LMA1 binds to vacuoles at Sec18p (NSF), transfers upon ATP hydrolysis to a t-SNARE (Vam3p) complex, and is released during fusion

Phosphatidic acid induces conformational changes in Sec18 protomers that prevent SNARE priming

Eukaryotic cell homeostasis requires transfer of cellular components among organelles and relies on membrane fusion catalyzed by SNARE proteins. Inactive SNARE bundles are reactivated by hexameric N-ethylmaleimide-sensitive factor, vesicle-fusing ATPase (Sec18/NSF)-driven disassembly that enables a new round of membrane fusion. We previously found that phosphatidic acid (PA) binds Sec18 and thereby sequesters it from SNAREs and that PA dephosphorylation dissociates Sec18 from the membrane, allowing it to engage SNARE complexes. We now report that PA also induces conformational changes in Sec18 protomers and that hexameric Sec18 cannot bind PA membranes. Molecular dynamics (MD) analyses revealed that the D1 and D2 domains of Sec18 contain PA-binding sites and that the residues needed for PA binding are masked in hexameric Sec18. Importantly, these simulations also disclosed that a major conformational change occurs in the linker region between the D1 and D2 domains, which is distinct from the conformational changes that occur in hexameric Sec18 during SNARE priming. Together, these findings indicate that PA regulates Sec18 function by altering its architecture and stabilizing membrane-bound Sec18 protomers.

A novel subtilase inhibitor in plants shows structural and functional similarities to protease propeptides

The propeptides of subtilisin-like serine proteinases (subtilases, SBTs) serve dual functions as intramolecular chaperones that are required for enzyme folding and as inhibitors of the mature proteases. SBT propeptides are homologous to the I9 family of protease inhibitors that have only been described in fungi. Here we report the identification and characterization of subtilisin propeptide-like inhibitor 1 (SPI-1) from Arabidopsis thaliana Sequence similarity and the shared β-α-β-β-α-β core structure identified SPI-1 as a member of the I9 inhibitor family and as the first independent I9 inhibitor in higher eukaryotes. SPI-1 was characterized as a high-affinity, tight-binding inhibitor of Arabidopsis subtilase SBT4.13 with K_d and K_i values in the picomolar range. SPI-1 acted as a stable inhibitor of SBT4.13 over the physiologically relevant range of pH, and its inhibitory profile included many other SBTs from plants but not bovine chymotrypsin or bacterial subtilisin A. Upon binding to SBT4.13, the C-terminal extension of SPI-1 was proteolytically cleaved. The last four amino acids at the newly formed C terminus of SPI-1 matched both the cleavage specificity of SBT4.13 and the consensus sequence of Arabidopsis SBTs at the junction of the propeptide with the catalytic domain. The data suggest that the C terminus of SPI-1 acts as a competitive inhibitor of target proteases as it remains bound to the active site in a product-like manner. SPI-1 thus resembles SBT propeptides with respect to its mode of protease inhibition. However, in contrast to SBT propeptides, SPI-1 could not substitute as a folding assistant for SBT4.13.

Membrane fusion: five lipids, four SNAREs, three chaperones, two nucleotides, and a Rab, all dancing in a ring on yeast vacuoles

Although fusion mechanisms are highly conserved in evolution and among organelles of the exocytic and endocytic pathways, yeast vacuole homotypic fusion offers unique technical advantages: excellent genetics, clear organelle cytology, in vitro colorimetric fusion assays, and reconstitution of fusion from all-pure components, including a Rab GTPase, HOPS (homotypic fusion and vacuole protein sorting complex), four SNAREs [soluble N-ethylmaleimide-sensitive factor (NSF) attachment receptors] that snare (bind) each other, SNARE-complex disassembly chaperones, and vacuolar lipids. Vacuole fusion studies offer paradigms of the interdependence of lipids and fusion proteins to assemble a fusion microdomain, distinct lipid functions, SNARE complex proofreading through the synergy between HOPS and the SNARE disassembly chaperones, and the role of each fusion protein in promoting radical bilayer restructuring for fusion without lysis.

Interaction of SNAREs with ArfGAPs precedes recruitment of Sec18p/NSF

Soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins are key components of the fusion machinery in vesicular transport and in homotypic membrane fusion. We previously found that ADP-ribosylation factor GTPase activating proteins (ArfGAPs) promoted a conformational change on SNAREs that allowed recruitment of the small GTPase Arf1p in stoichiometric amounts. Here, we show that the ArfGAP Gcs1p accelerates vesicle (v)-target membrane (t)-SNARE complex formation in vitro, indicating that ArfGAPs may act as folding chaperones. These SNARE complexes were resolved in the presence of ATP by the yeast homologues of alpha-soluble N-ethylmaleimide-sensitive factor attachment protein and N-ethylmaleimide-sensitive factor, Sec17p and Sec18p, respectively. In addition, Sec18p and Sec17p also recognized the "activated" SNAREs even when they were not engaged in v-t-SNARE complexes. Here again, the induction of a conformational change by ArfGAPs was essential. Surprisingly, recruitment of Sec18p to SNAREs did not require Sec17p or ATP hydrolysis. Moreover, Sec18p displaced prebound Arf1p from SNAREs, indicating that Sec18p may have more than one function: first, to ensure that all vesicle coat proteins are removed from the SNAREs before the engagement in a trans-SNARE complex; and second, to resolve cis-SNARE complexes after fusion has occurred.

