Tethering guides fusion-competent trans-SNARE assembly

Reconstitution of autophagosomal membrane tethering reveals that the ability of Atg11 to bind membranes is important for mitophagy

Autophagy is essential for the degradation of mitochondria from yeast to humans. Mitochondrial autophagy in yeast is initiated when the selective autophagy scaffolding protein Atg11 is recruited to mitochondria through its interaction with the selective autophagy receptor Atg32. This also results in the recruitment of small 30 nm vesicles that fuse to generate the initial autophagosomal membrane. We demonstrate that Atg11 can bind to autophagosomal-like membranes in vitro in a curvature dependent manner via a predicted amphipathic helix. Deletion of the amphipathic helix from Atg11 results in a delay in mitophagy in yeast. Furthermore, using a novel biochemical approach we demonstrate that the interaction between Atg11 and Atg32 results in the tethering of autophagosomal-like vesicles to giant unilamellar vesicles containing a lipid composition designed to mimic the outer mitochondrial membrane. Taken together our results demonstrate an important role for autophagosomal membrane binding by Atg11 in the initiation of mitochondrial autophagy.

SM protein Sly1 and a SNARE Habc domain promote membrane fusion through multiple mechanisms

SM proteins including Sly1 are essential cofactors of SNARE-mediated membrane fusion. Using SNARE and Sly1 mutants and chemically defined in vitro assays, we separate and assess proposed mechanisms through which Sly1 augments fusion: (i) opening the closed conformation of the Qa-SNARE Sed5; (ii) close-range tethering of vesicles to target organelles, mediated by the Sly1-specific regulatory loop; and (iii) nucleation of productive trans-SNARE complexes. We show that all three mechanisms are important and operate in parallel, and that close-range tethering promotes trans-complex assembly when cis-SNARE assembly is a competing process. Further, we demonstrate that the autoinhibitory N-terminal Habc domain of Sed5 has at least two positive activities: it is needed for correct Sed5 localization, and it directly promotes Sly1-dependent fusion. "Split Sed5," with Habc presented solely as a soluble fragment, can function both in vitro and in vivo. Habc appears to facilitate events leading to lipid mixing rather than promoting opening or stability of the fusion pore.

Sec18 binds the tethering/SM complex HOPS to engage the Qc-SNARE for membrane fusion

Membrane fusion is regulated by Rab GTPases, their tethering effectors such as HOPS, SNARE proteins on each fusion partner, SM proteins to catalyze SNARE assembly, Sec17 (SNAP), and Sec18 (NSF). Though concentrated HOPS can support fusion without Sec18, we now report that fusion falls off sharply at lower HOPS levels, where direct Sec18 binding to HOPS restores fusion. This Sec18-dependent fusion needs adenine nucleotide but neither ATP hydrolysis nor Sec17. Sec18 enhances HOPS recognition of the Qc-SNARE. With high levels of HOPS, Qc has a Km for fusion of a few nM. Either lower HOPS levels, or substitution of a synthetic tether for HOPS, strikingly increases the Km for Qc to several hundred nM. With dilute HOPS, Sec18 returns the Km for Qc to low nM. In contrast, HOPS concentration and Sec18 have no effect on Qb-SNARE recognition. Just as Qc is required for fusion but not for the initial assembly of SNAREs in trans, impaired Qc recognition by limiting HOPS without Sec18 still allows substantial trans-SNARE assembly. Thus, in addition to the known Sec18 functions of disassembling SNARE complexes, oligomerizing Sec17 for membrane association, and allowing Sec17 to drive fusion without complete SNARE zippering, we report a fourth Sec18 function, the Sec17-independent binding of Sec18 to HOPS to enhance functional Qc-SNARE engagement.

Model-based inference of a plant-specific dual role for HOPS in regulating guard cell vacuole fusion

