Defining the functions of trans-SNARE pairs

Structure of proteins involved in synaptic vesicle fusion in neurons

The fusion of vesicles with target membranes is controlled by a complex network of protein-protein and protein-lipid interactions. Structures of the SNARE complex, synaptotagmin III, nSec1, domains of the NSF chaperone and its adaptor SNAP, and Rab3 and some of its effectors provide the framework for developing molecular models of vesicle fusion and for designing experiments to test these models.

Ergosterol is required for the Sec18/ATP-dependent priming step of homotypic vacuole fusion

In vitro homotypic fusion of yeast vacuoles occurs in three stages: priming, the Sec18 (NSF)-mediated changes that precede vacuole association; docking, the Ypt7 and SNARE-mediated pairing of vacuoles; and fusion, mediated by calmodulin/V0/t-SNARE interactions. Defects in catalysts of each stage result in fragmented (unfused) vacuoles. Strains with deletions in any of ERG genes 3-6, lacking normal ergosterol biosynthesis, have fragmented vacuoles. The ergosterol ligands filipin, nystatin and amphotericin B block the in vitro fusion of vacuoles from wild-type cells. Each of these inhibitors acts at the priming stage to inhibit Sec17p release from vacuoles. A reversible delay in Sec18p action prevents vacuoles from acquiring resistance to any of these three drugs, confirming that their action is on the normal fusion pathway. Ergosterol or cholesterol delivery to wild-type vacuoles stimulates their in vitro fusion, and the in vitro fusion of ergDelta vacuoles requires added sterol. The need for ergosterol for vacuole priming underscores the role of lipids in organizing the membrane elements of this complex reaction.

A developmentally regulated Rab11 homologue in Trypanosoma brucei is involved in recycling processes

Endocytosis in the parasitic protozoan Trypanosoma brucei, a deeply divergent eukaryote, is implicated as important in both general cellular function and virulence, and is strongly developmentally regulated. We report the characterisation of a previously undefined endosomal compartment in T. brucei based on identification of a new trypanosome gene (TbRAB11) homologous to Rab11/Ypt31. Northern and western analyses indicated that TbRAB11 expression was significantly upregulated in the bloodstream stage of the parasite, the first trypanosome Rab to be identified with a developmentally regulated expression profile. In procyclic form parasites TbRAB11 localised to a compartment positioned close to the basal body, similar to mammalian Rab11. By contrast, in bloodstream form parasites, TbRAB11-containing structures were more extensive and the TbRAB11 compartment extended towards the posterior face of the nucleus, was more elaborate and was not always adjacent to the basal body. Colocalisation studies by light and confocal microscopy demonstrated that TbRAB11 was located on a compartment that did not correspond to other established trypanosomal organelles or markers. Using concanavalin A internalisation and temperature block procedures, TbRAB11 was observed on endomembranes anterior to the flagellar pocket that are juxtaposed to the collecting tubules. TbRAB11 colocalised with the trypanosomal transferrin receptor and internalised antivariant surface glycoprotein. Further, we show that the collecting tubules contain TbRAB5A, suggesting that they are the trypanosomatid early endosome. Hence, TbRAB11 is present on endosomal structures that contain recycling cargo molecules and is under developmental regulation, suggesting a role in stage-dependent endocytic processes.

Cycling of synaptic vesicles: how far? How fast!

Synaptic transmission is based on the regulated exocytotic fusion of synaptic vesicles filled with neurotransmitter. In order to sustain neurotransmitter release, these vesicles need to be recycled locally. Recent data suggest that two tracks for the cycling of synaptic vesicles coexist: a slow track in which vesicles fuse completely with the presynaptic plasma membrane, followed by clathrin-mediated recycling of the vesicular components, and a fast track that may correspond to the transient opening and closing of a fusion pore. In this review, we attempt to provide an overview of the components involved in both tracks of vesicle cycling, as well as to identify possible mechanistic links between these two pathways.

Functional and physical coupling of voltage-sensitive calcium channels with exocytotic proteins: ramifications for the secretion mechanism

The secretion of neurotransmitters is a rapid Ca(2+)-regulated process that brings about vesicle fusion with the plasma membrane. This rapid process (< 100 microseconds) involves multiple proteins located at the plasma and vesicular membranes. Because of their homology to proteins participating in constitutive secretion and protein trafficking, they have been characterized extensively. The sequential events that lead these proteins to vesicle docking and fusion are still unclear. We will review recent studies that demonstrate the operative role played by voltage-sensitive Ca(2+) channels and discuss the relevance for the process of evoked transmitter release. The regulation of Ca(2+) influx by syntaxin, synaptosome-associated protein of 25 kDa (SNAP-25) and synaptotagmin, and the reciprocity of these proteins in controlling the kinetic properties of the channel will be discussed. Calcium channel and synaptic proteins expressed in Xenopus oocytes demonstrate a strong functional interaction, which could be pertinent to the mechanism of secretion. First, the voltage-sensitive Ca(2+) channels are negatively modulated by syntaxin: this inhibition is reversed by synaptotagmin. Second, the modulation of N-type Ca(2+) channel activation kinetics strongly suggests that the vesicle could be docked at the plasma membrane through direct interaction with synaptotagmin. Finally, these interactions provide evidence for the assembly of the voltage-sensitive Ca(2+) channel with syntaxin 1A, SNAP-25 and synaptotagmin into an excitosome complex: a putative fusion complex with a potential role in the final stages of secretion. Studies suggest that cross-talk between the synaptic proteins and the channel in a tightly organized complex may enable a rapid secretory response to an incoming signal such as membrane depolarization.

Vac8p release from the SNARE complex and its palmitoylation are coupled and essential for vacuole fusion

Activated fatty acids stimulate budding and fusion in several cell-free assays for vesicular transport. This stimulation is thought to be due to protein palmitoylation, but relevant substrates have not yet been identified. We now report that Vac8p, a protein known to be required for vacuole inheritance, becomes palmitoylated when isolated yeast vacuoles are incubated under conditions that allow membrane fusion. Similar requirements for Vac8p palmitoylation and vacuole fusion, the inhibition of vacuole fusion by antibodies to Vac8p and the strongly reduced fusion of vacuoles lacking Vac8p suggest that palmitoylated Vac8p is essential for homotypic vacuole fusion. Strikingly, palmitoylation of Vac8p is blocked by the addition of antibodies to Sec18p (yeast NSF) only. Consistent with this, a portion of Vac8p is associated with the SNARE complex on vacuoles, which is lost during Sec18p- and ATP-dependent priming. During or after SNARE complex disassembly, palmitoylation occurs and anchors Vac8p to the vacuolar membrane. We propose that palmitoylation of Vac8p is regulated by the same machinery that controls membrane fusion.