A signal from inside the peroxisome initiates its division by promoting the remodeling of the peroxisomal membrane

We define the dynamics of spatial and temporal reorganization of the team of proteins and lipids serving peroxisome division. The peroxisome becomes competent for division only after it acquires the complete set of matrix proteins involved in lipid metabolism. Overloading the peroxisome with matrix proteins promotes the relocation of acyl-CoA oxidase (Aox), an enzyme of fatty acid beta-oxidation, from the matrix to the membrane. The binding of Aox to Pex16p, a membrane-associated peroxin required for peroxisome biogenesis, initiates the biosynthesis of phosphatidic acid and diacylglycerol (DAG) in the membrane. The formation of these two lipids and the subsequent transbilayer movement of DAG initiate the assembly of a complex between the peroxins Pex10p and Pex19p, the dynamin-like GTPase Vps1p, and several actin cytoskeletal proteins on the peroxisomal surface. This protein team promotes membrane fission, thereby executing the terminal step of peroxisome division.

Cellular functions of NSF: not just SNAPs and SNAREs

N-ethylmaleimide sensitive factor (NSF) is an ATPases associated with various cellular activities protein (AAA), broadly required for intracellular membrane fusion. NSF functions as a SNAP receptor (SNARE) chaperone which binds, through soluble NSF attachment proteins (SNAPs), to SNARE complexes and utilizes the energy of ATP hydrolysis to disassemble them thus facilitating SNARE recycling. While this is a major function of NSF, it does seem to interact with other proteins, such as the AMPA receptor subunit, GluR2, and beta2-AR and is thought to affect their trafficking patterns. New data suggest that NSF may be regulated by transient post-translational modifications such as phosphorylation and nitrosylation. These new aspects of NSF function as well as its role in SNARE complex dynamics will be discussed.

Synaptic vesicle mobility and presynaptic F-actin are disrupted in a N-ethylmaleimide-sensitive factor allele of Drosophila

N-ethylmaleimide sensitive factor (NSF) can dissociate the soluble NSF attachment receptor (SNARE) complex, but NSF also participates in other intracellular trafficking functions by virtue of SNARE-independent activity. Drosophila that express a neural transgene encoding a dominant-negative form of NSF2 show an 80% reduction in the size of releasable synaptic vesicle pool, but no change in the number of vesicles in nerve terminal boutons. Here we tested the hypothesis that vesicles in the NSF2 mutant terminal are less mobile. Using a combination of genetics, pharmacology, and imaging we find a substantial reduction in vesicle mobility within the nerve terminal boutons of Drosophila NSF2 mutant larvae. Subsequent analysis revealed a decrease of filamentous actin in both NSF2 dominant-negative and loss-of-function mutants. Lastly, actin-filament disrupting drugs also decrease vesicle movement. We conclude that a factor contributing to the NSF mutant phenotype is a reduction in vesicle mobility, which is associated with decreased presynaptic F-actin. Our data are consistent with a model in which actin filaments promote vesicle mobility and suggest that NSF participates in establishing or maintaining this population of actin.

Subcompartmentalizing the Golgi apparatus

The subcompartmentalized structure of the Golgi apparatus contributes to efficient glycosylation in the secretory pathway. Subcompartmentalization driven by maturation relies primarily on constant and accurate vesicle-mediated local recycling of Golgi residents. The precision of this vesicle transport is dependent on the interplay between the key factors that mediate vesicle budding and fusion--the coat proteins and the SNARE fusion machinery. These alone, however, may not be sufficient to ensure establishment of compartments de novo, and additional regulatory mechanisms operate to modify their activity.

Phenotypic switching in Candida glabrata accompanied by changes in expression of genes with deduced functions in copper detoxification and stress

Most strains of Candida glabrata switch spontaneously between a number of phenotypes distinguishable by graded brown coloration on agar containing 1 mM CuSO4, a phenomenon referred to as "core switching." C. glabrata also switches spontaneously and reversibly from core phenotypes to an irregular wrinkle (IWr) phenotype, a phenomenon referred to as "irregular wrinkle switching." To identify genes differentially expressed in the core phenotypes white (Wh) and dark brown (DB), a cDNA subtraction strategy was employed. Twenty-three genes were identified as up-regulated in DB, four in Wh, and six in IWr. Up-regulation was verified in two unrelated strains, one a and one alpha strain. The functions of these genes were deduced from the functions of their Saccharomyces cerevisiae orthologs. The majority of genes up-regulated in DB (78%) played deduced roles in copper assimilation, sulfur assimilation, and stress responses. These genes were differentially up-regulated in DB even though the conditions of growth for Wh and DB, including CuSO4 concentration, were identical. Hence, the regulation of these genes, normally regulated by environmental cues, has been usurped by switching, presumably as an adaptation to the challenging host environment. These results are consistent with the suggestion that switching provides colonizing populations with a minority of cells expressing a phenotype that allows them to enrich in response to an environmental challenge, a form of rapid adaptation. However, DB is the most commonly expressed phenotype at sites of host colonization, in the apparent absence of elevated copper levels. Hence, up-regulation of these genes by switching suggests that in some cases they may play roles in colonization and virulence not immediately obvious from the roles played by their orthologs in S. cerevisiae.

