Selective formation of Sed5p-containing SNARE complexes is mediated by combinatorial binding interactions

Sed5p is the only syntaxin family member required for protein transport thr ough the yeast Golgi and it is known to bind up to nine other soluble N-eth ylmaleimide-sensitive factor attachment receptor (SNARE) proteins in vivo. We describe in vitro binding experiments in which we identify ternary and q uaternary Sed5p-containing SNARE complexes. The formation of SNARE complexe s among these endoplasmic reticulum- and Golgi-localized proteins requires Sed5p and is syntaxin-selective. In addition, Sed5p-containing SNARE comple xes form selectively and this selectivity is mediated by Sed5p-containing i ntermediates that discriminate among subsequent binding partners. Although many of these SNAREs have overlapping distributions in vivo, the SNAREs tha t form complexes with Sed5p in vitro reflect their functionally distinct lo cales. Although SNARE-SNARE interactions are promiscuous and a single SNARE protein is often found in more than one complex, both the biochemical as w ell as genetic analyses reported here suggest that this is not a result of nonselective direct substitution of one SNARE for another. Rather our data are consistent with the existence of multiple (perhaps parallel) traffickin g pathways where Sed5p-containing SNARE complexes play overlapping and/or d istinct functional roles.