Stomata are the pores on a leaf surface that regulate gas exchange. Each stoma consists of two guard cells whose movements regulate pore opening and thereby control CO2 fixation and water loss. Guard cell movements depend in part on the remodeling of vacuoles, which have been observed to change from a highly fragmented state to a fused morphology during stomata opening. This change in morphology requires a membrane fusion mechanism that responds rapidly to environmental signals, allowing plants to respond to diurnal and stress cues. With guard cell vacuoles being both large and responsive to external signals, stomata represent a unique system in which to delineate mechanisms of membrane fusion. Fusion of vacuole membranes is a highly conserved process in eukaryotes, with key roles played by two multi-subunit complexes: HOPS (homotypic fusion and vacuolar protein sorting) and SNARE (soluble NSF attachment protein receptor). HOPS is a vacuole tethering factor that is thought to chaperone SNAREs from apposing vacuole membranes into a fusion-competent complex capable of rearranging membranes. To resolve a counter-intuitive observation regarding the role of HOPS in regulating plant vacuole morphology, we derived a quantitative model of vacuole fusion dynamics and used it to generate testable predictions about HOPS-SNARE interactions. We derived our model by applying simulation-based inference to integrate prior knowledge about molecular interactions with limited, qualitative observations of emergent vacuole phenotypes. By constraining the model parameters to yield the emergent outcomes observed for stoma opening - as induced by two distinct chemical treatments - we predicted a dual role for HOPS and identified a stalled form of the SNARE complex that differs from phenomena reported in yeast. We predict that HOPS has contradictory actions at different points in the fusion signaling pathway, promoting the formation of SNARE complexes, but limiting their activity.

Synergism of vesicle trafficking and cytoskeleton during regulation of plant growth and development: A mechanistic outlook

The cytoskeleton is a fundamental component found in all eukaryotic organisms, serving as a critical factor in various essential cyto-biological mechanisms, particularly in the locomotion and morphological transformations of plant cells. The cytoskeleton is comprised of three main components: microtubules (MT), microfilaments (MF), and intermediate filaments (IF). The cytoskeleton plays a crucial role in the process of cell wall formation and remodeling throughout the growth and development of cells. It is a highly organized and regulated network composed of filamentous components. In the basic processes of intracellular transport, such as mitosis, cytokinesis, and cell polarity, the plant cytoskeleton plays a crucial role according to recent studies. The major flaws in the organization of the cytoskeletal framework are at the root of the aberrant organogenesis currently observed in plant mutants. The regulation of protein compartmentalization and abundance within cells is predominantly governed by the process of vesicle/membrane transport, which plays a crucial role in several signaling cascades.The regulation of membrane transport in eukaryotic cells is governed by a diverse array of proteins. Recent developments in genomics have provided new tools to study the evolutionary relationships between membrane proteins in different plant species. It is known that members of the GTPases, COP, SNAREs, Rabs, tethering factors, and PIN families play essential roles in vesicle transport between plant, animal, and microbial species. This Review presents the latest research on the plant cytoskeleton, focusing on recent developments related to the cytoskeleton and summarizing the role of various proteins in vesicle transport. In addition, the report predicts future research direction of plant cytoskeleton and vesicle trafficking, potential research priorities, and provides researchers with specific pointers to further investigate the significant link between cytoskeleton and vesicle trafficking.

MARCKS Effector Domain, a reversible lipid ligand, illuminates late stages of membrane fusion

Yeast vacuolar HOPS tethers membranes, catalyzes trans-SNARE assembly between R- and Q-SNAREs, and shepherds SNAREs past early inhibition by Sec17. After partial SNARE zippering, fusion is driven slowly by either completion of SNARE zippering or by Sec17/Sec18, but rapid fusion needs zippering and Sec17/Sec18. Using reconstituted-vacuolar fusion, we find that MARCKS Effector Domain (MED) peptide, a lipid ligand, blocks fusion reversibly at a late reaction stage. The MED fusion blockade is overcome by either salt extraction, inactivation with the MED ligand calmodulin, or addition of Sec17/Sec18. During incubation with MED, SNAREs assemble stable complexes in trans and fusion becomes resistant to antibody to the Qa SNARE. When Q-SNAREs are preassembled, a synthetic tether can replace HOPS for fusion. With a synthetic tether, fusion needs both complete SNARE zippering and Sec17/Sec18 to overcome a MED block. In contrast, when SNARE domains are only two-third zippered, only HOPS will support Sec17/Sec18 driven fusion without needing complete zippering. HOPS thus remains engaged with SNAREs during zippering. MED facilitates the study of distinct fusion stages: tethering, initial trans-SNARE assembly and its sensitivity to Sec17, SNARE zippering, Sec17/Sec18 engagement, and lipid and lumenal mixing.