Disruption of synaptic development and ultrastructure byDrosophila NSF2 alleles

First identified as the cytosolic component that restored intra-Golgi vesicle trafficking following N-ethylmaleimide poisoning, N-ethylmaleimide-sensitive factor (NSF) was later shown to be an ATPase that participates in many vesicular trafficking events. Current models hold that NSF disassembles postfusion SNARE protein complexes, allowing them to participate in further rounds of vesicle cycling. To further understand the role of NSF in neural function, we have embarked on genetic studies of Drosophila NSF2. In one approach, we employed transgenic flies that carry a dominant-negative form of NSF2 (NSF(E/Q)). When expressed in neurons this construct suppresses synaptic transmission, increases activity-dependent fatigue of transmitter release, and reduces the functional size of the pool of vesicles available for release. Unexpectedly, it also induced pronounced overgrowth of the neuromuscular junction. The aim of the present study was twofold. First, we sought to determine if the neuromuscular junction (NMJ) overgrowth phenotype is present throughout development. Second, we examined NSF2(E/Q) larval synapses by serial section electron microscopy in order to determine if there are ultrastructural correlates to the observed physiological and morphological phenotypes. We indeed found that the NMJ overgrowth phenotype is present at the embryonic neuromuscular synapse. Likewise, at the ultrastructural level, we found considerable alterations in the number and distribution of synapses and active zones, whereas the number of vesicles present was not changed. From these data we conclude that a primary phenotype of the NSF2(E/Q) transgene is a developmental one and that alteration in the number and distribution of active zones contributes to the NSF2(E/Q) physiological phenotype.

A genetic screen for suppressors of Drosophila NSF2 neuromuscular junction overgrowth

The Drosophila larval neuromuscular system serves as a valuable model for studying the genes required for synaptic development and function. N-Ethylmaleimide sensitive factor (NSF) is a molecule known to be important in vesicular trafficking but neural expression of a dominant negative form of NSF2 induces an unexpected overgrowth of the Drosophila larval neuromuscular synapse. We have taken a genetic approach to understanding this novel phenotype by conducting a gain-of-function modifier screen to isolate genes that interact with the overgrowth phenotype. Our approach was to directly visualize the neuromuscular junction (NMJ) using a GFP transgene and screen for suppressors of NMJ overgrowth using the Gene Search collection of P-element insertions. Of the 3000 lines screened, we identified 99 lines that can partially restore the normal phenotype. Analysis of the GS element insertion sites by inverse PCR and comparison of the flanking DNA sequence to the Drosophila genome sequence revealed nearby genes for all but 10 of the 99 lines. The recovered genes, both known and predicted, include transcription factors, cytoskeletal elements, components of the ubiquitin pathway, and several signaling molecules. This collection of genes that suppress the NSF2 neuromuscular junction overgrowth phenotype is a valuable resource in our efforts to further understand the role of NSF at the synapse.

Genome-wide analysis reveals inositol, not choline, as the major effector of Ino2p-Ino4p and unfolded protein response target gene expression in yeast

In the yeast Saccharomyces cerevisiae, the transcription of many genes encoding enzymes of phospholipid biosynthesis are repressed in cells grown in the presence of the phospholipid precursors inositol and choline. A genome-wide approach using cDNA microarray technology was used to profile the changes in the expression of all genes in yeast that respond to the exogenous presence of inositol and choline. We report that the global response to inositol is completely distinct from the effect of choline. Whereas the effect of inositol on gene expression was primarily repressing, the effect of choline on gene expression was activating. Moreover, the combination of inositol and choline increased the number of repressed genes compared with inositol alone and enhanced the repression levels of a subset of genes that responded to inositol. In all, 110 genes were repressed in the presence of inositol and choline. Two distinct sets of genes exhibited differential expression in response to inositol or the combination of inositol and choline in wild-type cells. One set of genes contained the UASINO sequence and were bound by Ino2p and Ino4p. Many of these genes were also negatively regulated by OPI1, suggesting a common regulatory mechanism for Ino2p, Ino4p, and Opi1p. Another nonoverlapping set of genes was coregulated by the unfolded protein response pathway, an ER-localized stress response pathway, but was not dependent on OPI1 and did not show further repression when choline was present together with inositol. These results suggest that inositol is the major effector of target gene expression, whereas choline plays a minor role.

A soluble SNARE drives rapid docking, bypassing ATP and Sec17/18p for vacuole fusion

Membrane fusion requires priming, the disassembly of cis-SNARE complexes by the ATP-driven chaperones Sec18/17p. Yeast vacuole priming releases Vam7p, a soluble SNARE. Vam7p reassociation during docking allows trans-SNARE pairing and fusion. We now report that recombinant Vam7p (rVam7p) enters into complex with other SNAREs in vitro and bypasses the need for Sec17p, Sec18p, and ATP. Thus, the sole essential function of vacuole priming in vitro is the release of Vam7p from cis-SNARE complexes. In 'bypass fusion', without ATP but with added rVam7p, there are sufficient unpaired vacuolar SNAREs Vam3p, Vti1p, and Nyv1p to interact with Vam7p and support fusion. However, active SNARE proteins are not sufficient for bypass fusion. rVam7p does not bypass requirements for Rho GTPases,Vps33p, Vps39p, Vps41p, calmodulin, specific lipids, or Vph1p, a subunit of the V-ATPase. With excess rVam7p, reduced levels of PI(3)P or functional Ypt7p suffice for bypass fusion. High concentrations of rVam7p allow the R-SNARE Ykt6p to substitute for Nyv1p for fusion; this functional redundancy among vacuole SNAREs may explain why nyv1delta strains lack the vacuole fragmentation seen with mutants in other fusion catalysts.

Multiple binding proteins suggest diverse functions for the N-ethylmaleimide sensitive factor

The hexameric ATPase, N-ethylmaleimide sensitive factor (NSF), is essential to vesicular transport and membrane fusion because it affects the conformations and associations of the soluble NSF attachment protein receptor (SNARE) proteins. NSF binds SNAREs through adaptors called soluble NSF attachment proteins (alpha- or beta-SNAP) and disassembles SNARE complexes to recycle the monomers. NSF contains three domains, two nucleotide-binding domains (NSF-D1 and -D2) and an amino terminal domain (NSF-N) that is required for SNAP-SNARE complex binding. Mutagenesis studies indicate that a cleft between the two sub-domains of NSF-N is critical for binding. The structural conservation of N domains in NSF, p97/VCP, and VAT suggests that a similar type of binding site could mediate substrate recognition by other AAA proteins. In addition to SNAP-SNARE complexes, NSF also binds other proteins and protein complexes such as AMPA receptor subunits (GluR2), beta2-adrenergic receptor, beta-Arrestin1, GATE-16, LMA1, rabs, and rab-containing complexes. The potential for these interactions indicates a broader role for NSF in the assembly/disassembly cycles of several cellular complexes and suggests that NSF may have specific regulatory effects on the functions of the proteins involved in these complexes. The structural requirements for these interactions and their physiological significance will be discussed.

