition to SNAP-25 monomers, stable SNAP-25/syntaxin heterodimers were found in chromaffin cells. Subfractionation studies revealed the presence of SNAP-25/syntaxin heterodimers in an enriched fraction of chromaffin vesicles. This complex proved to be stable in SDS, and SNAP-25 within heterodimers was resistant to proteolytic attack by botulinum neurotoxin A. We suggest that these preexisting heterodimers may serve as receptors of soluble NSF attachment proteins (SNAP receptors) during chromaffin vesicle exocytosis.

A yeast protein related to a mammalian Ras-binding protein, Vps9p, is required for localization of vacuolar proteins

In the yeast Saccharomyces cerevisiae, mutations in vacuolar protein sorting (VPS) genes result in secretion of proteins normally localized to the vacuole. Characterization of the VPS pathway has provided considerable insight into mechanisms of protein sorting and vesicle-mediated intracellular transport. We have cloned VPS9 by complementation of the vacuolar protein sorting defect of vps9 cells, characterized its gene product, and investigated its role in vacuolar protein sorting. Cells with a vps9 disruption exhibit severe vacuolar protein sorting defects and a temperature-sensitive growth defect at 38 degrees C. Electron microscopic examination of delta vps9 cells revealed the appearance of novel reticular membrane structures as well as an accumulation of 40- to 50-nm-diameter vesicles, suggesting that Vps9p may be required for the consumption of transport vesicles containing vacuolar protein precursors. A temperature-conditional allele of vps9 was constructed and used to investigate the function of Vps9p. Immediately upon shifting of temperature-conditional vps9 cells to the nonpermissive temperature, newly synthesized carboxypeptidase Y was secreted, indicating that Vps9p function is directly required in the VPS pathway. Antibodies raised against Vps9p immunoprecipitate a rare 52-kDa protein that fractionates with cytosolic proteins following cell lysis and centrifugation. Analysis of the VPS9 DNA sequence predicts that Vps9p is related to human proteins that bind Ras and negatively regulate Ras-mediated signaling. We term the related regions of Vps9p and these Ras-binding proteins a GTPase binding homology domain and suggest that it defines a family of proteins that bind monomeric GTPases. Vps9p may bind and serve as an effector of a rab GTPase, like Vps2lp, required for vacuolar protein sorting.

Sec18p (NSF)-driven release of Sec17p (alpha-SNAP) can precede docking and fusion of yeast vacuoles

S. cerevisiae inherits its vacuole by projecting vacuole-derived membrane vesicles and tubules into the bud, where they fuse to establish the daughter vacuole. This homotypic fusion event can be assayed in vitro. It requires Sec17p and Sec18p, the homologs of the mammalian alpha-SNAP and NSF, which cooperate in multiple steps of membrane trafficking. We now report that Sec17p, Sec18p, and ATP are only needed for an early stage of the reaction that results in Sec17p release. Sec17p and Sec18p actions precede, and are needed for, the step employing the Ras-like GTPase Ypt7p. Sec18p-driven release of Sec17p can even precede vacuole docking, as it can occur prior to mixing of vacuoles and is insensitive to vacuole concentration. Sec17p and Sec18p thus may function in a predocking stage of the reaction, rather than in bilayer fusion per se.