Homotypic fusion of immature secretory granules during maturation requires syntaxin 6

Homotypic fusion of immature secretory granules (ISGs) gives rise to mature secretory granules (MSGs), the storage compartment in endocrine and neuroendocrine cells for hormones and neuropeptides. With the use of a cell-free fusion assay, we investigated which soluble N-ethylmaleimide-sensitive fusion protein attachment receptor (SNARE) molecules are involved in the homotypic fusion of ISGs. Interestingly, the SNARE molecules mediating the exocytosis of MSGs in neuroendocrine cells, syntaxin 1, SNAP-25, and VAMP2, were not involved in homotypic ISG fusion. Instead, we have identified syntaxin 6 as a component of the core machinery responsible for homotypic ISG fusion. Subcellular fractionation studies and indirect immunofluorescence microscopy show that syntaxin 6 is sorted away during the maturation of ISGs to MSGs. Although, syntaxin 6 on ISG membranes is associated with SNAP-25 and SNAP-29/GS32, we could not find evidence that these target (t)-SNARE molecules are involved in homotypic ISG fusion. Nor could we find any involvement for the vesicle (v)-SNARE VAMP4, which is known to be associated with syntaxin 6. Importantly, we have shown that homotypic fusion requires the function of syntaxin 6 on both donor as well as acceptor membranes, which suggests that t-t-SNARE interactions, either direct or indirect, may be required during fusion of ISG membranes.

Sequential involvement of p115, SNAREs, and Rab proteins in intra-Golgi protein transport

Delivery of transport vesicles to their receptor compartment involves tethering, priming, and fusion. Soluble NSF attachment protein-alpha (alphaSNAP) mediates the disruption of SNAREs by N-ethylmaleimide sensitive factor (NSF) and was employed to determine the hierarchy of proteins responsible for intra-Golgi protein transport. The N-terminal 23 amino acids of alphaSNAP are necessary for SNARE binding. The antibody 2F10 recognizes this SNARE interaction domain of alphaSNAP and inhibits intra-Golgi protein transport reversibly. This antibody was applied to modify the transport assay to determine the protein requirements relative to the action of alphaSNAP and NSF. We found that 1) p115 acts independently of alphaSNAP and NSF, 2) SNAREs are required after tethering and interact selectively after activation by alphaSNAP and NSF, and 3) Rab proteins act after SNARE activation and before fusion.

Molecular mechanisms of neurotransmitter release

The release of neurotransmitter from neurons represents one of the pivotal events in synaptic transmission. Neurotransmitters are released from synaptic vesicles in presynaptic neurons in response to neural activity, diffuse across the synaptic cleft, and bind specific receptors in order to bring about changes in postsynaptic neurons. Some of the molecular processes that govern neurotransmitter release are now becoming better understood. The steps involved can be broken down into two partially overlapping presynaptic cycles, the neurotransmitter cycle and the synaptic vesicle cycle. The neurotransmitter cycle involves transmitter biosynthesis, storage, reuptake, and degradation. The synaptic vesicle cycle involves targeting to the nerve terminal, docking, fusion, endocytosis, and recycling. Biochemical and structural studies have yielded important insight into our understanding of each of these two cycles. Further, both pharmacological and genetic interference with either of these cycles results in profound alterations in synaptic transmission and behavior, demonstrating the crucial role of neurotransmitter release.

Gaf-1, a gamma -SNAP-binding protein associated with the mitochondria

The role of alpha/beta-SNAP (Soluble NSF Attachment Protein) in vesicular trafficking is well established; however, the function of the ubiquitously expressed gamma-SNAP remains unclear. To further characterize the cellular role of this enigmatic protein, a two-hybrid screen was used to identify new, gamma-SNAP-binding proteins and to uncover potentially novel functions for gamma-SNAP. One such SNAP-binding protein, termed Gaf-1 (gamma-SNAP associate factor-1) specifically binds gamma- but not alpha-SNAP. The full-length Gaf-1 (75 kDa) is ubiquitously expressed and is found stoichiometrically associated with gamma-SNAP in cellular extracts. This binding is distinct from other SNAP interactions since no alpha-SNAP or NSF coprecipitated with Gaf-1. Subcellular fractionation and immunofluorescence analysis show that Gaf-1 is peripherally associated with the outer mitochondrial membrane. Only a fraction of gamma-SNAP was mitochondrial with the balance being either cytosolic or associated with other membrane fractions. GFP-gamma-SNAP and the C-terminal domain of Gaf-1 both show a reticular distribution in HEK-293 cells. This reticular structure colocalizes with Gaf-1 and mitochondria as well as with microtubules but not with other cytoskeletal elements. These data identify a class of gamma-SNAP interactions that is distinct from other members of the SNAP family and point to a potential role for gamma-SNAP in mitochondrial dynamics.

Activation of annexin 7 by protein kinase C in vitro and in vivo

Annexin 7, a Ca(2+)/GTP-activated membrane fusion protein, is preferentially phosphorylated in intact chromaffin cells, and the levels of annexin 7 phosphorylation increase quantitatively in proportion to the extent of catecholamine secretion. Consistently, various protein kinase C inhibitors proportionately reduce both secretion and phosphorylation of annexin 7 in these cells. In vitro, annexin 7 is quantitatively phosphorylated by protein kinase C to a mole ratio of 2.0, and phosphorylation is extraordinarily sensitive to variables such as pH, calcium, phospholipid, phorbol ester, and annexin 7 concentration. Phosphorylation of annexin 7 by protein kinase C significantly potentiates the ability of the protein to fuse phospholipid vesicles and lowers the half-maximal concentration of calcium needed for this fusion process. Furthermore, other protein kinases, including cAMP-dependent protein kinase, cGMP-dependent protein kinase, and protein-tyrosine kinase pp60(c-)(src), also label annexin 7 with high efficiency but do not have this effect on membrane fusion. In the case of pp60(c-)(src), we note that this kinase, if anything, modestly suppresses the membrane fusion activity of annexin 7. These results thus lead us to hypothesize that annexin 7 may be a positive mediator for protein kinase C action in the exocytotic membrane fusion reaction in chromaffin cells.