Peroxisome division in the yeast Yarrowia lipolytica is regulated by a signal from inside the peroxisome

We describe an unusual mechanism for organelle division. In the yeast Yarrowia lipolytica, only mature peroxisomes contain the complete set of matrix proteins. These mature peroxisomes assemble from several immature peroxisomal vesicles in a multistep pathway. The stepwise import of distinct subsets of matrix proteins into different immature intermediates along the pathway causes the redistribution of a peroxisomal protein, acyl-CoA oxidase (Aox), from the matrix to the membrane. A significant redistribution of Aox occurs only in mature peroxisomes. Inside mature peroxisomes, the membrane-bound pool of Aox interacts with Pex16p, a membrane-associated protein that negatively regulates the division of early intermediates in the pathway. This interaction inhibits the negative action of Pex16p, thereby allowing mature peroxisomes to divide.

SNARE regulators: matchmakers and matchbreakers

SNAREs (soluble N-ethylmaleimide-sensitive fusion protein attachment protein receptors) are membrane-associated proteins that participate in the fusion of internal membranes in eukaryotic cells. SNAREs comprise three distinct and well-conserved families of molecules that act directly as membrane fusogens or, at the least, as elements that bring membranes into close apposition and allow for subsequent fusion events to occur. While the molecular events leading to fusion are still under debate, it is clear that a number of additional factors are required to bring about SNARE-mediated membrane fusion in vivo. Many of these factors, which collectively can be called SNARE regulators (e.g. Sec1/Munc18, synaptotagmin, GATE-16, LMA1, Munc13/UNC-13, synaptophysin, tomosyn, Vsm1, etc.), bind directly to SNAREs and are involved in the regulation of SNARE assembly as well as the ability of SNAREs to participate in trafficking events. In addition, recent studies have suggested a role for posttranslational modification (e.g., phosphorylation) in the regulation of SNARE functions. In this review the possible role of SNARE regulators in SNARE assembly and the involvement of SNARE phosphorylation in the regulation of intracellular membrane trafficking will be discussed.

Involvement of LMA1 and GATE-16 family members in intracellular membrane dynamics

Intracellular membrane fusion is conserved from yeast to man as well as among different intracellular trafficking pathways. This process can be generally divided into several well-defined biochemical reactions. First, an early recognition (or tethering) takes place between donor and acceptor membranes, mediated by ypt/rab GTPases and complexes of tethering factors. Subsequently, a closer association between the two membranes is achieved by a docking process, which involves tight association between membrane proteins termed SNAREs. The formation of such a trans-SNARE complex leads to the final membrane fusion, resulting in an accumulation of cis-SNARE complexes on the acceptor membrane. Thus, multiple rounds of transport and delivery of the donor SNARE back to its original membrane require dissociation of the SNARE complexes. SNARE dissociation, termed priming, is mediated by the AAA ATPase, N-ethylmaleimide-sensitive factor (NSF) and its partner, soluble NSF attachment protein (SNAP), in a reaction that requires ATP hydrolysis. In the present review we focus on LMA1 and GATE-16, two low-molecular-weight proteins, which assist in priming SNARE molecules in the vacuole in yeast and the Golgi complex in mammals, respectively. LMA1 and GATE-16 are suggested to keep the dissociated cis-SNAREs apart from each other, allowing multiple fusion processes to take place. GATE-16 belongs to a novel family of ubiquitin-like proteins conserved from yeast to man. We discuss here the involvement of this family in multiple intracellular trafficking pathways.

Phosphorylation of the autoinhibitory domain of the Sso t-SNAREs promotes binding of the Vsm1 SNARE regulator in yeast

We have shown that protein kinase A phosphorylation of t-SNAREs inhibits SNARE assembly and suppresses endo- and exocytosis in yeast. Herein, we show that protein kinase A phosphorylation of the Sso exocytic t-SNAREs promotes the binding of Vsm1, a potential SNARE regulator identified previously in our laboratory. Phosphorylation of Sso increases its affinity for Vsm1 by more than fivefold in vitro and both phosphorylated Sso1, as well as Sso1 bearing an aspartate substitution at position 79, interact tightly with Vsm1. Vsm1 binding is dependent upon the NH2-terminal autoinhibitory domain of Sso, and constitutively "open" forms of the t-SNARE show a reduction in Vsm1 binding in vivo. The substitution of serine-79 in Sso1 with an alanine residue or the treatment of yeast with C2-ceramide, which results in the dephosphorylation of serine-79, both inhibit Vsm1 binding in vivo. Importantly, Vsm1 binding to Sso seems to preclude Sso binding to its partner t-SNARE, Sec9, and vice versa. This is consistent with the idea that Vsm1 is an inhibitor of SNARE assembly in yeast. Thus, one way by which phosphorylation inhibits SNARE assembly could be by regulating the association of inhibitory factors that control the ability of t-SNAREs to form complexes in vivo.

