Promiscuity in Rab-SNARE interactions.
Fusion of post-Golgi secretory vesicles with the plasma membrane in yeast requires 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 selective 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 is 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 genes that block the secretory pathway before the formation of secretory vesicles. In 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-free forms of six Rab proteins bind with similar low efficiency to three SNARE proteins, Ssop, Pep12p, and Sncp. We conclude that Rabs and SNAREs do not cooperate to specify the target membrane.
Grote
E
E
Department of Cell Biology, Yale University School of Medicine, New Haven, Connecticut 06520-8002, USA.
Novick
P J
PJ
D005089:Exocytosis / Q000502:physiology; D005656:Fungal Proteins / Q000378:metabolism*; D006056:Golgi Apparatus / Q000378:metabolism*; D008565:Membrane Proteins / Q000378:metabolism*; D009154:Mutation; D011485:Protein Binding; D050765:Qa-SNARE Proteins; D050767:Qc-SNARE Proteins; D050683:R-SNARE Proteins; D050600:SNARE Proteins; D029701:Saccharomyces cerevisiae Proteins; D033921:Vesicular Transport Proteins; D015003:Yeasts / Q000378:metabolism*; D020691:rab GTP-Binding Proteins / Q000235:genetics / Q000378:metabolism*
D005656:Fungal Proteins; D008565:Membrane Proteins; D050765:Qa-SNARE Proteins; D050767:Qc-SNARE Proteins; D050683:R-SNARE Proteins; C090117:SEC9 protein, S cerevisiae; D050600:SNARE Proteins; C083241:SNC1 protein, S cerevisiae; C083242:SNC2 protein, S cerevisiae; C084006:SSO1 protein, S cerevisiae; C084007:SSO2 protein, S cerevisiae; D029701:Saccharomyces cerevisiae Proteins; D033921:Vesicular Transport Proteins; C064320:SEC4 protein, S cerevisiae; D020691:rab GTP-Binding Proteins