Structural insights into the molecular mechanism of calcium-dependent vesicle-membrane fusion

The fusion of vesicles with target membranes is controlled by a complex network of protein-protein and protein-lipid interactions. Recently determined structures of the SNARE complex, synaptotagmin III, nSec1, domains of the NSF chaperone and its adaptor (SNAP), and Rab3 and some of its effectors provide the framework for developing molecular models of vesicle fusion and for designing experiments to test these models. Ultimately, knowledge of the structures of higher-order complexes and their dynamic behavior will be required to obtain a full understanding of the vesicle fusion protein machinery.

Mechanisms of synaptic vesicle exocytosis

Chemical synaptic transmission serves as the main form of cell to cell communication in the nervous system. Neurotransmitter release occurs through the process of regulated exocytosis, in which a synaptic vesicle releases its contents in response to an increase in calcium. The use of genetic, biochemical, structural, and functional studies has led to the identification of factors important in the synaptic vesicle life cycle. Here we focus on the prominent role of SNARE (soluble NSF attachment protein receptor) proteins during membrane fusion and the regulation of SNARE function by Rab3a, nSec1, and NSF. Many of the proteins important for transmitter release have homologs involved in intracellular vesicle transport, and all forms of vesicle trafficking share common basic principles. Finally, modifications to the synaptic exocytosis pathway are very likely to underlie certain forms of synaptic plasticity and therefore contribute to learning and memory.

Yeast homotypic vacuole fusion: a window on organelle trafficking mechanisms

Homotypic (self) fusion of yeast vacuoles, which is essential for the low copy number of this organelle, uses catalytic elements similar to those used in heterotypic vesicular trafficking reactions between different organelles throughout nature. The study of vacuole inheritance has benefited from the ease of vacuole isolation, the availability of the yeast genome sequence and numerous mutants, and from a rapid, quantitative in vitro assay of fusion. The soluble proteins and small molecules that support fusion are being defined, conserved membrane proteins that catalyze the reaction have been identified, and the vacuole membrane has been solubilized and reconstituted into fusion-competent proteoliposomes, allowing the eventual purification of all needed factors. Studies of homotypic vacuole fusion have suggested a modified paradigm of membrane fusion in which integral membrane proteins termed "SNAREs" can form stable complexes in cis (when on the same membrane) as well as in trans (when anchored to opposing membranes). Chaperones (NSF/Sec18p, LMA1, and -SNAP/Sec17p) disassemble cis-SNARE complexes to prepare for the docking of organelles rather than to drive fusion. The specificity of organelle docking resides in a cascade of trans-interactions (involving Rab-like GTPases), "tethering factors," and trans-SNARE pairing. Fusion itself, the mixing of the membrane bilayers and the organelle contents, is triggered by calcium signaling.

SNARE complex oligomerization by synaphin/complexin is essential for synaptic vesicle exocytosis

Synaphin/complexin is a cytosolic protein that preferentially binds to syntaxin within the SNARE complex. We find that synaphin promotes SNAREs to form precomplexes that oligomerize into higher order structures. A peptide from the central, syntaxin binding domain of synaphin competitively inhibits these two proteins from interacting and prevents SNARE complexes from oligomerizing. Injection of this peptide into squid giant presynaptic terminals inhibited neurotransmitter release at a late prefusion step of synaptic vesicle exocytosis. We propose that oligomerization of SNARE complexes into a higher order structure creates a SNARE scaffold for efficient, regulated fusion of synaptic vesicles.

SNARE-mediated membrane fusion

SNARE proteins have been proposed to mediate all intracellular membrane fusion events. There are over 30 SNARE family members in mammalian cells and each is found in a distinct subcellular compartment. It is likely that SNAREs encode aspects of membrane transport specificity but the mechanism by which this specificity is achieved remains controversial. Functional studies have provided exciting insights into how SNARE proteins interact with each other to generate the driving force needed to fuse lipid bilayers.

Trans-complex formation by proteolipid channels in the terminal phase of membrane fusion

SNAREs (soluble N-ethylmaleimide-sensitive factor attachment protein receptors) and Rab-GTPases, together with their cofactors, mediate the attachment step in the membrane fusion of vesicles. But how bilayer mixing--the subsequent core process of fusion--is catalysed remains unclear. Ca2+/calmodulin controls this terminal process in many intracellular fusion events. Here we identify V0, the membrane-integral sector of the vacuolar H+-ATPase, as a target of calmodulin on yeast vacuoles. Between docking and bilayer fusion, V0 sectors from opposing membranes form complexes. V0 trans-complex formation occurs downstream from trans-SNARE pairing, and depends on both the Rab-GTPase Ypt7 and calmodulin. The maintenance of existing complexes and completion of fusion are independent of trans-SNARE pairs. Reconstituted proteolipids form sealed channels, which can expand to form aqueous pores in a Ca2+/calmodulin-dependent fashion. V0 trans-complexes may therefore form a continuous, proteolipid-lined channel at the fusion site. We propose that radial expansion of such a protein pore may be a mechanism for intracellular membrane fusion.

Small GTP-binding proteins

Small GTP-binding proteins (G proteins) exist in eukaryotes from yeast to human and constitute a superfamily consisting of more than 100 members. This superfamily is structurally classified into at least five families: the Ras, Rho, Rab, Sar1/Arf, and Ran families. They regulate a wide variety of cell functions as biological timers (biotimers) that initiate and terminate specific cell functions and determine the periods of time for the continuation of the specific cell functions. They furthermore play key roles in not only temporal but also spatial determination of specific cell functions. The Ras family regulates gene expression, the Rho family regulates cytoskeletal reorganization and gene expression, the Rab and Sar1/Arf families regulate vesicle trafficking, and the Ran family regulates nucleocytoplasmic transport and microtubule organization. Many upstream regulators and downstream effectors of small G proteins have been isolated, and their modes of activation and action have gradually been elucidated. Cascades and cross-talks of small G proteins have also been clarified. In this review, functions of small G proteins and their modes of activation and action are described.

N-ethylmaleimide sensitive factor (NSF) structure and function

Our understanding of the molecular mechanisms of membrane trafficking advanced at a rapid rate during the 1990s. As one of the initial protein components of the trafficking machinery to be identified, N-ethylmaleimide sensitive factor (NSF) has served as a reference point in many of these recent studies. This hexameric ATPase is essential for most of the membrane-trafficking events in a cell. Initially, due to its ATPase activity, NSF was thought to be the motor that drove membrane fusion. Subsequent studies have shown that NSF actually plays the role of a chaperone by activating SNAP receptor proteins (SNAREs) so that they can participate in membrane fusion. In this review we will examine the initial characterization of NSF, its role in membrane fusion events, and what new structural information can tell us about NSF's mechanism of action.

