Promiscuity in Rab-SNARE interactions

Fusion of post-Golgi secretory vesicles with the plasma membrane in yeast r equires the function of:a Rab protein, Sec4p, and a set of v- and t-SNAREs, the Snc, Sso, and Sec9 proteins. We have tested the hypothesis that a sele ctive interaction between Sec4p and the exocytic SNAREs is responsible for ensuring that secretory vesicles fuse with the plasma membrane but not with intracellular organelles. Assembly of Sncp and Ssop into a SNARE complex i s defective in a sec4-8 mutant strain. However, Snc2p binds in vivo to-many other syntaxin-like t-SNAREs, and binding of Sncp to the endosomal/Golgi t -SNARE Tlg2p is also reduced in sec4-8 cells. In addition, binding of Sncp to Ssop is reduced by mutations in two other Rab genes and four non-Rab gen es that block the secretory pathway before the formation of secretory vesic les. Ln an alternate approach to look for selective Rab-SNARE interactions, we report that the nucleotide-free form of Sec4p coimmunoprecipitates with Ssop. However, Rab-SNARE binding is nonselective, because the nucleotide-f ree forms of six Rab proteins bind with similar low efficiency to three SNA RE proteins, Ssop, Pep12p, and Sncp. We conclude that Rabs and SNAREs do no t cooperate to specify the target membrane.