NME3 binds to phosphatidic acid and mediates PLD6-induced mitochondrial tethering

Mitochondria are dynamic organelles regulated by fission and fusion processes. The fusion of membranes requires elaborative coordination of proteins and lipids and is particularly crucial for the function and quality control of mitochondria. Phosphatidic acid (PA) on the mitochondrial outer membrane generated by PLD6 facilitates the fusion of mitochondria. However, how PA promotes mitochondrial fusion remains unclear. Here, we show that a mitochondrial outer membrane protein, NME3, is required for PLD6-induced mitochondrial tethering or clustering. NME3 is enriched at the contact interface of two closely positioned mitochondria depending on PLD6, and NME3 binds directly to PA-exposed lipid packing defects via its N-terminal amphipathic helix. The PA binding function and hexamerization confer NME3 mitochondrial tethering activity. Importantly, nutrient starvation enhances the enrichment efficiency of NME3 at the mitochondrial contact interface, and the tethering ability of NME3 contributes to fusion efficiency. Together, our findings demonstrate NME3 as a tethering protein promoting selective fusion between PLD6-remodeled mitochondria for quality control.

Efficient fusion requires a membrane anchor on the vacuolar Qa-SNARE

As a prelude to fusion, the R-SNARE on one membrane zippers with Qa-, Qb-, and Qc-SNAREs from its apposed fusion partner, forming a four-helical bundle that draws the two membranes together. Because Qa- and Qb-SNAREs are anchored to the same membrane and are adjacent in the 4-SNARE bundle, their two anchors might be redundant. Using the recombinant pure protein catalysts of yeast vacuole fusion, we now report that the specific distribution of transmembrane (TM) anchors on the Q-SNAREs is critical for efficient fusion. A TM anchor on the Qa-SNARE supports rapid fusion even when the other two Q-SNAREs are unanchored, while a TM anchor on the Qb-SNARE is dispensable and is insufficient for rapid fusion as the sole Q-SNARE anchor. This does not depend on which specific TM domain is attached to the Qa-SNARE but rather is due to the Qa-SNARE being anchored per se. The need for Qa-SNARE anchoring is even seen when the homotypic fusion and vacuole protein sorting protein (HOPS), the physiological catalyst of tethering and SNARE assembly, is replaced by an artificial tether. The need for a Qa TM anchor is thus a fundamental property of vacuolar SNARE zippering-induced fusion and may reflect the need for the Qa juxtamembrane (JxQa) region to be anchored between its SNARE and TM domains. This requirement for Qa-SNARE anchoring and correct JxQa position is bypassed by Sec17/Sec18, exploiting a platform of partially zippered SNAREs. Because Qa is the only synaptic Q-SNARE with a TM anchor, the need for Qa-specific anchoring may reflect a general requirement for SNARE-mediated fusion.

PI3P regulates multiple stages of membrane fusion

The conserved catalysts of intracellular membrane fusion are Rab-family GTPases, effector complexes that bind Rabs for membrane tethering, SNARE proteins of the R, Qa, Qb, and Qc families, and SNARE chaperones of the SM, Sec17/SNAP, and Sec18/NSF families. Yeast vacuole fusion is regulated by phosphatidylinositol-3-phosphate (PI3P). PI3P binds directly to the vacuolar Qc-SNARE and to HOPS, the vacuolar tethering/SM complex. We now report several distinct functions of PI3P in fusion. PI3P binds the N-terminal PX domain of the Qc-SNARE to enhance its engagement for fusion. Even when Qc has been preassembled with the Qa- and Qb-SNAREs, PI3P still promotes trans-SNARE assembly and fusion between these 3Q proteoliposomes and those with R-SNARE, whether with the natural HOPS tether or with a synthetic tether. With HOPS, efficient trans-SNARE complex formation needs PI3P on the 3Q-SNARE proteoliposomes, in cis to the Qc. PI3P is also needed for HOPS to confer resistance to Sec17/Sec18. With a synthetic tether, fusion is supported by PI3P on either fusion partner membrane, but this fusion is blocked by Sec17/Sec18. PI3P thus supports multiple stages of fusion: the engagement of the Qc-SNARE, trans-SNARE complex formation with preassembled Q-SNAREs, HOPS protection of SNARE complexes from Sec17/Sec18, and fusion per se after tethering and Q-SNARE assembly.

