The yeast endosomal t-SNARE, Pep12p, functions in the absence of its transmembrane domain

The t-SNARE protein FgPep12, associated with FgVam7, is essential for ascospore discharge and plant infection by trafficking Ca2+ ATPase FgNeo1 between Golgi and endosome/vacuole in Fusarium graminearum

Soluble N-ethylmaleimide-sensitive factor attachment receptors (SNAREs) play a crucial role in the development and virulence through mediation of membrane fusion and vesicle trafficking in pathogens. Our previous studies reported that the SNARE protein FgVam7 and its binding proteins FgVps39/41 are involved in vesicle trafficking and are important for vegetative growth, asexual/sexual development, deoxynivalenol production and virulence in the Fusarium head blight fungus Fusarium graminearum. Here, we identified and characterized another FgVam7 binding protein in F. graminearum, FgPep12, an ortholog of yeast t-SNARE Pep12 with both the SNARE and TM domains being essential for its localization and function. Deletion of FgPep12 caused defects in vegetative growth, conidiogenesis, deoxynivalenol production and virulence. Cytological observation revealed that FgPep12 localizes to the Golgi apparatus, late endosomes and vacuoles, and is necessary for transport from the vacuole to prevacuolar compartment. Further investigation revealed that both FgPep12 and FgVam7 are essential for ascospore discharge through interaction with and trafficking of the Ca²⁺ ATPase FgNeo1 between the Golgi and endosomal/vacuolar system. FgNeo1 has similar biological roles to FgPep12 and is required for ascospore discharge in F. graminearum. Together, these results provide solid evidence to help unravel the mechanisms underlying the manipulation of ascospore discharge and plant infection by SNARE proteins in F. graminearum.

SNARE proteins which mediate fusion of transport vesicles with the correct target membrane, are essential components of vesicle trafficking machinery. Together with the cognate effectors, SNAREs coordinate the dynamics of trafficking pathway and determines the cargo proteins destination. Here, we found that SNARE protein FgPep12 is important for fungal development and virulence through its involvement in vesicle trafficking between the Golgi and endosomal/vacuolar system. We further provide multiple lines of evidence showing that SNARE proteins modulate development and ascospore discharge in pathogenic fungi. FgPep12, associated with FgVam7, is required for the trafficking of the Ca²⁺ ATPase FgNeo1 between the Golgi and endosomal/vacuolar system, thus controlling growth, asexual development, ascospore discharge and plant infection in F. graminearum.

Characterization of a Novel Prevacuolar Compartment in Neurospora crassa

Using confocal microscopy, we observed ring-like organelles, similar in size to nuclei, in the hyphal tip of the filamentous fungus Neurospora crassa. These organelles contained a subset of vacuolar proteins. We hypothesize that they are novel prevacuolar compartments (PVCs). We examined the locations of several vacuolar enzymes and of fluorescent compounds that target the vacuole. Vacuolar membrane proteins, such as the vacuolar ATPase (VMA-1) and the polyphosphate polymerase (VTC-4), were observed in the PVCs. A pigment produced by adenine auxotrophs, used to visualize vacuoles, also accumulated in PVCs. Soluble enzymes of the vacuolar lumen, alkaline phosphatase and carboxypeptidase Y, were not observed in PVCs. The fluorescent molecule Oregon Green 488 carboxylic acid diacetate, succinimidyl ester (carboxy-DFFDA) accumulated in vacuoles and in a subset of PVCs, suggesting maturation of PVCs from the tip to distal regions. Three of the nine Rab GTPases in N. crassa, RAB-2, RAB-4, and RAB-7, localized to the PVCs. RAB-2 and RAB-4, which have similar amino acid sequences, are present in filamentous fungi but not in yeasts, and no function has previously been reported for these Rab GTPases in fungi. PVCs are highly pleomorphic, producing tubular projections that subsequently become detached. Dynein and dynactin formed globular clusters enclosed inside the lumen of PVCs. The size, structure, dynamic behavior, and protein composition of the PVCs appear to be significantly different from those of the well-studied prevacuolar compartment of yeasts.

