HOMOTYPIC VACUOLAR FUSION MEDIATED BY T-SNARE AND V-SNARE

Membrane fusion is necessary both in the eukaryotic secretory pathway and for the inheritance of organelles during the cell cycle. In the se cretory pathway, heterotypic fusion takes place between small transpor t vesicles and organelles. It requires N-ethylmaleimide-sensitive fusi on protein (NSF/Sec18p), soluble NSF attachment proteins (SNAPs/Sec17p ) and SNAP receptors (SNAREs). SNAREs are integral membrane proteins ( v-SNAREs on vesicles, t-SNAREs on the target organelles) and are thoug ht to provide specificity to the fusion process(1-5). It has been sugg ested that Sec17p and Sec18p bind to v-SNARE/t-SNARE complexes and med iate the membrane fusion event(1-3). Homotypic fusion of yeast vacuole s also requires Sec17p and Sec18p (ref, 6), but in vitro they are need ed only to 'prime' the vacuoles, not for subsequent docking or fusion( 7,8). It has been unclear whether these reactions involve SNAREs that are similar to those previously identified in heterotypic fusion syste ms and, hence, whether the actions of Sec18p/NSF and Sec17p/alpha SNAP in these systems can be compared, Here we identify typical v- and t-S NAREs on the yeast vacuolar membrane, Although both are normally prese nt, vacuoles containing only the v-SNARE can fuse with those containin g only the t-SNARE. Vacuoles containing neither SNARE cannot fuse with those containing both, demonstrating that docking is mediated by cogn ate SNAREs on the two organelle membranes. Even when t- and v-SNAREs a re on separate membranes, Sec17p and Sec18p act at the priming stage. Their action is not required at the point of assembly of the SNARE com plex, nor for the fusion event itself.