SEPARATE GTP-BINDING AND GTPASE-ACTIVATING DOMAINS OF A G-ALPHA-SUBUNIT

Most members of the guanosine triphosphatase (GTPase) superfamily hydr olyze guanosine triphosphate (GTP) quite slowly unless stimulated by a GTPase activating protein or GAP. The alpha subunits (Galpha) of the heterotrimeric G proteins hydrolyze GTP much more rapidly and contain an approximately 120-residue insert not found in other GTPases. Intera ctions between a Galpha insert domain and a Galpha GTP-binding core do main, both expressed as recombinant proteins, show that the insert act s biochemically as a GAP. The results suggest a general mechanism for GAP-dependent hydrolysis of GTP by other GTPases.