The Gos28 SNARE protein mediates intra-Golgi transport of rhodopsin and is required for photoreceptor survival

SNARE proteins play indispensable roles in membrane fusion events in many cellular processes, including synaptic transmission and protein trafficking. Here, we characterize the Golgi SNARE protein, Gos28, and its role in rhodopsin (Rh1) transport through Drosophila photoreceptors. Mutations in gos28 lead to defective Rh1 trafficking and retinal degeneration. We have pinpointed a role for Gos28 in the intra-Golgi transport of Rh1, downstream from α-mannosidase-II in the medial- Golgi. We have confirmed the necessity of key residues in Gos28's SNARE motif and demonstrate that its transmembrane domain is not required for vesicle fusion, consistent with Gos28 functioning as a t-SNARE for Rh1 transport. Finally, we show that human Gos28 rescues both the Rh1 trafficking defects and retinal degeneration in Drosophila gos28 mutants, demonstrating the functional conservation of these proteins. Our results identify Gos28 as an essential SNARE protein in Drosophila photoreceptors and provide mechanistic insights into the role of SNAREs in neurodegenerative disease.

LegC3, an effector protein from Legionella pneumophila, inhibits homotypic yeast vacuole fusion in vivo and in vitro

During infection, the intracellular pathogenic bacterium Legionella pneumophila causes an extensive remodeling of host membrane trafficking pathways, both in the construction of a replication-competent vacuole comprised of ER-derived vesicles and plasma membrane components, and in the inhibition of normal phagosome:endosome/lysosome fusion pathways. Here, we identify the LegC3 secreted effector protein from L. pneumophila as able to inhibit a SNARE- and Rab GTPase-dependent membrane fusion pathway in vitro, the homotypic fusion of yeast vacuoles (lysosomes). This vacuole fusion inhibition appeared to be specific, as similar secreted coiled-coiled domain containing proteins from L. pneumophila, LegC7/YlfA and LegC2/YlfB, did not inhibit vacuole fusion. The LegC3-mediated fusion inhibition was reversible by a yeast cytosolic extract, as well as by a purified soluble SNARE, Vam7p. LegC3 blocked the formation of trans-SNARE complexes during vacuole fusion, although we did not detect a direct interaction of LegC3 with the vacuolar SNARE protein complexes required for fusion. Additionally, LegC3 was incapable of inhibiting a defined synthetic model of vacuolar SNARE-driven membrane fusion, further suggesting that LegC3 does not directly inhibit the activity of vacuolar SNAREs, HOPS complex, or Sec17p/18p during membrane fusion. LegC3 is likely utilized by Legionella to modulate eukaryotic membrane fusion events during pathogenesis.

The Sec1/Munc18 protein Vps45 regulates cellular levels of its SNARE binding partners Tlg2 and Snc2 in Saccharomyces cerevisiae

Intracellular membrane trafficking pathways must be tightly regulated to ensure proper functioning of all eukaryotic cells. Central to membrane trafficking is the formation of specific SNARE (soluble N-ethylmeleimide-sensitive factor attachment protein receptor) complexes between proteins on opposing lipid bilayers. The Sec1/Munc18 (SM) family of proteins play an essential role in SNARE-mediated membrane fusion, and like the SNAREs are conserved through evolution from yeast to humans. The SM protein Vps45 is required for the formation of yeast endosomal SNARE complexes and is thus essential for traffic through the endosomal system. Here we report that, in addition to its role in regulating SNARE complex assembly, Vps45 regulates cellular levels of its SNARE binding partners: the syntaxin Tlg2 and the v-SNARE Snc2: Cells lacking Vps45 have reduced cellular levels of Tlg2 and Snc2; and elevation of Vps45 levels results in concomitant increases in the levels of both Tlg2 and Snc2. As well as regulating traffic through the endosomal system, the Snc v-SNAREs are also required for exocytosis. Unlike most vps mutants, cells lacking Vps45 display multiple growth phenotypes. Here we report that these can be reversed by selectively restoring Snc2 levels in vps45 mutant cells. Our data indicate that as well as functioning as part of the machinery that controls SNARE complex assembly, Vps45 also plays a key role in determining the levels of its cognate SNARE proteins; another key factor in regulation of membrane traffic.