Vacuole membrane fusion: V0 functions after trans-SNARE pairing and is coupled to the Ca2+-releasing channel

Pore models of membrane fusion postulate that cylinders of integral membrane proteins can initiate a fusion pore after conformational rearrangement of pore subunits. In the fusion of yeast vacuoles, V-ATPase V0 sectors, which contain a central cylinder of membrane integral proteolipid subunits, associate to form a transcomplex that might resemble an intermediate postulated in some pore models. We tested the role of V0 sectors in vacuole fusion. V0 functions in fusion and proton translocation could be experimentally separated via the differential effects of mutations and inhibitory antibodies. Inactivation of the V0 subunit Vph1p blocked fusion in the terminal reaction stage that is independent of a proton gradient. Deltavph1 mutants were capable of docking and trans-SNARE pairing and of subsequent release of lumenal Ca2+, but they did not fuse. The Ca2+-releasing channel appears to be tightly coupled to V0 because inactivation of Vph1p by antibodies blocked Ca2+ release. Vph1 deletion on only one fusion partner sufficed to severely reduce fusion activity. The functional requirement for Vph1p correlates to V0 transcomplex formation in that both occur after docking and Ca2+ release. These observations establish V0 as a crucial factor in vacuole fusion acting downstream of trans-SNARE pairing.

Golgi architecture and inheritance

Golgi inheritance proceeds via sequential biogenesis and partitioning phases. Although little is known about Golgi growth and replication (biogenesis), ultrastructural and fluorescence analyses have provided a detailed, though still controversial, perspective of Golgi partitioning during mitosis in mammalian cells. Partitioning requires the fragmentation of the juxtanuclear ribbon of interconnected Golgi stacks into a multitude of tubulovesicular clusters. This process is choreographed by a cohort of mitotic kinases and an inhibition of heterotypic and homotypic Golgi membrane-fusion events. Our model posits that accurate partitioning occurs early in mitosis by the equilibration of Golgi components on either side of the metaphase plate. Disseminated Golgi components then coalesce to regenerate Golgi stacks during telophase. Semi-intact cell and cell-free assays have accurately recreated these processes and allowed their molecular dissection. This review attempts to integrate recent findings to depict a more coherent, synthetic molecular picture of mitotic Golgi fragmentation and reassembly. Of particular importance is the emerging concept of a highly regulated and dynamic Golgi structural matrix or template that interfaces with cargo receptors, Golgi enzymes, Rab-GTPases, and SNAREs to tightly couple biosynthetic transport to Golgi architecture. This structural framework may be instructive for Golgi biogenesis and may encode sufficient information to ensure accurate Golgi inheritance, thereby helping to resolve some of the current discrepancies between different workers.

Membrane fusion in eukaryotic cells

Membrane fusion is a fundamental biochemical reaction and the final step in all vesicular trafficking events. It is crucial for the transfer of proteins and lipids between different compartments and for exo- and endocytic traffic of signaling molecules and receptors. It leads to the reconstruction of organelles such as the Golgi or the nuclear envelope, which decay into fragments during mitosis. Hence, controlled membrane fusion reactions are indispensible for the compartmental organization of eukaryotic cells; for their communication with the environment via hormones, neurotransmitters, growth factors, and receptors; and for the integration of cells into multicellular organisms. Intracellular pathogenic bacteria, such as Mycobacteria or Salmonellae, have developed means to control fusion reactions in their host cells. They persist in phagosomes whose fusion with lysosomes they actively suppress-a means to ensure survival inside host cells. The past decade has witnessed rapid progress in the elucidation of parts of the molecular machinery involved in these membrane fusion reactions. Whereas some elements of the fusion apparatus are remarkably similar in several compartments, there is an equally striking divergence of others. The purpose of this review is to highlight common features of different fusion reactions and the concepts that emerged from them but also to stress the differences and challenge parts of the current hypotheses. This review covers only the endoplasmic fusion reactions mentioned above, i.e., reactions initiated by contacts of membranes with their cytoplasmic faces. Ectoplasmic fusion events, which depend on an initial contact of the fusion partners via the membrane surfaces exposed to the surrounding medium are not discussed, nor are topics such as the entry of enveloped viruses, formation of syncytia, gamete fusion, or vesicle scission (a fusion reaction that leads to the fission of, e.g., transport vesicles).

Secretory granule exocytosis

Regulated exocytosis of secretory granules or dense-core granules has been examined in many well-characterized cell types including neurons, neuroendocrine, endocrine, exocrine, and hemopoietic cells and also in other less well-studied cell types. Secretory granule exocytosis occurs through mechanisms with many aspects in common with synaptic vesicle exocytosis and most likely uses the same basic protein components. Despite the widespread expression and conservation of a core exocytotic machinery, many variations occur in the control of secretory granule exocytosis that are related to the specialized physiological role of particular cell types. In this review we describe the wide range of cell types in which regulated secretory granule exocytosis occurs and assess the evidence for the expression of the conserved fusion machinery in these cells. The signals that trigger and regulate exocytosis are reviewed. Aspects of the control of exocytosis that are specific for secretory granules compared with synaptic vesicles or for particular cell types are described and compared to define the range of accessory control mechanisms that exert their effects on the core exocytotic machinery.

Involvement of glycosylation in the intracellular trafficking of glycoproteins in polarized epithelial cells

The surface of epithelial cells is composed of apical and basolateral domains with distinct structure and function. This polarity is maintained by specific sorting mechanisms occurring in the Trans-Golgi Network. Peptidic signals are responsible for the trafficking via clathrin-coated vesicles by means of an interaction with an adaptor complex (AP). The basolateral targeting is mediated by AP-1B, which is specifically expressed in epithelial cells. In contrast, the apical targeting is proposed to occur via apical raft carriers. It is thought that apically targeted glycoproteins contain glycan signals that would be responsible for their association with rafts and for apical targeting. However, the difficulty in terms of acting specifically on a single step of glycosylation did not allow one to identify such a specific signal. The complete inhibition of the processing of N-glycans by tunicamycin often results in an intracellular accumulation of unfolded proteins in the Golgi. Similarly, inhibition of O-glycosylation can be obtained by competitive substrates which gave a complex pattern of inhibition. Therefore, it is still unknown if glycosylation acts in an indirect manner, i.e. by modifying the folding of the protein, or in a specific manner, such as an association with specific lectins.