Sequential action of two GTPases to promote vacuole docking and fusion

Homotypic vacuole fusion occurs by sequential priming, docking and fusion reactions. Priming frees the HOPS complex (Vps 11, 16, 18, 33, 39 and 41) to activate Ypt7p for docking. Here we explore the roles of the GDP and GTP states of Ypt7p using Gdi1p (which extracts Ypt7:GDP), Gyp7p (a GTPase-activating protein for Ypt7p:GTP), GTPgammaS or GppNHp (non-hydrolyzable nucleotides), and mutant forms of Ypt7p that favor either GTP or GDP states. GDP-bound Ypt7p on isolated vacuoles can be extracted by Gdi1p, although only the GTP-bound state allows docking. Ypt7p is converted to the GTP-bound state after priming and stably associates with HOPS. Gyp7p can cause Ypt7p to hydrolyze bound GTP to GDP, driving HOPS release and accelerating Gdi1p-mediated release of Ypt7p. Ypt7p extraction does not inhibit the Ca(2+)-triggered cascade that leads to fusion. However, in the absence of Ypt7p, fusion is still sensitive to GTPgammaS and GppNHp, indicating that there is a second specific GTPase that regulates the calcium flux and hence fusion. Thus, two GTPases sequentially govern vacuole docking and fusion.

Functional morphology of the secretory pathway organelles in yeast

The glycoprotein secretory pathway of yeast serves mainly for cell surface growth and cell division. It involves a centrifugal transport of transit macromolecules among organelles, whose membranes contain resident proteins needed for driving the transport. These resident membrane proteins return by retrograde vesicular transport. Apart from this, the pathway involves endocytosis. The model yeast Saccharomyces cerevisiae and vertebrate cells were found to contain very similar gene products regulating the molecular mechanism of glycoprotein transport, and the cellular mechanism of their secretion pathways was therefore also presumed to be identical. Biochemists have postulated that, in S. cerevisiae, the translocation of peptides through the endoplasmic reticulum membranes into the lumen of ER cisternae and the core glycosylation is followed by a vector-mediated transport into the functional cascade of the Golgi system cisternae and between them. This is the site of maturation and sorting of glycoproteins, before the ultimate transport by other vectors involving either secretion from the cells (exocytosis across the plasmalemma into the cell wall) or transport into the lysosome-like vacuole via a prevacuolar compartment, which serves at the same time as a primary endosome. The established cellular model of secretion deals with budding yeast; interphase yeast cells, in which the secretion is limited and which predominate in exponential cultures, have not been taken into consideration. The quality of organelle imaging in S. cerevisiae ultra-thin sections depends on the fixation technique used and on specimen contrasting by metals. The results achieved by combinations of different techniques differ mostly in the imaging of bilayers of membrane interfaces and the transparence of the matrix phase. Fixation procedures are decisive for the results of topochemical localisations of cellular antigenic components or enzyme activities, which form the basis of the following survey of functional morphology of organelles involved in the yeast secretory pathway. The existing results of these studies do not confirm all aspects of the vertebrate model of the Golgi apparatus proposed by molecular geneticists to hold for S. cerevisiae, and alternative models of the cellular mechanism of secretion in this yeast are, therefore, also discussed.

Exocytosis requires asymmetry in the central layer of the SNARE complex

Assembly of SNAREs (soluble N:-ethylmaleimide- sensitive factor attachment protein receptors) mediates membrane fusions in all eukaryotic cells. The synaptic SNARE complex is represented by a twisted bundle of four alpha-helices. Leucine zipper-like layers extend through the length of the complex except for an asymmetric and ionic middle layer formed by three glutamines (Q) and one arginine (R). We have examined the functional consequences of Q-R exchanges in the conserved middle layer using the exocytotic SNAREs of yeast as a model. Exchanging Q for R in Sso2p drastically reduces cell growth and protein secretion. When a 3Q/1R ratio is restored by a mirror R-->Q substitution in the R-SNARE Snc2p, wild-type functionality is observed. Secretion is near normal when all four helices contain Q, but defects become apparent when additional mutations are present in other layers. Using molecular dynamics free energy perturbation simulations, these findings are rationalized in structural and energetic terms. We conclude that the asymmetric arrangement of the polar amino acids in the central layer is essential for normal function of SNAREs in membrane fusion.

New component of the vacuolar class C-Vps complex couples nucleotide exchange on the Ypt7 GTPase to SNARE-dependent docking and fusion

The class C subset of vacuolar protein sorting (Vps) proteins (Vps11, Vps18, Vps16 and Vps33) assembles into a vacuole/prevacuole-associated complex. Here we demonstrate that the class C-Vps complex contains two additional proteins, Vps39 and Vps41. The COOH-terminal 148 amino acids of Vps39 direct its association with the class C-Vps complex by binding to Vps11. A previous study has shown that a large protein complex containing Vps39 and Vps41 functions as a downstream effector of the active, GTP-bound form of Ypt7, a rab GTPase required for the fusion of vesicular intermediates with the vacuole (Price, A., D. Seals, W. Wickner, and C. Ungermann. 2000. J. Cell Biol. 148:1231-1238). Here we present data that indicate that this complex also functions to stimulate nucleotide exchange on Ypt7. We show that Vps39 directly binds the GDP-bound and nucleotide-free forms of Ypt7 and that purified Vps39 stimulates nucleotide exchange on Ypt7. We propose that the class C-Vps complex both promotes Vps39-dependent nucleotide exchange on Ypt7 and, based on the work of Price et al., acts as a Ypt7 effector that tethers transport vesicles to the vacuole. Thus, the class C-Vps complex directs multiple reactions during the docking and fusion of vesicles with the vacuole, each of which contributes to the overall specificity and efficiency of this transport process.

Geranylgeranylated SNAREs are dominant inhibitors of membrane fusion

Exocytosis in yeast requires the assembly of the secretory vesicle soluble N-ethylmaleimide-sensitive factor attachment protein receptor (v-SNARE) Sncp and the plasma membrane t-SNAREs Ssop and Sec9p into a SNARE complex. High-level expression of mutant Snc1 or Sso2 proteins that have a COOH-terminal geranylgeranylation signal instead of a transmembrane domain inhibits exocytosis at a stage after vesicle docking. The mutant SNARE proteins are membrane associated, correctly targeted, assemble into SNARE complexes, and do not interfere with the incorporation of wild-type SNARE proteins into complexes. Mutant SNARE complexes recruit GFP-Sec1p to sites of exocytosis and can be disassembled by the Sec18p ATPase. Heterotrimeric SNARE complexes assembled from both wild-type and mutant SNAREs are present in heterogeneous higher-order complexes containing Sec1p that sediment at greater than 20S. Based on a structural analogy between geranylgeranylated SNAREs and the GPI-HA mutant influenza virus fusion protein, we propose that the mutant SNAREs are fusion proteins unable to catalyze fusion of the distal leaflets of the secretory vesicle and plasma membrane. In support of this model, the inverted cone-shaped lipid lysophosphatidylcholine rescues secretion from SNARE mutant cells.