The pmr gene, encoding a Ca2+-ATPase, is required for calcium and manganese homeostasis and normal development of hyphae and conidia in Neurospora crassa

The pmr gene is predicted to encode a Ca(2+)-ATPase in the secretory pathway. We examined two strains of Neurospora crassa that lacked PMR: the Δpmr strain, in which pmr was completely deleted, and pmr(RIP), in which the gene was extensively mutated. Both strains had identical, complex phenotypes. Compared to the wild type, these strains required high concentrations of calcium or manganese for optimal growth and had highly branched, slow-growing hyphae. They conidiated poorly, and the shape and size of the conidia were abnormal. Calcium accumulated in the Δpmr strains to only 20% of the wild-type level. High concentrations of MnCl(2) (1 to 5 mM) in growth medium partially suppressed the morphological defects but did not alter the defect in calcium accumulation. The Δpmr Δnca-2 double mutant (nca-2 encodes a Ca(2+)-ATPase in the plasma membrane) accumulated 8-fold more calcium than the wild type, and the morphology of the hyphae was more similar to that of wild-type hyphae. Previous experiments failed to show a function for nca-1, which encodes a SERCA-type Ca(2+)-ATPase in the endoplasmic reticulum (B. J. Bowman, S. Abreu, E. Margolles-Clark, M. Draskovic, and E. J. Bowman, Eukaryot. Cell 10:654-661, 2011). The pmr(RIP) Δnca-1 double mutant accumulated small amounts of calcium, like the Δpmr strain, but exhibited even more extreme morphological defects. Thus, PMR can apparently replace NCA-1 in the endoplasmic reticulum, but NCA-1 cannot replace PMR. The morphological defects in the Δpmr strain are likely caused, in part, by insufficient concentrations of calcium and manganese in the Golgi compartment; however, PMR is also needed to accumulate normal levels of calcium in the whole cell.

Distinct contributions of vacuolar Qabc- and R-SNARE proteins to membrane fusion specificity

In eukaryotic endomembrane systems, Qabc-SNAREs (soluble N-ethylmaleimide-sensitive factor attachment protein receptors) on one membrane and R-SNARE on the opposing membrane assemble into a trans-QabcR-SNARE complex to drive membrane fusion. However, it remains ambiguous whether pairing of Qabc- and R-SNAREs mediates membrane fusion specificity. Here, we explored the fusion specificity of reconstituted proteoliposomes bearing purified SNAREs in yeast vacuoles and other organelles. We found that not only vacuolar R-SNARE Nyv1p but also the non-cognate R-SNAREs, endosomal Snc2p, and endoplasmic reticulum-Golgi Sec22p caused efficient fusion with vacuolar Qabc-SNAREs. In contrast, their fusion is blocked completely by replacing vacuolar Qc-SNARE Vam7p with the non-cognate endosomal Tlg1p and Syn8p, although these endosomal Qc-SNAREs fully retained the ability to form cis-SNARE complexes with vacuolar SNAREs in solution and on membranes. Thus, our current study establishes that an appropriate assembly of Qabc-SNAREs is crucial for regulating fusion specificity, whereas R-SNARE itself has little contribution to specificity.

Schizosaccharomyces pombe Pep12p is required for vacuolar protein transport and vacuolar homotypic fusion

In eukaryotic cells, SNARE proteins are essential for intracellular vesicle trafficking. Several SNARE proteins are required for vacuolar protein transport and vacuolar biogenesis in Saccharomyces cerevisiae. Previously we demonstrated that one of the fission yeast SNARE proteins, Pep12p, is not required for vacuolar fusion process in Schizosaccharomyces pombe. We have re-examined the function of S. pombe Pep12p using the newly created pep12(+) deletion strain. Deletion of the fission yeast pep12(+) gene results in pleiotropic phenotypes consistent with the absence of normal vacuoles, including missorting of vacuolar carboxypeptidase Y-and various ion- and drug-sensitivities. GFP-Pep12 fusion protein is mostly localized at the vacuolar membrane and the prevacuolar compartment. The S. pombe pep12Δ mutation phenocopies that of vps33Δ, suggesting that both Pep12p and Vps33p act at the same membrane fusion step in S. pombe, and both mutations cause vacuolar deficiency.