Sequential SNARE disassembly and GATE-16-GOS-28 complex assembly mediated by distinct NSF activities drives Golgi membrane fusion

Characterization of mammalian NSF (G274E) and Drosophila NSF (comatose) mutants revealed an evolutionarily conserved NSF activity distinct from ATPase-dependent SNARE disassembly that was essential for Golgi membrane fusion. Analysis of mammalian NSF function during cell-free assembly of Golgi cisternae from mitotic Golgi fragments revealed that NSF disassembles Golgi SNAREs during mitotic Golgi fragmentation. A subsequent ATPase-independent NSF activity restricted to the reassembly phase is essential for membrane fusion. NSF/alpha-SNAP catalyze the binding of GATE-16 to GOS-28, a Golgi v-SNARE, in a manner that requires ATP but not ATP hydrolysis. GATE-16 is essential for NSF-driven Golgi reassembly and precludes GOS-28 from binding to its cognate t-SNARE, syntaxin-5. We suggest that this occurs at the inception of Golgi reassembly to protect the v-SNARE and regulate SNARE function.

Redox reactions of regulatory proteins: do kinetics promote specificity?

Signaling by redox state regulates the transcriptional and post-transcriptional events that control gene expression. To elucidate redox signaling in vivo, the effects of the reductive intracellular redox environment on regulatory redox events must be taken into account. This article focuses on proteins that contain regulatory disulfides, considering whether regulatory proteins can be oxidized and how the redox state of regulatory proteins can be uniquely controlled to allow redox signaling via specific pathways. It is possible that the favored kinetics of the redox reactions of regulatory proteins are important for attaining specificity in redox signaling.

Yeast vacuoles and membrane fusion pathways

Selective membrane fusion underlies subcellular compartmentation, cell growth, neurotransmission and hormone secretion. Its fundamental mechanisms are conserved among organelles, tissues and organisms. As befits a conserved process, reductionism led to its study in microorganisms. Homotypic fusion of the vacuole of Saccharomyces cerevisiae is particularly accessible to study as vacuoles are readily visualized, there is a rapid and quantitative in vitro assay of vacuole fusion, and the genetics and genomics of this organism and of vacuole fusion are highly advanced. Recent progress is reviewed in the context of general questions in the membrane fusion field.

Genomic analysis of homotypic vacuole fusion

Yeast vacuoles undergo fission and homotypic fusion, yielding one to three vacuoles per cell at steady state. Defects in vacuole fusion result in vacuole fragmentation. We have screened 4828 yeast strains, each with a deletion of a nonessential gene, for vacuole morphology defects. Fragmented vacuoles were found in strains deleted for genes encoding known fusion catalysts as well as 19 enzymes of lipid metabolism, 4 SNAREs, 12 GTPases and GTPase effectors, 9 additional known vacuole protein-sorting genes, 16 protein kinases, 2 phosphatases, 11 cytoskeletal proteins, and 28 genes of unknown function. Vacuole fusion and vacuole protein sorting are catalyzed by distinct, but overlapping, sets of proteins. Novel pathways of vacuole priming and docking emerged from this deletion screen. These include ergosterol biosynthesis, phosphatidylinositol (4,5)-bisphosphate turnover, and signaling from Rho GTPases to actin remodeling. These pathways are supported by the sensitivity of the late stages of vacuole fusion to inhibitors of phospholipase C, calcium channels, and actin remodeling. Using databases of yeast protein interactions, we found that many nonessential genes identified in our deletion screen interact with essential genes that are directly involved in vacuole fusion. Our screen reveals regulatory pathways of vacuole docking and provides a genomic basis for studies of this reaction.

Vacuole fusion at a ring of vertex docking sites leaves membrane fragments within the organelle

Three membrane microdomains can be identified on docked vacuoles: "outside" membrane, not in contact with other vacuoles, "boundary" membrane that contacts adjacent vacuoles, and "vertices," where boundary and outside membrane meet. In living cells and in vitro, vacuole fusion occurs at vertices rather than from a central pore expanding radially. Vertex fusion leaves boundary membrane within the fused organelle and is an unexpected pathway for the formation of intralumenal membranes. Proteins that regulate docking and fusion (Vac8p, the GTPase Ypt7p, its HOPS/Vps-C effector complex, the t-SNARE Vam3p, and protein phosphatase 1) accumulate at these vertices during docking. Their vertex enrichment requires cis-SNARE complex disassembly and is thus part of the normal fusion pathway.

The Vtc proteins in vacuole fusion: coupling NSF activity to V(0) trans-complex formation

The fusion of cellular membranes comprises several steps; membrane attachment requires priming of SNAREs and tethering factors by Sec18p/NSF (N-ethylmaleimide sensitive factor) and LMA1. This leads to trans-SNARE pairing, i.e. formation of SNARE complexes between apposed membranes. The yeast vacuole system has revealed two subsequent molecular events: trans-complex formation of V-ATPase proteolipid sectors (V(0)) and release of LMA1 from the membrane. We have now identified a hetero-oligomeric membrane integral complex of vacuolar transporter chaperone (Vtc) proteins integrating these events. The Vtc complex associates with the R-SNARE Nyv1p and with V(0). Subunits Vtc1p and Vtc4p control the initial steps of fusion. They are required for Sec18p/NSF activity in SNARE priming, membrane binding of LMA1 and V(0) trans-complex formation. In contrast, subunit Vtc3p is required for the latest step, LMA1 release, but dispensible for all preceding steps, including V(0) trans-complex formation. This suggests that Vtc3p might act close to or at fusion pore opening. We propose that Vtc proteins may couple ATP-dependent NSF activity to a subset of V(0) sectors in order to activate them for V(0) trans-complex formation and/or control fusion pore opening.

