SPECIFIC BINDING TO A NOVEL AND ESSENTIAL GOLGI MEMBRANE-PROTEIN (YIP1P) FUNCTIONALLY LINKS THE TRANSPORT GTPASES YPT1P AND YPT31P

The regulation of vesicular transport in eukaryotic cells involves Ras -like GTPases of the Ypt/Rab family. Studies in yeast and mammalian ce lls indicate that individual family members act in vesicle docking/fus ion to specific target membranes. Using the two-hybrid system, we have now identified a 248 amino acid, integral membrane protein, termed Yi p1, that specifically binds to the transport GTPases Ypt1p and Ypt31p, Evidence for physical interaction of these GTPases with Yip1p was als o demonstrated by affinity chromatography and/or co-immunoprecipitatio n, Like the two GTPases, Yip1p is essential for yeast cell viability a nd, according to subcellular fractionation and indirect immunofluoresc ence, is located to Golgi membranes at steady state. Mutant cells depl eted of Yip1p and conditionally lethal yip1 mutants at the nonpermissi ve temperature massively accumulate endoplasmic reticulum membranes an d display aberrations in protein secretion and glycosylation of secret ed invertase, The results suggests for a role for Yip1p in recruiting the two GTPases to Golgi target membranes in preparation for fusion.