Arabidopsis Qa-SNARE SYP2 proteins localized to different subcellular regions function redundantly in vacuolar protein sorting and plant development

SYP2 proteins are a sub-family of Qa-SNAREs (soluble N-ethylmaleimide-sensitive factor attachment protein receptors) that may be responsible for protein trafficking between pre-vacuolar compartments (PVC) and vacuoles. Arabidopsis thaliana SYP22/VAM3/SGR3 and SYP21/PEP12 proteins function independently, but are both reported to be essential for male gametophytic viability. Here, we systematically examined the redundancy of three SYP2 paralogs (i.e. SYP21, 22 and 23) using a Col-0 ecotype harboring a SYP2 paralog (SYP23/PLP) that lacked a transmembrane domain. Surprisingly, no visible phenotypes were observed, even in the double knockout syp21/pep12 syp23/plp. Deficiency of either SYP21/PEP12 or SYP23/PLP in the syp22 background resulted in a defect in vacuolar protein sorting, characterized by abnormal accumulation of protein precursors in seeds. SYP21/PEP12 knockdown enhanced the syp22 phenotype (i.e. semi-dwarfism, poor leaf vein development and abnormal development of myrosin cells), and additional knockout of SYP23/PLP further aggravated the phenotype. A GFP-SYP23/PLP fusion localized to the cytosol, but not to the PVC or vacuolar membrane, where SYP21/PEP12 or SYP22/VAM3, respectively, were localized. Immunoprecipitation analysis showed that SYP23/PLP interacted with the vacuolar Qb- and Qc-SNAREs, VTI11 and SYP5, respectively, suggesting that SYP23/PLP is able to form a SNARE complex anchoring the membrane. Unexpectedly, we found that expression of multiple copies of a genomic fragment of SYP23/PLP suppressed the abnormal syp22-3 phenotype. Thus, SYP2 proteins, including cytosolic SYP23/PLP, appear to function redundantly in vacuolar trafficking and plant development.

Vacuolar/pre-vacuolar compartment Qa-SNAREs VAM3/SYP22 and PEP12/SYP21 have interchangeable functions in Arabidopsis

SNAREs (soluble N-ethylmaleimide sensitive factor attachment protein receptors) mediate specific membrane fusion between transport vesicles or organelles and target membranes. VAM3/SYP22 and PEP12/SYP21 are Qa-SNAREs that act in the vacuolar transport pathway of Arabidopsis thaliana, and are localized predominantly on the vacuolar membrane and the pre-vacuolar compartment (PVC), respectively. Previous studies have shown that loss-of-function mutants of VAM3/SYP22 or PEP12/SYP21 showed male gametophytic lethality, suggesting that VAM3/SYP22 and PEP12/SYP21 possess different, non-redundant functions. We have re-evaluated the effects of mutations in these genes using T-DNA insertion mutants in the Columbia accession. We found that a mutation in VAM3/SYP22 (vam3-1) caused pleiotropic abnormalities, including semi-dwarfism and wavy leaves. In contrast, a loss-of-function mutant of PEP12/SYP21 (pep12) showed no apparent abnormal phenotype. We also found that the double vam3-1 pep12 mutant had severely reduced fertilization competence, although male and female gametophytes (vam3-1(-) pep12(-) ) maintained the ability to fertilize. Moreover, promoter swapping analysis revealed that expression of a GFP-PEP12/SYP21 fusion under the control of the VAM3/SYP22 promoter suppressed all phenotypes of the vam3-1 mutant. These results indicate that the functions of VAM3/SYP22 and PEP12/SYP21 were redundant and interchangeable.

Minimal membrane docking requirements revealed by reconstitution of Rab GTPase-dependent membrane fusion from purified components

Rab GTPases and their effectors mediate docking, the initial contact of intracellular membranes preceding bilayer fusion. However, it has been unclear whether Rab proteins and effectors are sufficient for intermembrane interactions. We have recently reported reconstituted membrane fusion that requires yeast vacuolar SNAREs, lipids, and the homotypic fusion and vacuole protein sorting (HOPS)/class C Vps complex, an effector and guanine nucleotide exchange factor for the yeast vacuolar Rab GTPase Ypt7p. We now report reconstitution of lysis-free membrane fusion that requires purified GTP-bound Ypt7p, HOPS complex, vacuolar SNAREs, ATP hydrolysis, and the SNARE disassembly catalysts Sec17p and Sec18p. We use this reconstituted system to show that SNAREs and Sec17p/Sec18p, and Ypt7p and the HOPS complex, are required for stable intermembrane interactions and that the three vacuolar Q-SNAREs are sufficient for these interactions.