The N-terminal domain of the t-SNARE Vam3p coordinates priming and docking in yeast vacuole fusion

Homotypic fusion of yeast vacuoles requires a regulated sequence of events. During priming, Sec18p disassembles cis-SNARE complexes. The HOPS complex, which is initially associated with the cis-SNARE complex, then mediates tethering. Finally, SNAREs assemble into trans-complexes before the membranes fuse. The t-SNARE of the vacuole, Vam3p, plays a central role in the coordination of these processes. We deleted the N-terminal region of Vam3p to analyze the role of this domain in membrane fusion. The truncated protein (Vam3 Delta N) is sorted normally to the vacuole and is functional, because the vacuolar morphology is unaltered in this strain. However, in vitro vacuole fusion is strongly reduced due to the following reasons: Assembly, as well as disassembly of the cis-SNARE complex is more efficient on Vam3 Delta N vacuoles; however, the HOPS complex is not associated well with the Vam3 Delta N cis-complex. Thus, primed SNAREs from Vam3 Delta N vacuoles cannot participate efficiently in the reaction because trans-SNARE pairing is substantially reduced. We conclude that the N-terminus of Vam3p is required for coordination of priming and docking during homotypic vacuole fusion.

Cdc42p functions at the docking stage of yeast vacuole membrane fusion

Membrane fusion reactions have been considered to be primarily regulated by Rab GTPases. In the model system of homotypic vacuole fusion in the yeast Saccharomyces cerevisiae, we show that Cdc42p, a member of the Rho family of GTPases, has a direct role in membrane fusion. Genetic evidence suggested a relationship between Cdc42p and Vtc1p/Nrf1p, a central part of the vacuolar membrane fusion machinery. Vacuoles from cdc42 temperature-sensitive mutants are deficient for fusion at the restrictive temperature. Specific amino acid changes on the Cdc42p protein surface in these mutants define the putative interaction domain that is crucial for its function in membrane fusion. Affinity-purified antibodies to this domain inhibited the in vitro fusion reaction. Using these antibodies in kinetic analyses and assays for subreactions of the priming, docking and post-docking phase of the reaction, we show that Cdc42p action follows Ypt7p-dependent tethering, but precedes the formation of trans-SNARE complexes. Thus, our data define an effector binding domain of Cdc42p by which it regulates the docking reaction of vacuole fusion.

Fusion of docked membranes requires the armadillo repeat protein Vac8p

The discovery of molecules required for membrane fusion has revealed a remarkably conserved mechanism that centers upon the formation of a complex of SNARE proteins. However, whether the SNARE proteins or other components catalyze the final steps of membrane fusion in vivo remains unclear. Understanding this last step depends on the identification of molecules that act late in the fusion process. Here we demonstrate that in Saccharomyces cerevisiae, Vac8p, a myristoylated and palmitoylated armadillo repeat protein, is required for homotypic vacuole fusion. Vac8p is palmitoylated during the fusion reaction, and the ability of Vac8p to be palmitoylated appears to be necessary for its function in fusion. Both in vivo and in vitro analyses show that Vac8p functions after both Rab-dependent vacuole docking and the formation of trans-SNARE pairs. We propose that Vac8p may bind the fusion machinery through its armadillo repeats and that palmitoylation brings this machinery to a specialized lipid domain that facilitates bilayer mixing.

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.

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-replenishment mechanism of exocytosis in pancreatic acini

Here we report exocytosis of zymogen granules, as examined by multiphoton excitation imaging in intact pancreatic acini. Cholecystokinin induces Ca 2+ oscillations that trigger exocytosis when the cytosolic Ca 2+ concentration exceeds 1 microM. Zymogen granules fused with the plasma membrane maintain their Omega-shaped profile for an average of 220 s and serve as targets for sequential fusion of granules that are located within deeper layers of the cell. This secondary exocytosis occurs as rapidly as the primary exocytosis and accounts for most exocytotic events. Granule-granule fusion does not seem to precede primary exocytosis, indicating that secondary fusion events may require a plasma-membrane factor. This sequential-replenishment mechanism of exocytosis allows the cell to take advantage of a large supply of fusion-ready granules without needing to transport them to the plasma membrane.

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.

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.

Aut7p, a soluble autophagic factor, participates in multiple membrane trafficking processes

Aut7p, a protein recently implicated in autophagic events in the yeast Saccharomyces cerevisiae, exhibits significant homology to a mammalian protein, p16, herein termed GATE-16 (Golgi-associated ATPase Enhancer of 16 kDa), a novel intra-Golgi transport factor. Here we provide evidence for the involvement of Aut7p in different membrane trafficking processes. Aut7p largely substitutes for the activity of GATE-16 in mammalian intra-Golgi transport in vitro. In vivo, AUT7 interacts genetically with endoplasmic reticulum to Golgi SNAREs, specifically with BET1 and SEC22. Aut7p interacts physically with the following two v-SNAREs: Bet1p, which is involved in endoplasmic reticulum to Golgi vesicular transport, and Nyv1p, implicated in vacuolar inheritance. We suggest that, in addition to its role in autophagocytosis, Aut7p has pleiotropic effects and participates in at least two membrane traffic events.