Ordering the final events in yeast exocytosis

In yeast, assembly of exocytic soluble N-ethylmaleimide-sensitive fusion protein (NSF) attachment protein receptor (SNARE) complexes between the secretory vesicle SNARE Sncp and the plasma membrane SNAREs Ssop and Sec9p occurs at a late stage of the exocytic reaction. Mutations that block either secretory vesicle delivery or tethering prevent SNARE complex assembly and the localization of Sec1p, a SNARE complex binding protein, to sites of secretion. By contrast, wild-type levels of SNARE complexes persist in the sec1-1 mutant after a secretory block is imposed, suggesting a role for Sec1p after SNARE complex assembly. In the sec18-1 mutant, cis-SNARE complexes containing surface-accessible Sncp accumulate in the plasma membrane. Thus, one function of Sec18p is to disassemble SNARE complexes on the postfusion membrane.

SNAP-25 functional domains in SNARE core complex assembly and glutamate release of cerebellar granule cells

Synaptosomal associated protein of 25 kDa (SNAP-25) is a member of the SNARE protein complex that has been implicated in synaptic vesicle docking and fusion. In this report, we have generated SNAP-25 mutants and assayed their functions in SNARE complex formation and glutamate release from cultured rat cerebellar granule cells. In vitro binding studies show that a deletion mutant lacking the C-terminal 181-206 amino acid sequence inhibits the formation of the SNARE core complex. Additional deletion of an N-terminal 1-31 amino acid sequence abolished this inhibitory activity. Adenovirus-mediated gene transfer is used to overexpress wild-type and mutant SNAP-25 in cerebellar granule cells. Neurons overexpressing the wild-type protein show slight reductions in glutamate release, ranging from 10 to 15% in both the developing and mature granule cells. A 30-35% inhibition is obtained with the C-terminal deletion mutant, and the inhibitory effect is abolished in the N- and C-terminal double deletion mutant. These results demonstrate that the SNARE core complex exists in a dynamic and reversible state, and the formation of the core complex is necessary for neurotransmitter release in neurons.

Functional architecture of an intracellular membrane t-SNARE

Lipid bilayer fusion is mediated by SNAREs (soluble N-ethylmaleimide-sensitive factor attachment protein receptors) located on the vesicle membrane (v-SNAREs) and the target membrane (t-SNAREs). The assembled v-SNARE/t-SNARE complex consists of a bundle of four helices, of which one is supplied by the v-SNARE and the other three by the t-SNARE. For t-SNAREs on the plasma membrane, the protein syntaxin supplies one helix and a SNAP-25 protein contributes the other two. Although there are numerous homologues of syntaxin on intracellular membranes, there are only two SNAP-25-related proteins in yeast, Sec9 and Spo20, both of which are localized to the plasma membrane and function in secretion and sporulation, respectively. What replaces SNAP-25 in t-SNAREs of intracellular membranes? Here we show that an intracellular t-SNARE is built from a 'heavy chain' homologous to syntaxin and two separate non-syntaxin 'light chains'. SNAP-25 may thus be the exception rather than the rule, having been derived from genes that encoded separate light chains that fused during evolution to produce a single gene encoding one protein with two helices.

Trypanosoma cruzi: cloning and characterization of a RAB7 gene

The small monomeric GTP-binding proteins of the RAB subfamily are key regulatory elements of the machinery that controls membrane traffic in eukaryotic cells. These proteins have been localized to many different intracellular organelles, on both endocytic and exocytic compartments, suggesting that each step of vesicular traffic can involve a specific RAB protein. The presence of conserved amino acid domains in these proteins has allowed the cloning of their genes from several organisms, including yeast, plants, humans, and parasites. In this work we describe the identification, cloning, and characterization of a RAB7 gene homologue in Trypanosoma cruzi (TcRAB7). Our data indicate that this gene is present as a single copy in the T. cruzi genome, located on a 2.25-Mb chromosomal DNA. TcRAB7 is expressed in T. cruzi epimastigotes, metacyclic trypomastigotes, and spheromastigotes. We established transformed cell lines that express two versions of an epitope-tagged TcRAB7 protein: one wild type (pTAG) and one deleted at the C-terminal cysteines (pDeltaCXC). Wild-type TcRAB7 protein (pTAG) appears to be localized exclusively in the membrane fraction, while the mutated TcRAB7 protein (pDeltaCXC) loses the ability to associate with the membrane, showing only cytosolic localization. Also, we produced the recombinant TcRAB7 protein and demonstrated that it binds GTP. The identification of exo- and endocytic machinery components in T. cruzi and their function would provide specific markers of these subcellular compartments, thereby unveiling important aspects of vesicular traffic in this parasite.

Peroxisomal membrane fusion requires two AAA family ATPases, Pex1p and Pex6p

Two AAA family ATPases, NSF and p97, have been implicated in membrane fusion during assembly and inheritance of organelles of the secretory pathway. We have now investigated the roles of AAA ATPases in membrane fusion during assembly of the peroxisome, an organelle outside the classical secretory system. Here, we show that peroxisomal membrane fusion in the yeast Yarrowia lipolytica requires two AAA ATPases, Pex1p and Pex6p. Release of membrane- associated Pex1p and Pex6p drives the asymmetric priming of two fusion partners. The next step, peroxisome docking, requires release of Pex1p from one partner. Subsequent fusion of the peroxisomal membranes is independent of both Pex1p and Pex6p.

The mechanisms of aquaporin control in the renal collecting duct

The antidiuretic hormone arginine-vasopressin (AVP) regulates water reabsorption in renal collecting duct principal cells. Central to its antidiuretic action in mammals is the exocytotic insertion of the water channel aquaporin-2 (AQP2) from intracellular vesicles into the apical membrane of principal cells, an event initiated by an increase in cAMP and activation of protein kinase A. Water is then reabsorbed from the hypotonic urine of the collecting duct. The water channels aquaporin-3 (AQP3) and aquaporin-4 (AQP4), which are constitutively present in the basolateral membrane, allow the exit of water from the cell into the hypertonic interstitium. Withdrawal of the hormone leads to endocytotic retrieval of AQP2 from the cell membrane. The hormone-induced rapid redistribution between the interior of the cell and the cell membrane establishes the basis for the short term regulation of water permeability. In addition water channels (AQP2 and 3) of principal cells are regulated at the level of expression (long term regulation). This review summarizes the current knowledge on the molecular mechanisms underlying the short and long term regulation of water channels in principal cells. In the first part special emphasis is placed on the proteins involved in short term regulation of AQP2 (SNARE proteins, Rab proteins, cytoskeletal proteins, G proteins, protein kinase A anchoring proteins and endocytotic proteins). In the second part, physiological and pathophysiological stimuli determining the long term regulation are discussed.