Vps-C complexes: gatekeepers of endolysosomal traffic

Genetic studies in yeast, plants, insects, and mammals have identified four universally conserved proteins, together called Vps Class C, that are essential for late endosome and lysosome assembly and for numerous endolysosomal trafficking pathways, including the terminal stages of autophagy. Two Vps-C complexes, HOPS and CORVET, incorporate diverse biochemical functions: they tether membranes, stimulate Rab nucleotide exchange, guide SNARE assembly to drive membrane fusion, and possibly act as ubiquitin ligases. Recent studies offer new insight into the complex relationships between Vps-C complexes and their cognate Rab small GTP-binding (G-)proteins at endosomes and lysosomes. Accumulating evidence supports the view that Vps-C complexes implement a regulatory logic that governs endomembrane identity and dynamics.

Sec18p and Vam7p remodel trans-SNARE complexes to permit a lipid-anchored R-SNARE to support yeast vacuole fusion

Intracellular membrane fusion requires SNARE proteins in a trans-complex, anchored to apposed membranes. Proteoliposome studies have suggested that SNAREs drive fusion by stressing the lipid bilayer via their transmembrane domains (TMDs), and that SNARE complexes require a TMD in each docked membrane to promote fusion. Yeast vacuole fusion is believed to require three Q-SNAREs from one vacuole and the R-SNARE Nyv1p from its fusion partner. In accord with this model, we find that fusion is abolished when the TMD of Nyv1p is replaced by lipid anchors, even though lipid-anchored Nyv1p assembles into trans-SNARE complexes. However, normal fusion is restored by the addition of both Sec18p and the soluble SNARE Vam7p. In restoring fusion, Sec18p promotes the disassembly of trans-SNARE complexes, and Vam7p enhances their assembly. Thus, either the TMD of this R-SNARE is not essential for fusion, and TMD-mediated membrane stress is not the only mode of trans-SNARE complex action, or these SNAREs have more flexibility than heretofore appreciated to form alternate functional complexes that violate the 3Q:1R rule.

The ergosterol biosynthesis inhibitor zaragozic acid promotes vacuolar degradation of the tryptophan permease Tat2p in yeast

Ergosterol is the yeast functional equivalent of cholesterol in mammalian cells. Deletion of the ERG6 gene, which encodes an enzyme catalyzing a late step of ergosterol biosynthesis, impedes targeting of the tryptophan permease Tat2p to the plasma membrane, but does not promote vacuolar degradation. It is unknown whether similar features appear when other steps of ergosterol biogenesis are inhibited. We show herein that the ergosterol biosynthesis inhibitor zaragozic acid (ZA) evoked massive vacuolar degradation of Tat2p, accompanied by a decrease in tryptophan uptake. ZA inhibits squalene synthetase (SQS, EC 2.5.1.21), which catalyzes the first committed step in the formation of cholesterol/ergosterol. The degradation of Tat2p was dependent on the Rsp5p-mediated ubiquitination of Tat2p and was not suppressed by deletions of VPS1, VPS27, VPS45 or PEP12. We will discuss ZA-mediated Tat2p degradation in the context of lipid rafts.

Exo70p mediates the secretion of specific exocytic vesicles at early stages of the cell cycle for polarized cell growth

In budding yeast, two classes of post-Golgi secretory vesicles carrying different sets of cargoes typified by Bgl2p and invertase are delivered to the plasma membrane for secretion. The exocyst is implicated in tethering these vesicles to the daughter cell membrane for exocytosis. In this study, we report that mutations in the exocyst component Exo70p predominantly block secretion of the Bgl2p vesicles. Furthermore, a defect in invertase vesicle trafficking caused by vps1Δ or pep12Δ in the exo70 mutant background is detrimental to the cell. The secretion defect in exo70 mutants was most pronounced during the early budding stage, which affected daughter cell growth. The selective secretion block does not occur at the vesicle formation or sorting stage because the exocytic vesicles are properly generated and protein processing is normal in the exo70 mutants. Our study suggests that Exo70p functions primarily at early stages of the cell cycle in Bgl2p vesicle secretion, which is critical for polarized cell growth.