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.

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.

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.

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.

GATE-16, a membrane transport modulator, interacts with NSF and the Golgi v-SNARE GOS-28

Membrane proteins located on vesicles (v-SNAREs) and on the target membrane (t-SNAREs) mediate specific recognition and, possibly, fusion between a transport vesicle and its target membrane. The activity of SNARE molecules is regulated by several soluble cytosolic proteins. We have cloned a bovine brain cDNA encoding a conserved 117 amino acid polypeptide, denoted Golgi-associated ATPase Enhancer of 16 kDa (GATE-16), that functions as a soluble transport factor. GATE-16 interacts with N-ethylmaleimidesensitive factor (NSF) and significantly stimulates its ATPase activity. It also interacts with the Golgi v-SNARE GOS-28 in an NSF-dependent manner. We propose that GATE-16 modulates intra-Golgi transport through coupling between NSF activity and SNAREs activation.

A screen for dominant negative mutants of SEC18 reveals a role for the AAA protein consensus sequence in ATP hydrolysis

An evolutionarily ancient mechanism is used for intracellular membrane fusion events ranging from endoplasmic reticulum-Golgi traffic in yeast to synaptic vesicle exocytosis in the human brain. At the heart of this mechanism is the core complex of N-ethylmaleimide-sensitive fusion protein (NSF), soluble NSF attachment proteins (SNAPs), and SNAP receptors (SNAREs). Although these proteins are accepted as key players in vesicular traffic, their molecular mechanisms of action remain unclear. To illuminate important structure-function relationships in NSF, a screen for dominant negative mutants of yeast NSF (Sec18p) was undertaken. This involved random mutagenesis of a GAL1-regulated SEC18 yeast expression plasmid. Several dominant negative alleles were identified on the basis of galactose-inducible growth arrest, of which one, sec18-109, was characterized in detail. The sec18-109 phenotype (abnormal membrane trafficking through the biosynthetic pathway, accumulation of a membranous tubular network, growth suppression, increased cell density) is due to a single A-G substitution in SEC18 resulting in a missense mutation in Sec18p (Thr(394)-->Pro). Thr(394) is conserved in most AAA proteins and indeed forms part of the minimal AAA consensus sequence that serves as a signature of this large protein family. Analysis of recombinant Sec18-109p indicates that the mutation does not prevent hexamerization or interaction with yeast alpha-SNAP (Sec17p), but instead results in undetectable ATPase activity that cannot be stimulated by Sec17p. This suggests a role for the AAA protein consensus sequence in regulating ATP hydrolysis. Furthermore, this approach of screening for dominant negative mutants in yeast can be applied to other conserved proteins so as to highlight important functional domains in their mammalian counterparts.

The docking stage of yeast vacuole fusion requires the transfer of proteins from a cis-SNARE complex to a Rab/Ypt protein

The homotypic fusion of yeast vacuoles requires Sec18p (NSF)-driven priming to allow vacuole docking, but the mechanism that links priming and docking is unknown. We find that a large multisubunit protein called the Vam2/6p complex is bound to cis-paired SNAP receptors (SNAREs) on isolated vacuoles. This association of the Vam2/6p complex with the cis-SNARE complex is disrupted during priming. The Vam2/6p complex then binds to Ypt7p, a guanosine triphosphate binding protein of the Rab family, to initiate productive contact between vacuoles. Thus, cis-SNARE complexes can contain Rab/Ypt effectors, and these effectors can be mobilized by NSF/Sec18p-driven priming, allowing their direct association with a Rab/Ypt protein to activate docking.

Proteins needed for vesicle budding from the Golgi complex are also required for the docking step of homotypic vacuole fusion

Vam2p/Vps41p is known to be required for transport vesicles with vacuolar cargo to bud from the Golgi. Like other VAM-encoded proteins, which are needed for homotypic vacuole fusion, we now report that Vam2p and its associated protein Vam6p/Vps39p are needed on each vacuole partner for homotypic fusion. In vitro vacuole fusion occurs in successive steps of priming, docking, and membrane fusion. While priming does not require Vam2p or Vam6p, the functions of these two proteins cannot be fulfilled until priming has occurred, and each is required for the docking reaction which culminates in trans-SNARE pairing. Consistent with their dual function in Golgi vesicle budding and homotypic fusion of vacuoles, approximately half of the Vam2p and Vam6p of the cell are recovered from cell lysates with purified vacuoles.

Phosphatidylinositol 4,5-bisphosphate regulates two steps of homotypic vacuole fusion

Yeast vacuoles undergo cycles of fragmentation and fusion as part of their transmission to the daughter cell and in response to changes of nutrients and the environment. Vacuole fusion can be reconstituted in a cell free system. We now show that the vacuoles synthesize phosphoinositides during in vitro fusion. Of these phosphoinositides, phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate (PI(4,5)P(2)) are important for fusion. Monoclonal antibodies to PI(4,5)P(2), neomycin (a phosphoinositide ligand), and phosphatidylinositol-specific phospholipase C interfere with the reaction. Readdition of PI(4, 5)P(2) restores fusion in each case. Phosphatidylinositol 3-phosphate and PI(3,5)P(2) synthesis are not required. PI(4,5)P(2) is necessary for priming, i.e., for the Sec18p (NSF)-driven release of Sec17p (alpha-SNAP), which activates the vacuoles for subsequent tethering and docking. Therefore, it represents the kinetically earliest requirement identified for vacuole fusion so far. Furthermore, PI(4,5)P(2) is required at a step that can only occur after docking but before the BAPTA sensitive step in the latest stage of the reaction. We hence propose that PI(4,5)P(2) controls two steps of vacuole fusion.