Msb4p, a protein involved in Cdc42p-dependent organization of the actin cytoskeleton, is a Ypt/Rab-specific GAP

Ypt/Rab proteins of the Has superfamily are regulators of protein transport in exo- and endocytosis. Like pas and Rho proteins, they have a slow intri nsic GTPase activity that can be accelerated by several orders of magnitude by GTPase-activating proteins (GAP). Here we describe a new member of a fa mily of Ypt/Rab-specific GAPs, Msb4p/Gyp4p, that shares with other Gyp fami ly members significant homology in the catalytic domain, recently identifie d in Gyp1p and Gyp7p. Purified Msb4p/Gyp4p acts primarily on Sec4p, Ypt6p a nd Ypt7p and might have a role in polarized secretion.