Overexpression of the Arabidopsis syntaxin PEP12/SYP21 inhibits transport from the prevacuolar compartment to the lytic vacuole in vivo

Golgi-mediated transport to the lytic vacuole involves passage through the prevacuolar compartment (PVC), but little is known about how vacuolar proteins exit the PVC. We show that this last step is inhibited by overexpression of Arabidopsis thaliana syntaxin PEP12/SYP21, causing an accumulation of soluble and membrane cargo and the plant vacuolar sorting receptor BP80 in the PVC. Anterograde transport proceeds normally from the endoplasmic reticulum to the Golgi and the PVC, although export from the PVC appears to be compromised, affecting both anterograde membrane flow to the vacuole and the recycling route of BP80 to the Golgi. However, Golgi-mediated transport of soluble and membrane cargo toward the plasma membrane is not affected, but a soluble BP80 ligand is partially mis-sorted to the culture medium. We also observe clustering of individual PVC bodies that move together and possibly fuse with each other, forming enlarged compartments. We conclude that PEP12/SYP21 overexpression specifically inhibits export from the PVC without affecting the Golgi complex or compromising the secretory branch of the endomembrane system. The results provide a functional in vivo assay that confirms PEP12/SYP21 involvement in vacuolar sorting and indicates that excess of this syntaxin in the PVC can be detrimental for further transport from this organelle.

Extracting Sequence Motifs and the Phylogenetic Features of SNARE-Dependent Membrane Traffic

The SNARE proteins are required for membrane fusion during intracellular vesicular transport and for its specificity. Only the unique combination of SNARE proteins (cognates) can be bound and can lead to membrane fusion, although the characteristics of the possible specificity of the binding combinations encoded in the SNARE sequences have not yet been determined. We discovered by whole genome sequence analysis that sequence motifs (conserved sequences) in the SNARE motif domains for each protein group correspond to localization sites or transport pathways. We claim that these motifs reflect the specificity of the binding combinations of SNARE motif domains. Using these motifs, we could classify SNARE proteins from 48 organisms into their localization sites or transport pathways. The classification result shows that more than 10 SNARE subgroups are kingdom specific and that the SNARE paralogs involved in the plasma membrane-related transport pathways have developed greater variations in higher animals and higher plants than those involved in the endoplasmic reticulum-related transport pathways throughout eukaryotic evolution.

Homotypic fusion of early endosomes: SNAREs do not determine fusion specificity

Membrane fusion in the secretory pathway is mediated by soluble N-ethylmaleimide-sensitive factor attachment receptor (SNARE) proteins. Different fusion steps are thought to be effected by independent sets of SNAREs, but it is unclear whether specificity is determined by an intrinsic specificity of SNARE pairing or by upstream factors. Using a newly developed microscopy-based assay, we have investigated the SNARE specificity of homotypic early endosomal fusion. We show that early endosomes contain multiple sets of SNAREs, including, in addition to the putative early endosomal SNAREs, those involved in exocytosis and in fusion of late endosomes. We demonstrate that fusion is largely mediated by a complex formed by syntaxin 13, syntaxin 6, vti1a, and VAMP4, whereas the exocytic and late endosomal SNAREs play little or no role in the reaction. In contrast, proteoliposomes reconstituted with early endosomal SNAREs promiscuously fuse with liposomes containing exocytotic or late endosomal SNAREs. We conclude that the specificity of SNARE pairing does not suffice to determine the specificity of organelle fusion.