Sec18 supports membrane fusion by promoting Sec17 membrane association

Membrane fusion is driven by Sec17, Sec18, and SNARE zippering. Sec17 bound to SNAREs promotes fusion through its membrane-proximal N-terminal apolar loop domain. At its membrane-distal end, Sec17 serves as a high-affinity receptor for Sec18. At that distance from the fusion site, it has been unclear how Sec18 can aid Sec17 to promote fusion. We now report that Sec18, with ATPγS, lowers the Km of Sec17 for fusion. A C-terminal and membrane-distal Sec17 mutation, L291A,L292A, diminishes Sec17 affinity for Sec18. High levels of wild-type Sec17 or Sec17-L291AL292A show equivalent fusion without Sec18, but Sec18 causes far less fusion enhancement with low levels of Sec17-L291AL292A than with wild-type Sec17. Another mutant, Sec17-F21SM22S, has reduced N-loop apolarity. Only very high levels of this mutant protein support fusion, but Sec18 still lowers the apparent fusion Km for Sec17-F21SM22S. Thus Sec18 stimulates fusion through Sec17 and acts at the well-described interface between Sec18 and Sec17. ATP acts as a ligand to activate Sec18 for Sec17-dependent fusion, but ATP hydrolysis is not required. Even without SNAREs, Sec18 and Sec17 exhibit interdependent stable association with lipids, with several Sec17 bound for each Sec18 hexamer, explaining how Sec18 stabilization of surface-concentrated clusters of Sec17 lowers the Sec17 Km for assembly with SNAREs. Each of the associations, between SNARE complex, Sec18, Sec17, and lipid, helps assemble the fusion machinery.

Emerging Concepts in Defective Macrophage Phagocytosis in Cystic Fibrosis

Cystic fibrosis (CF) is caused by mutations of the cystic fibrosis transmembrane conductance regulator (CFTR) gene. Chronic inflammation and decline in lung function are major reasons for morbidity in CF. Mutant CFTR expressed in phagocytic cells such as macrophages contributes to persistent infection, inflammation, and lung disease in CF. Macrophages play a central role in innate immunity by eliminating pathogenic microbes by a process called phagocytosis. Phagocytosis is required for tissue homeostasis, balancing inflammation, and crosstalk with the adaptive immune system for antigen presentation. This review focused on (1) current understandings of the signaling underlying phagocytic mechanisms; (2) existing evidence for phagocytic dysregulation in CF; and (3) the emerging role of CFTR modulators in influencing CF phagocytic function. Alterations in CF macrophages from receptor initiation to phagosome formation are linked to disease progression in CF. A deeper understanding of macrophages in the context of CFTR and phagocytosis proteins at each step of phagosome formation might contribute to the new therapeutic development of dysregulated innate immunity in CF. Therefore, the review also indicates future areas of research in the context of CFTR and macrophages.

Fusion with wild-type SNARE domains is controlled by juxtamembrane domains, transmembrane anchors, and Sec17

Membrane fusion requires tethers, SNAREs of R, Qa, Qb, and Qc families, and chaperones of the SM, Sec17/SNAP, and Sec18/NSF families. SNAREs have N-domains, SNARE domains that zipper into 4-helical RQaQbQc coiled coils, a short juxtamembrane (Jx) domain, and (often) a C-terminal transmembrane anchor. We reconstitute fusion with purified components from yeast vacuoles, where the HOPS protein combines tethering and SM functions. The vacuolar Rab, lipids, and R-SNARE activate HOPS to bind Q-SNAREs and catalyze trans-SNARE associations. With SNAREs initially disassembled, as they are on the organelle, we now report that R- and Qa-SNAREs require their physiological juxtamembrane (Jx) regions for fusion. Swap of the Jx domain between the R- and Qa-SNAREs blocks fusion after SNARE association in trans. This block is bypassed by either Sec17, which drives fusion without requiring complete SNARE zippering, or transmembrane-anchored Qb-SNARE in complex with Qa. The abundance of the trans-SNARE complex is not the sole fusion determinant, as it is unaltered by Sec17, Jx swap, or the Qb-transmembrane anchor. The sensitivity of fusion to Jx swap in the absence of a Qb transmembrane anchor is inherent to the SNAREs, because it remains when a synthetic tether replaces HOPS.

