GDI1 ENCODES A GDP DISSOCIATION INHIBITOR THAT PLAYS AN ESSENTIAL ROLE IN THE YEAST SECRETORY PATHWAY

GTP binding proteins of the Sec4/Ypt/rab family regulate distinct vesi cular traffic events in eukaryotic cells. We have cloned GD11, an esse ntial homolog of bovine rab GDI (GDP dissociation inhibitor) from the yeast Saccharomyces cervisiae. Analogous to the bovine protein, purifi ed Gdi1p slows the dissociationof GDP from Sec4p and releases the GDP- bound form from yeast membranes. Depletion of Gdi1p in vivo leads to l oss of the soluble pool of Sec4p and inhibition of protein transport a t multiple stages of the secretory pathway. Complementation analysis i ndicates that GD11 is allelic to sec19-1. These results establish that Gdi1p plays an essential function in membrane traffic and are consist ent with a role for Gdi1p in the recycling of proteins of the Sec3/Ypt /rab family from their target membranes back to their vesicular pools.