Three SNARE complexes cooperate to mediate membrane fusion

Soluble M-ethylmaleimide-sensitive factor attachment protein receptor (SNAR E) proteins of the syntaxin, SNAP-25, and VAMP families mediate intracellul ar membrane fusion through the formation of helical bundles that span oppos ing membranes. Soluble SNARE domains that lack their integral membrane anch ors inhibit membrane fusion by forming nonfunctional complexes with endogen ous SNARE proteins. In this study we investigate the dependence of membrane fusion on the concentration of a soluble SNARE coil domain derived from VA MP2, The increase in the inhibition of fusion observed with increasing conc entration of inhibitor is best fit to a function that suggests three SNARE complexes cooperate to mediate fusion of a single vesicle. These three comp lexes likely contribute part of a protein and lipidic fusion pore.