Phosphatidylinositol and phosphatidylinositol-3-phosphate activate HOPS to catalyze SNARE assembly, allowing small headgroup lipids to support the terminal steps of membrane fusion

Intracellular membrane fusion requires Rab GTPases, tethers, SNAREs of the R, Qa, Qb, and Qc families, and SNARE chaperones of the Sec17 (SNAP), Sec18 (NSF), and SM (Sec1/Munc18) families. The vacuolar HOPS complex combines the functions of membrane tethering and SM catalysis of SNARE assembly. HOPS is activated for this catalysis by binding to the vacuolar lipids and Rab. Of the eight major vacuolar lipids, we now report that phosphatidylinositol and phosphatidylinositol-3-phosphate are required to activate HOPS for SNARE complex assembly. These lipids plus ergosterol also allow full trans-SNARE complex assembly, yet do not support fusion, which is reliant on either phosphatidylethanolamine (PE) or on phosphatidic acid (PA), phosphatidylserine (PS), and diacylglycerol (DAG). Fusion with a synthetic tether and without HOPS, or even without SNAREs, still relies on either PE or on PS, PA, and DAG. These lipids are thus required for the terminal bilayer rearrangement step of fusion, distinct from the lipid requirements for the earlier step of activating HOPS for trans-SNARE assembly.

Fusion of tethered membranes can be driven by Sec18/NSF and Sec17/αSNAP without HOPS

Yeast vacuolar membrane fusion has been reconstituted with R, Qa, Qb, and Qc-family SNAREs, Sec17/αSNAP, Sec18/NSF, and the hexameric HOPS complex. HOPS tethers membranes and catalyzes SNARE assembly into RQaQbQc trans-complexes which zipper through their SNARE domains to promote fusion. Previously, we demonstrated that Sec17 and Sec18 can bypass the requirement of complete zippering for fusion (Song et al., 2021), but it has been unclear whether this activity of Sec17 and Sec18 is directly coupled to HOPS. HOPS can be replaced for fusion by a synthetic tether when the three Q-SNAREs are pre-assembled. We now report that fusion intermediates with arrested SNARE zippering, formed with a synthetic tether but without HOPS, support Sec17/Sec18-triggered fusion. This zippering-bypass fusion is thus a direct result of Sec17 and Sec18 interactions: with each other, with the platform of partially zippered SNAREs, and with the apposed tethered membranes. As these fusion elements are shared among all exocytic and endocytic traffic, Sec17 and Sec18 may have a general role in directly promoting fusion.

Sec17/Sec18 can support membrane fusion without help from completion of SNARE zippering

Membrane fusion requires R-, Qa-, Qb-, and Qc-family SNAREs that zipper into RQaQbQc coiled coils, driven by the sequestration of apolar amino acids. Zippering has been thought to provide all the force driving fusion. Sec17/αSNAP can form an oligomeric assembly with SNAREs with the Sec17 C-terminus bound to Sec18/NSF, the central region bound to SNAREs, and a crucial apolar loop near the N-terminus poised to insert into membranes. We now report that Sec17 and Sec18 can drive robust fusion without requiring zippering completion. Zippering-driven fusion is blocked by deleting the C-terminal quarter of any Q-SNARE domain or by replacing the apolar amino acids of the Qa-SNARE that face the center of the 4-SNARE coiled coils with polar residues. These blocks, singly or combined, are bypassed by Sec17 and Sec18, and SNARE-dependent fusion is restored without help from completing zippering.

Asymmetric Rab activation of vacuolar HOPS to catalyze SNARE complex assembly

Intracellular membrane fusion requires Rab-family GTPases, their effector tethers, soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins, and SNARE chaperones of the Sec1/Munc18 (SM), Sec17/α-SNAP, and Sec18/NSF families. We have developed an assay using fluorescence resonance energy transfer to measure SNARE complex formation in real time. We now show that yeast vacuolar SNAREs assemble spontaneously into RQaQbQc complexes when the R- and Qa-SNAREs are concentrated in the same micelles or in cis on the same membrane. When SNAREs are free in solution or are tethered to distinct membranes, assembly requires catalysis by HOPS, the vacuolar SM and tethering complex. The Rab Ypt7 and vacuole lipids together allosterically activate the bound HOPS for catalyzing SNARE assembly, even if none of the SNAREs are membrane bound. HOPS-dependent fusion between proteoliposomes bearing R- or Qa-SNAREs shows a strict requirement for Ypt7 on the R-SNARE proteoliposomes but not on the Qa-SNARE proteoliposomes. This asymmetry is reflected in the strikingly different capacity of Ypt7 in cis to either the R- or Qa-SNARE to stimulate SNARE complex assembly. Membrane-bound Ypt7 activates HOPS to catalyze 4-SNARE complex assembly when it is on the same membrane as the R-SNARE but not the Qa-SNARE, thus explaining the asymmetric need for Ypt7 for fusion.

