Figure 6.

A model for Snc v-SNARE functioning in exocytosis and endocytosis. (1) Secretory vesicles derived from the trans Golgi (TGN) or, perhaps, a late endosomal sorting compartment undergo docking and fusion at the PM. Snc v-SNAREs confer the docking and fusion of two classes of secretory vesicles by forming functional SNARE complexes in trans with partner t-SNAREs from the PM, Sso1,2, and Sec9. (2) After membrane fusion and subsequent disassembly of the cis SNARE complexes (catalyzed by Sec17/18 [not illustrated]), Snc proteins are recruited to newly forming endocytic vesicles. Next, they interact with Tlg2 at an early endosomal compartment and mediate fusion of the endocytic vesicles, as suggested from this and other studies (see DISCUSSION). (3) Finally, Snc proteins are retrieved back to the Golgi by entering an endosome-derived vesicle and subsequently conferring its ability to fuse with the TGN. This last step is probably mediated by Tlg1, which has been shown to localize to that compartment. Given the long half-life of Snc proteins (approximately 8 h; Couve et al., 1995), it is likely that these v-SNAREs participate in many rounds of exocytic and endocytic vesicle trafficking.