Protein kinase C phosphorylation of syntaxin 4 in thrombin-activated human platelets

We postulated that the syntaxins, because of their key role in SNARE complex formation and exocytosis, could be important targets for signaling by intracellular kinases involved in secretion. We found that syntaxin 4 was phosphorylated in human platelets treated with a physiologic agent that induces secretion (thrombin) but not when they were treated with an agent that prevents secretion (prostacyclin). Syntaxin 4 phosphorylation was blocked by inhibitors of activated protein kinase C (PKC), and, in parallel assays, PKC inhibitors also blocked secretion from thrombin-activated platelets. In platelets, cellular activation by thrombin or phorbol 12-myristate 13-acetate decreased the binding of syntaxin 4 with SNAP-23, another platelet t-SNARE. Phosphatase inhibitors increased syntaxin 4 phosphorylation and further decreased syntaxin 4-SNAP-23 binding induced by cell activation. Conversely, a PKC inhibitor blocked syntaxin 4 phosphorylation and returned binding of syntaxin 4-SNAP-23 to that seen in nonstimulated platelets. In vitro, PKC directly phosphorylated platelet syntaxin 4 and recombinant syntaxin 4. PKC phosphorylation in vitro inhibited (71 +/- 8%) the binding of syntaxin 4 to SNAP-23. These results provide evidence that extracellular activation can be coupled through intracellular PKC signaling so as to modulate SNARE protein interactions involved in platelet exocytosis.

Ca2+-dependent regulation of synaptic SNARE complex assembly via a calmodulin- and phospholipid-binding domain of synaptobrevin

Synaptic core complex formation is an essential step in exocytosis, and assembly into a superhelical structure may drive synaptic vesicle fusion. To ascertain how Ca(2+) could regulate this process, we examined calmodulin binding to recombinant core complex components. Surface plasmon resonance and pull-down assays revealed Ca(2+)-dependent calmodulin binding (K(d) = 500 nM) to glutathione S-transferase fusion proteins containing synaptobrevin (VAMP 2) domains but not to syntaxin 1 or synaptosomal-associated protein of 25 kDa (SNAP-25). Deletion mutations, tetanus toxin cleavage, and peptide synthesis localized the calmodulin-binding domain to VAMP(77-94), immediately C-terminal to the tetanus toxin cleavage site (Q(76)-F(77)). In isolated synaptic vesicles, Ca(2+)/calmodulin protected native membrane-inserted VAMP from proteolysis by tetanus toxin. Assembly of a (35)S-SNAP-25, syntaxin 1 GST-VAMP(1-96) complex was inhibited by Ca(2+)/calmodulin, but assembly did not mask subsequent accessibility of the calmodulin-binding domain. The same domain contains a predicted phospholipid interaction site. SPR revealed calcium-independent interactions between VAMP(77-94) and liposomes containing phosphatidylserine, which blocked calmodulin binding. Circular dichroism spectroscopy demonstrated that the calmodulin/phospholipid-binding peptide displayed a significant increase in alphahelical content in a hydrophobic environment. These data provide insight into the mechanisms by which Ca(2+) may regulate synaptic core complex assembly and protein interactions with membrane bilayers during exocytosis.

A Ypt/Rab effector complex containing the Sec1 homolog Vps33p is required for homotypic vacuole fusion

Yeast vacuoles undergo priming, docking, and homotypic fusion, although little has been known of the connections between these reactions. Vacuole-associated Vam2p and Vam6p (Vam2/6p) are components of a 65S complex containing SNARE proteins. Upon priming by Sec18p/NSF and ATP, Vam2/6p is released as a 38S subcomplex that binds Ypt7p to initiate docking. We now report that the 38S complex consists of both Vam2/6p and the class C Vps proteins [Reider, S. E. and Emr, S. D. (1997) Mol. Biol. Cell 8, 2307-2327]. This complex includes Vps33p, a member of the Sec1 family of proteins that bind t-SNAREs. We term this 38S complex HOPS, for homotypic fusion and vacuole protein sorting. This unexpected finding explains how Vam2/6p associates with SNAREs and provides a mechanistic link of the class C Vps proteins to Ypt/Rab action. HOPS initially associates with vacuole SNAREs in "cis" and, after release by priming, hops to Ypt7p, activating this Ypt/Rab switch to initiate docking.

Membrane tethering and fusion in the secretory and endocytic pathways

Studies of intracellular trafficking over the past decade or so have led to striking advances in our understanding of the molecular processes by which transport intermediates dock and fuse. SNARE proteins play a central role, assembling into complexes that bridge membranes and may catalyze membrane fusion directly. In general, different SNARE proteins operate in different intracellular trafficking pathways, so recent reports that SNARE assembly in vitro is promiscuous have come as something of a surprise. We propose a model in which proper SNARE assembly is under kinetic control, orchestrated by members of the Sec1 protein family, small GTP-binding Rab proteins, and a diverse assortment of tethering proteins.

Divide and multiply: organelle partitioning in yeast

The mechanisms ensuring accurate partitioning of yeast vacuoles and mitochondria are distinct, yet they share common elements. Both organelles move along actin filaments, and both organelles require fusion and fission to maintain normal morphology. Recent studies have revealed that while vacuolar inheritance requires a processive myosin motor, mitochondrial inheritance requires controlled actin polymerization. Distinct sets of proteins required for the fusion and fission of each organelle have also been identified.

Role of p97 and syntaxin 5 in the assembly of transitional endoplasmic reticulum

Transitional endoplasmic reticulum (tER) consists of confluent rough and smooth endoplasmic reticulum (ER) domains. In a cell-free incubation system, low-density microsomes (1.17 g cc(-1)) isolated from rat liver homogenates reconstitute tER by Mg(2+)GTP- and Mg(2+)ATP-hydrolysis-dependent membrane fusion. The ATPases associated with different cellular activities protein p97 has been identified as the relevant ATPase. The ATP depletion by hexokinase or treatment with either N-ethylmaleimide or anti-p97 prevented assembly of the smooth ER domain of tER. High-salt washing of low-density microsomes inhibited assembly of the smooth ER domain of tER, whereas the readdition of purified p97 with associated p47 promoted reconstitution. The t-SNARE syntaxin 5 was observed within the smooth ER domain of tER, and antisyntaxin 5 abrogated formation of this same membrane compartment. Thus, p97 and syntaxin 5 regulate assembly of the smooth ER domain of tER and hence one of the earliest membrane differentiated components of the secretory pathway.