Re-examining how Munc13-1 facilitates opening of syntaxin-1

Munc13-1 is crucial for neurotransmitter release and, together with Munc18-1, orchestrates assembly of the neuronal SNARE complex formed by syntaxin-1, SNAP-25, and synaptobrevin. Assembly starts with syntaxin-1 folded into a self-inhibited closed conformation that binds to Munc18-1. Munc13-1 is believed to catalyze the opening of syntaxin-1 to facilitate SNARE complex formation. However, different types of Munc13-1-syntaxin-1 interactions have been reported to underlie this activity, and the critical nature of Munc13-1 for release may arise because of its key role in bridging the vesicle and plasma membranes. To shed light into the mechanism of action of Munc13-1, we have used NMR spectroscopy, SNARE complex assembly experiments, and liposome fusion assays. We show that point mutations in a linker region of syntaxin-1 that forms intrinsic part of the closed conformation strongly impair stimulation of SNARE complex assembly and liposome fusion mediated by Munc13-1 fragments, even though binding of this linker region to Munc13-1 is barely detectable. Conversely, the syntaxin-1 SNARE motif clearly binds to Munc13-1, but a mutation that disrupts this interaction does not affect SNARE complex assembly or liposome fusion. We also show that Munc13-1 cannot be replaced by an artificial tethering factor to mediate liposome fusion. Overall, these results emphasize how very weak interactions can play fundamental roles in promoting conformational transitions and strongly support a model whereby the critical nature of Munc13-1 for neurotransmitter release arises not only from its ability to bridge two membranes but also from an active role in opening syntaxin-1 via interactions with the linker.

HOPS recognizes each SNARE, assembling ternary trans-complexes for rapid fusion upon engagement with the 4th SNARE

Yeast vacuole fusion requires R-SNARE, Q-SNAREs, and HOPS. A HOPS SM-family subunit binds the R- and Qa-SNAREs. We now report that HOPS binds each of the four SNAREs. HOPS catalyzes fusion when the Q-SNAREs are not pre-assembled, ushering them into a functional complex. Co-incubation of HOPS, proteoliposomes bearing R-SNARE, and proteoliposomes with any two Q-SNAREs yields a rapid-fusion complex with 3 SNAREs in a trans-assembly. The missing Q-SNARE then induces sudden fusion. HOPS can 'template' SNARE complex assembly through SM recognition of R- and Qa-SNAREs. Though the Qa-SNARE is essential for spontaneous SNARE assembly, HOPS also assembles a rapid-fusion complex between R- and QbQc-SNARE proteoliposomes in the absence of Qa-SNARE, awaiting Qa for fusion. HOPS-dependent fusion is saturable at low concentrations of each Q-SNARE, showing binding site functionality. HOPS thus tethers membranes and recognizes each SNARE, assembling R+Qa or R+QbQc rapid fusion intermediates.

Sec17 (α-SNAP) and Sec18 (NSF) restrict membrane fusion to R-SNAREs, Q-SNAREs, and SM proteins from identical compartments

Membrane fusion at each organelle requires conserved proteins: Rab-GTPases, effector tethering complexes, Sec1/Munc18 (SM)-family SNARE chaperones, SNAREs of the R, Qa, Qb, and Qc families, and the Sec17/α-SNAP and ATP-dependent Sec18/NSF SNARE chaperone system. The basis of organelle-specific fusion, which is essential for accurate protein compartmentation, has been elusive. Rab family GTPases, SM proteins, and R- and Q-SNAREs may contribute to this specificity. We now report that the fusion supported by SNAREs alone is both inefficient and promiscuous with respect to organelle identity and to stimulation by SM family proteins or complexes. SNARE-only fusion is abolished by the disassembly chaperones Sec17 and Sec18. Efficient fusion in the presence of Sec17 and Sec18 requires a tripartite match between the organellar identities of the R-SNARE, the Q-SNAREs, and the SM protein or complex. The functions of Sec17 and Sec18 are not simply negative regulation; they stimulate fusion with either vacuolar SNAREs and their SM protein complex HOPS or endoplasmic reticulum/cis-Golgi SNAREs and their SM protein Sly1. The fusion complex of each organelle is assembled from its own functionally matching pieces to engage Sec17/Sec18 for fusion stimulation rather than inhibition.