Identification of an allele of VAM3/SYP22 that confers a semi-dwarf phenotype in Arabidopsis thaliana

The short stem and midrib (ssm) mutants of Arabidopsis thaliana show both semi-dwarf and wavy leaf phenotypes due to defects in the elongation of the stem internodes and leaves. Moreover, these abnormalities cannot be recovered by exogenous phytohormones. ssm was originally identified as a single recessive mutant of the ecotype Columbia (Col-0), but genetic crossing experiments have revealed that this mutant phenotype is restored by another gene that is functional in the ecotype Landsberg erecta (Ler) and not in Col-0. Map-based cloning of the gene that is defective in ssm mutants has uncovered a small deletion in the sixth intron of a gene encoding a syntaxin, VAM3/SYP22, which has been implicated in vesicle transport to the vacuole. This mutation appears to cause a peptide insertion in the deduced VAM3/SYP22 polypeptide sequence due to defective splicing of the shortened sixth intron. Significantly, when compared with the wild-type Ler genome, the wild-type Col-0 genome has a single base pair deletion causing a frameshift mutation in SYP23, a gene with the highest known homology to VAM3/SYP22. These findings suggest that VAM3/SYP22 and SYP23 have overlapping functions and that the vesicle transport mediated by these syntaxins is important for shoot morphogenesis.

Identification of cytoplasmic residues of Sec61p involved in ribosome binding and cotranslational translocation

The cytoplasmic surface of Sec61p is the binding site for the ribosome and has been proposed to interact with the signal recognition particle receptor during targeting of the ribosome nascent chain complex to the translocation channel. Point mutations in cytoplasmic loops six (L6) and eight (L8) of yeast Sec61p cause reductions in growth rates and defects in the translocation of nascent polypeptides that use the cotranslational translocation pathway. Sec61 heterotrimers isolated from the L8 sec61 mutants have a greatly reduced affinity for 80S ribosomes. Cytoplasmic accumulation of protein precursors demonstrates that the initial contact between the large ribosomal subunit and the Sec61 complex is important for efficient insertion of a nascent polypeptide into the translocation pore. In contrast, point mutations in L6 of Sec61p inhibit cotranslational translocation without significantly reducing the ribosome-binding activity, indicating that the L6 and L8 sec61 mutants affect different steps in the cotranslational translocation pathway.

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.

What is the role of SNARE proteins in membrane fusion?

Soluble N-ethylmaleimide-sensitive factor activating protein receptor (SNARE) proteins have been at the fore-front of research on biological membrane fusion for some time. The subcellular localization of SNAREs and their ability to form the so-called SNARE complex may be integral to determining the specificity of intracellular fusion (the SNARE hypothesis) and/or serving as the minimal fusion machinery. Both the SNARE hypothesis and the idea of the minimal fusion machinery have been challenged by a number of experimental observations in various model systems, suggesting that SNAREs may have other functions. Considering recent advances in the SNARE literature, it appears that SNAREs may actually function as part of a complex fusion "machine." Their role in the machinery could be any one or a combination of roles, including establishing tight membrane contact, formation of a scaffolding on which to build the machine, binding of lipid surfaces, and many others. It is also possible that complexations other than the classic SNARE complex participate in membrane fusion.

The amino-terminal domain of the vacuolar proton-translocating ATPase a subunit controls targeting and in vivo dissociation, and the carboxyl-terminal domain affects coupling of proton transport and ATP hydrolysis

The 100-kDa "a" subunit of the vacuolar proton-translocating ATPase (V-ATPase) is encoded by two genes in yeast, VPH1 and STV1. The Vph1p-containing complex localizes to the vacuole, whereas the Stv1p-containing complex resides in some other intracellular compartment, suggesting that the a subunit contains information necessary for the correct targeting of the V-ATPase. We show that Stv1p localizes to a late Golgi compartment at steady state and cycles continuously via a prevacuolar endosome back to the Golgi. V-ATPase complexes containing Vph1p and Stv1p also differ in their assembly properties, coupling of proton transport to ATP hydrolysis, and dissociation in response to glucose depletion. To identify the regions of the a subunit that specify these different properties, chimeras were constructed containing the cytosolic amino-terminal domain of one isoform and the integral membrane, carboxyl-terminal domain from the other isoform. Like the Stv1p-containing complex, the V-ATPase complex containing the chimera with the amino-terminal domain of Stv1p localized to the Golgi and the complex did not dissociate in response to glucose depletion. Like the Vph1p-containing complex, the V-ATPase complex containing the chimera with the amino-terminal domain of Vph1p localized to the vacuole and the complex exhibited normal dissociation upon glucose withdrawal. Interestingly, the V-ATPase complex containing the chimera with the carboxyl-terminal domain of Vph1p exhibited a higher coupling of proton transport to ATP hydrolysis than the chimera containing the carboxyl-terminal domain of Stv1p. Our results suggest that whereas targeting and in vivo dissociation are controlled by sequences located in the amino-terminal domains of the subunit a isoforms, coupling efficiency is controlled by the carboxyl-terminal region.