A new role for a SNARE protein as a regulator of the Ypt7/Rab-dependent stage of docking

The homotypic fusion of yeast vacuoles occurs in an ordered cascade of priming, docking, and fusion. The linkage between these steps has so far remained unclear. We now report that Vam7p (the vacuolar SNAP-23/25 homolog) signals from the cis-SNARE complex to Ypt7p (the vacuolar Rab/Ypt) to initiate the docking process. After Vam7p has been released from the cis-SNARE complex by Sec18p-mediated priming, it is still required for Ypt7p-dependent docking and it needs Ypt7p to remain on the vacuole. Thus, after priming, Vam7p is released from the vacuole altogether if Ypt7p has been extracted by Gdi1p or inactivated by antibody but is not released if docking is blocked simply by vacuole dilution; it is therefore Ypt7p function, and not docking per se, that retains Vam7p. In accord with this finding, cells deleted for the gene encoding Ypt7 have normal amounts of Vam7p but have little Vam7p on their isolated vacuoles. Interaction of Vam7p and Ypt7p is further indicated by two-hybrid analysis [Uetz, P., Giot, L., Cagney, G., Mansfield, T. A., Judson, R. S., Knight, J. R., Lockshon, D., Narayan, V., Srinivasan, M., Pochart, P., et al. (2000) Nature (London) 403, 623-627] and by the effect of Vam7p on the association of the Rab/Ypt-effector HOPS complex (homotypic fusion and vacuole protein sorting; Vam2p and Vam6p plus four vacuole protein sorting class C proteins) with Ypt7p. Vam7p provides a functional link between the priming step, which releases it from the cis-SNARE complex, and docking.

The docking of primed vacuoles can be reversibly arrested by excess Sec17p (alpha-SNAP)

Homotypic vacuole fusion occurs in ordered stages of priming, docking, and fusion. Priming, which prepares vacuoles for productive association, requires Sec17p (the yeast homolog of alpha-SNAP), Sec18p (the yeast NSF, an ATP-driven chaperone), and ATP. Sec17p is initially an integral part of the cis-SNARE complex together with vacuolar SNARE proteins and Sec18p (NSF). Previous studies have shown that Sec17p is rapidly released from the vacuole membrane during priming as the cis-SNARE complex is disassembled, but the order and causal relationship of these subreactions has not been known. We now report that the addition of excess recombinant his(6)-Sec17p to primed vacuoles can block subsequent docking. This inhibition is reversible by Sec18p, but the reaction cannot proceed to the tethering and trans-SNARE pairing steps of docking while the Sec17p block is in place. Once docking has occurred, excess Sec17p does not inhibit membrane fusion per se. Incubation of cells with thermosensitive Sec17-1p at nonpermissive temperature causes SNARE complex disassembly. These data suggest that Sec17p can stabilize vacuolar cis-SNARE complexes and that the release of Sec17p by Sec18p and ATP allows disassembly of this complex and activates its components for docking.

Cell biology. ER-to-Golgi traffic--this bud's for you

How do protein-transporting vesicles, which bud from the endoplasmic reticulum (ER), specifically dock to, and fuse with, the Golgi apparatus? In their Perspective, Brittle and Waters discuss new work (Allan et al.) suggesting that some vesicle-associated docking and fusion proteins are "programmed" during vesicle budding from the ER and direct downstream events that occur during fusion of these transport vesicles with the membranes of the Golgi.

Membrane fusion and exocytosis

Membrane fusion involves the merger of two phospholipid bilayers in an aqueous environment. In artificial lipid bilayers, fusion proceeds by means of defined transition states, including hourglass-shaped intermediates in which the proximal leaflets of the fusing membranes are merged whereas the distal leaflets are separate (fusion stalk), followed by the reversible opening of small aqueous fusion pores. Fusion of biological membranes requires the action of specific fusion proteins. Best understood are the viral fusion proteins that are responsible for merging the viral with the host cell membrane during infection. These proteins undergo spontaneous and dramatic conformational changes upon activation. In the case of the paradigmatic fusion proteins of the influenza virus and of the human immunodeficiency virus, an amphiphilic fusion peptide is inserted into the target membrane. The protein then reorients itself, thus forcing the fusing membranes together and inducing lipid mixing. Fusion of intracellular membranes in eukaryotic cells involves several protein families including SNAREs, Rab proteins, and Sec1/Munc-18 related proteins (SM-proteins). SNAREs form a novel superfamily of small and mostly membrane-anchored proteins that share a common motif of about 60 amino acids (SNARE motif). SNAREs reversibly assemble into tightly packed helical bundles, the core complexes. Assembly is thought to pull the fusing membranes closely together, thus inducing fusion. SM-proteins comprise a family of soluble proteins that bind to certain types of SNAREs and prevent the formation of core complexes. Rab proteins are GTPases that undergo highly regulated GTP-GDP cycles. In their GTP form, they interact with specific proteins, the effector proteins. Recent evidence suggests that Rab proteins function in the initial membrane contact connecting the fusing membranes but are not involved in the fusion reaction itself.

Close is not enough: SNARE-dependent membrane fusion requires an active mechanism that transduces force to membrane anchors

Is membrane fusion an essentially passive or an active process? It could be that fusion proteins simply need to pin two bilayers together long enough, and the bilayers could do the rest spontaneously. Or, it could be that the fusion proteins play an active role after pinning two bilayers, exerting force in the bilayer in one or another way to direct the fusion process. To distinguish these alternatives, we replaced one or both of the peptidic membrane anchors of exocytic vesicle (v)- and target membrane (t)-SNAREs (soluble N-ethylmaleimide-sensitive fusion protein [NSF] attachment protein [SNAP] receptor) with covalently attached lipids. Replacing either anchor with a phospholipid prevented fusion of liposomes by the isolated SNAREs, but still allowed assembly of trans-SNARE complexes docking vesicles. This result implies an active mechanism; if fusion occurred passively, simply holding the bilayers together long enough would have been sufficient. Studies using polyisoprenoid anchors ranging from 15-55 carbons and multiple phospholipid-containing anchors reveal distinct requirements for anchors of v- and t-SNAREs to function: v-SNAREs require anchors capable of spanning both leaflets, whereas t-SNAREs do not, so long as the anchor is sufficiently hydrophobic. These data, together with previous results showing fusion is inhibited as the length of the linker connecting the helical bundle-containing rod of the SNARE complex to the anchors is increased (McNew, J.A., T. Weber, D.M. Engelman, T.H. Sollner, and J.E. Rothman, 1999. Mol. Cell. 4:415-421), suggests a model in which one activity of the SNARE complex promoting fusion is to exert force on the anchors by pulling on the linkers. This motion would lead to the simultaneous inward movement of lipids from both bilayers, and in the case of the v-SNARE, from both leaflets.

