Primary structure and biochemical characterization of yeast GTPase-activating proteins with substrate preference for the transport GTPase Ypt7p

Small GTPases of the Ypt/Rab family are regulators of vesicular protein tra fficking in exo-and endocytosis. GTPase-activating proteins (GAP) play an i mportant role as down regulators of GTPases. We here report the molecular c loning of a novel GAP-encoding gene (GYP7, for GAP for Ypt7) by high expres sion from a Saccharomyces cerevisiae genomic library. The GYP7 gene encodes a hydrophilic protein with a molecular mass of 87 kDa. Comparison of its p rimary sequence with that of the three other known GAPs for transport GTPas es, the yeast Gyp6 and Gyp1 proteins and the Rab3A-GAP from rat brain, show s similarity between the yeast GAPs only. Like GYP6 and GYP1, GYP7 is not e ssential for yeast cell viability. Gyp7p was able to most effectively accel erate the intrinsic GTPase activity of Ypt7p. It was also active, but to a lesser extent, on Ypt31p, Ypt32p and Ypt1p. Ypt6p, Sec4p and the human H-Ra s protein did not serve as substrates. We also report the identification an d cloning of a gene from the dimorphic yeast Yarrowia lipolytica that encod es a protein whose primary structure and biochemical activity are significa ntly related to those of Gyp7p from baker's yeast.