Identification of a Sec4p GTPase-activating protein (GAP) as a novel member of a Rab GAP family

A yeast open reading frame sharing homology with the two known yeast Rab GTPase-activating proteins (GAPs), Gyp6p and Gyp7p, was found in a data base search. We have named the gene containing this open reading frame GYP1. Recombinant Gyp1p showed GAP activity on Sec4p, increasing both its steady-state rate and single turnover GTPase activity. Gyp1p also stimulated the GTPase activity of several other yeast Rab proteins including Ypt1p, Ypt7p, and Ypt51p but showed no GAP activity on Ypt6p and Ypt32p. Deletion of the GYP1 gene or overexpression of Gyp1p did not alter the growth rate of yeast. However, overexpression of Gyp1p was inhibitory in combination with a subset of secretory mutants including sec4-8 and several ypt1 mutants. This effect is probably due to the increase in GAP activity, which can be observed in a lysate from cells overexpressing Gyp1p. The finding that yeast Rab GAPs share homology with proteins in other species, such as Caenorhabditis elegans and human, suggests the existence of a conserved Rab GAP family.