A conserved membrane-spanning amino acid motif drives homomeric and supports heteromeric assembly of presynaptic SNARE proteins

Assembly of the SNARE proteins synaptobrevin/VAMP, syntaxin, and SNAP-25 to binary and ternary complexes is important for docking and/or fusion of presynaptic vesicles to the neuronal plasma membrane prior to regulated neurotransmitter release. Despite the well characterized structure of their cytoplasmic assembly domains, little is known about the role of the transmembrane segments in SNARE protein assembly and function. Here, we identified conserved amino acid motifs within the transmembrane segments that are required for homodimerization of synaptobrevin II and syntaxin 1A. Minimal motifs of 6-8 residues grafted onto an otherwise monomeric oligoalanine host sequence were sufficient for self-interaction of both transmembrane segments in detergent solution or membranes. These motifs constitute contiguous areas of interfacial residues assuming alpha-helical secondary structures. Since the motifs are conserved, they also contributed to heterodimerization of synaptobrevin II and syntaxin 1A and therefore appear to constitute interaction domains independent of the cytoplasmic coiled coil regions. Interactions between the transmembrane segments may stabilize the SNARE complex, cause its multimerization to previously observed multimeric superstructures, and/or be required for the fusogenic activity of SNARE proteins.

Structural insights into the molecular mechanism of Ca(2+)-dependent exocytosis

The fusion of vesicles with target membranes is controlled by a complex network of protein-protein and protein-lipid interactions. Recent structures of the SNARE complex, synaptotagmin III, nSec1, domains of NSF and its adaptor SNAP, along with Rab3 and some of its effectors, provide the framework for developing molecular models of vesicle fusion and for designing experiments to test these models. Ultimately, this knowledge of the structures of higher-order complexes and their dynamic behavior will allow us to obtain a full understanding of the vesicle fusion protein machinery.

Patching plasma membrane disruptions with cytoplasmic membrane

Vesicle-vesicle fusion initiated in cell cytoplasm by high Ca(2+) can rapidly erect large membrane boundaries. These might be used as a ‘patch’ for resealing plasma membrane disruptions. Three central predictions of this ‘patch’ hypothesis are here established in sea urchin eggs. First, we show that surface markers for plasma membrane protein and lipid are initially absent over disruption sites after resealing is complete. Second, we demonstrate that resealing capacity is strongly dependent upon local availability of fusion competent cytoplasmic organelles, specifically the reserve or yolk granule. Lastly, we demonstrate that the reserve granule is capable of rapid (t(1/2) &lt;1 second), Ca(2+)-regulated (high threshold) fusion capable of erecting large (&gt;1000 μm(2)), continuous membrane boundaries. Production of patch vesicles for resealing may proceed by an ‘emergency’ fusion mechanism distinct from that utilized for the much slower, highly regulated, cytosol-requiring organelle-organelle fusion events typical of constitutive membrane trafficking pathways.

SNAREpins are functionally resistant to disruption by NSF and alphaSNAP

SNARE (SNAP [soluble NSF (N-ethylmaleimide-sensitive fusion protein) attachment protein] receptor) proteins are required for many fusion processes, and recent studies of isolated SNARE proteins reveal that they are inherently capable of fusing lipid bilayers. Cis-SNARE complexes (formed when vesicle SNAREs [v-SNAREs] and target membrane SNAREs [t-SNAREs] combine in the same membrane) are disrupted by the action of the abundant cytoplasmic ATPase NSF, which is necessary to maintain a supply of uncombined v- and t-SNAREs for fusion in cells. Fusion is mediated by these same SNARE proteins, forming trans-SNARE complexes between membranes. This raises an important question: why doesn't NSF disrupt these SNARE complexes as well, preventing fusion from occurring at all? Here, we report several lines of evidence that demonstrate that SNAREpins (trans-SNARE complexes) are in fact functionally resistant to NSF, and they become so at the moment they form and commit to fusion. This elegant design allows fusion to proceed locally in the face of an overall environment that massively favors SNARE disruption.

Disassembly of membrane-associated NSF 20S complexes is slow relative to vesicle fusion and is Ca(2+)-independent

N-ethylmaleimide-sensitive fusion protein (NSF) and its co-factor soluble NSF attachment protein (alpha)-SNAP) are essential components of the synaptic vesicle fusion machinery and form part of a structurally-conserved 20S protein complex. However, their precise function, relative to fusion itself, is not clear. Using a UV-activated cross-linking approach, we have measured the rate at which a single round of NSF-driven ATP hydrolysis leads to 20S complex disassembly within synaptic membranes. Although this rate is substantially faster than previous estimates of NSF-dependent ATP hydrolysis, it remains much lower than published rates for fusion of synaptic vesicles. Furthermore, the stability of 20S complexes is unaffected by Ca(2+) at concentrations that elicit rapid membrane fusion. We conclude that the ATPase activity of NSF does not contribute directly to vesicle fusion, but more likely plays an earlier role in the synaptic vesicle cycle.

A 56-kDa selenium-binding protein participates in intra-Golgi protein transport

Transport of proteins between intracellular membrane compartments is a highly regulated process that depends on several cytosolic factors. By using the well characterized intra-Golgi cell-free transport assay, we purified from bovine brain cytosol a 56-kDa protein that shows a significant transport activity. Partial sequencing of four tryptic peptides obtained from the 56-kDa protein revealed its identity to a cytosolic protein previously characterized as a selenium-binding protein, SBP56. Recombinant SBP56 expressed in Escherichia coli exhibited transport activity when added to the cell-free intra-Golgi transport. Affinity purified anti-SBP56 polyclonal antibodies specifically inhibited intra-Golgi transport in vitro. Although SBP56 is predominantly localized in the cytosol, a significant amount is associated with membranes. Subcellular fractionation showed that this protein is peripherally associated with the Golgi membrane. The experiments presented in this study indicate that SBP56 participates in late stages of intra-Golgi protein transport.