Fig. 1.

SM protein–chaperoned SNARE assembly. According to the template model (4, 12, 13, 16, 17), a SM protein first interacts with Qa- and R-SNAREs to form a ternary template complex, which switches between inactive and active states (A–C). The active template complex rapidly recruits a specific Qbc-SNARE to complete SNARE complex assembly (D). For clarity, the diagram depicts exocytic vesicle fusion, in which a Qbc-SNARE contains two SNARE motifs connected by a linker. In other vesicle fusion pathways, Qb- and Qc-SNAREs are usually two separate proteins (14).