Two new members of a family of Ypt/Rab GTPase activating proteins - Promiscuity of substrate recognition

Monomeric GTPases of the Ras superfamily have a very slow intrinsic GTPase activity which is accelerated by specific GTPase-activating proteins. In co ntrast to Ras- and Rho-specific GTPase-activating proteins (GAPs) that have been studied in great detail, little is known about the functioning of GAP s specific for Ypt/Rab transport GTPases. We have identified two novel Ypt/ Rab-GAPs because of their sequence relatedness to the three known GAPs Gyp1 p, Gyp6p, and Gyp7p. Mdr1/Gyp2p is an efficient GAP for Ypt6p and Sec4p, wh ereas Msb3/Gyp3p is a potent GAP for Sec4p, Ypt6p, Ypt51p, Ypt31/Ypt32p, an d Ypt1p. Although the affinity of Msb3/Gyp3p for its preferred substrate Se c4p is low (K-m = 154 mu M), it accelerates the intrinsic GTPase activity o f Sec4p 5 x 10(5)-fold. Msb3/Gyp3p appears to be functionally linked to Cdc 42p-regulated pathway(s). The results demonstrate that in yeast there is a large family of Ypt/Rab-GAPs, members of which discriminate poorly between GTPases involved in regulating different steps of exo- and endocytic transp ort routes.