Defining the functions of trans-SNARE pairs

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.

Yeast Sec1p functions before and after vesicle docking

Sec1/Munc18 (SM) proteins bind cognate soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complexes and stimulate vesicle membrane fusion. Before fusion, vesicles are docked to specific target membranes. Regulation of vesicle docking is attributed to some but not all SM proteins, suggesting specialization of this earlier function. Yeast Sec1p seems to function only after vesicles are docked and SNARE complexes are assembled. Here, we show that yeast Sec1p is required before and after SNARE complex assembly, in support of general requirements for SM proteins in both vesicle docking and fusion. Two classes of sec1 mutants were isolated. Class A mutants are tightly blocked in cell growth and secretion at a step before SNARE complex assembly. Class B mutants have a SNARE complex binding defect, with a range in severity of cell growth and secretion defects. Mapping the mutations onto an SM protein structure implicates a peripheral bundle of helices for the early, docking function and a deep groove, opposite the syntaxin-binding cleft on nSec1/Munc-18, for the interaction between Sec1p and the exocytic SNARE complex.

A conserved membrane attachment site in alpha-SNAP facilitates N-ethylmaleimide-sensitive factor (NSF)-driven SNARE complex disassembly

The ATPase NSF (N-ethylmaleimide-sensitive factor) and its SNAP (soluble N-ethylmaleimide-sensitive factor attachment protein) cofactor constitute the ubiquitous enzymatic machinery responsible for recycling of the SNARE (SNAP receptor) membrane fusion machinery. The enzyme uses the energy of ATP hydrolysis to dissociate the constituents of the SNARE complex, which is formed during the fusion of a transport vesicle with the acceptor membrane. However, it is still unclear how NSF and the SNAP adaptor work together to take the tight SNARE bundle apart. SNAPs have been reported to attach to membranes independently from SNARE complex binding. We have investigated how efficient the disassembly of soluble and membrane-bound substrates are, comparing the two. We found that SNAPs support disassembly of membrane-bound SNARE complexes much more efficiently. Moreover, we identified a putative, conserved membrane attachment site in an extended loop within the N-terminal domain of alpha-SNAP. Mutation of two highly conserved, exposed phenylalanine residues on the extended loop prevent SNAPs from facilitating disassembly of membrane-bound SNARE complexes. This implies that the disassembly machinery is adapted to attack membrane-bound SNARE complexes, probably in their relaxed cis-configuration.

Model of SNARE-mediated membrane adhesion kinetics

SNARE proteins are conserved components of the core fusion machinery driving diverse membrane adhesion and fusion processes in the cell. In many cases micron-sized membranes adhere over large areas before fusion. Reconstituted in vitro assays have helped isolate SNARE mechanisms in small membrane adhesion-fusion and are emerging as powerful tools to study large membrane systems by use of giant unilamellar vesicles (GUVs). Here we model SNARE-mediated adhesion kinetics in SNARE-reconstituted GUV-GUV or GUV-supported bilayer experiments. Adhesion involves many SNAREs whose complexation pulls apposing membranes into contact. The contact region is a tightly bound rapidly expanding patch whose growth velocity v(patch) increases with SNARE density Gamma(snare). We find three patch expansion regimes: slow, intermediate, fast. Typical experiments belong to the fast regime where v(patch) ~ (Gamma(snare)(2/3) depends on SNARE diffusivities and complexation binding constant. The model predicts growth velocities ~10 - 300 microm/s. The patch may provide a close contact region where SNAREs can trigger fusion. Extending the model to a simple description of fusion, a broad distribution of fusion times is predicted. Increasing SNARE density accelerates fusion by boosting the patch growth velocity, thereby providing more complexes to participate in fusion. This quantifies the notion of SNAREs as dual adhesion-fusion agents.