Functional analysis of conserved structural elements in yeast syntaxin Vam3p

Vam3p, a syntaxin-like SNARE protein involved in yeast vacuole fusion, is composed of a three-helical N-terminal domain, a canonical SNARE motif, and a C-terminal transmembrane region (TMR). Surprisingly, we find that the N-terminal domain of Vam3p is not essential for fusion, although analogous domains in other syntaxins are indispensible for fusion and/or protein-protein interactions. In contrast to the N-terminal domain, mutations in the SNARE motif of Vam3p or replacement of the SNARE motif of Vam3p with the SNARE motif from other syntaxins inhibited fusion. Furthermore, the precise distance between the SNARE motif and the TMR was critical for fusion. Insertion of only three residues after the SNARE motif significantly impaired fusion and insertion of 12 residues abolished fusion. As judged by co-immunoprecipitation experiments, the SNARE motif mutations and the insertions did not alter the association of Vam3p with Vam7p, Vti1p, Nyv1p, and Ykt6p, other vacuolar SNARE proteins implicated in fusion. In contrast, the SNARE motif substitutions interfered with the stable formation of Vam3p complexes with Nyv1p and Vti1p, although Vam3p complexes with Vam7p and Ykt6p were still present. Our data suggest that in contrast to previously characterized syntaxins, Vam3p contains only two domains essential for fusion, the SNARE motif and the TMR, and these domains have to be closely coupled to function in fusion.

The sodium/proton exchanger Nhx1p is required for endosomal protein trafficking in the yeast Saccharomyces cerevisiae

We show that the vacuolar protein sorting gene VPS44 is identical to NHX1, a gene that encodes a sodium/proton exchanger. The Saccharomyces cerevisiae protein Nhx1p shows high homology to mammalian sodium/proton exchangers of the NHE family. Nhx1p is thought to transport sodium ions into the prevacuole compartment in exchange for protons. Pulse-chase experiments show that approximately 35% of the newly synthesized soluble vacuolar protein carboxypeptidase Y is missorted in nhx1 delta cells, and is secreted from the cell. nhx1 delta cells accumulate late Golgi, prevacuole, and lysosome markers in an aberrant structure next to the vacuole, and late Golgi proteins are proteolytically cleaved more rapidly than in wild-type cells. Our results show that efficient transport out of the prevacuolar compartment requires Nhx1p, and that nhx1 delta cells exhibit phenotypes characteristic of the "class E" group of vps mutants. In addition, we show that Nhx1p is required for protein trafficking even in the absence of the vacuolar ATPase. Our analysis of Nhx1p provides the first evidence that a sodium/proton exchange protein is important for correct protein sorting, and that intraorganellar ion balance may be important for endosomal function in yeast.

Pep12p is a multifunctional yeast syntaxin that controls entry of biosynthetic, endocytic and retrograde traffic into the prevacuolar compartment

Delivery of proteins to the vacuole of the yeast Saccharomyces cerevisiae requires the function of the endosomal syntaxin, Pep12p. Many vacuolar proteins, such as the soluble vacuolar hydrolase, carboxypeptidase Y (CPY), traverse the prevacuolar compartment (PVC) en route to the vacuole. Here we show that deletion of the carboxy-terminal transmembrane domain of Pep12p results in a temperature-conditional block in transport of CPY to the PVC. The PVC also receives traffic from the early endosome and the vacuole, and mutation in PEP12 also blocks these other trafficking pathways into the PVC. Therefore, Pep12p is a multifunctional syntaxin that is required for all known trafficking pathways into the yeast PVC. Finally, we found that the internalized pheromone receptor, Ste3p, can cycle out of the PVC in a VPS27-independent fashion.