The stimulatory GDP/GTP exchange protein for ras p21-related small GTP-binding proteins

We have purified a novel type of regulatory protein for smg p21s, designated as smg p21 GDP dissociation stimulator (GDS), to near homogeneity from bovine brain cytosol. Moreover, we have isolated the cDNA of this protein from a bovine brain cDNA library, determined the complete nucleotide and deduced amino acid sequences, and characterized the kinetic properties. The cDNA of smg p21 GDS has an open reading frame encoding a protein of 558 amino acids with a calculated M(r) value of 61,066, similar to the M(r) value of 53,000 estimated for the purified smg p21 GDS by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and sucrose density gradient ultracentrifugation. smg p21 GDS stimulates the dissociation of [3H]GDP from and also stimulates the binding of [35S]guanosine 5'-(3-O-thio)triphosphate (GTP gamma S) to smg p21s. These actions of smg p21 GDS were specific for smg p21s and inactive for other ras p21/ras p21-related small G proteins including c-Ha-ras p21 and smg p25A. Neither smg p21 GDS stimulate the GTPase activity of smg p21s nor by itself showed [35S]GTP gamma S-binding or GTPase activity. These results indicate that bovine brain contains regulatory proteins for smg p21s that stimulate the GDP/GTP exchange reaction of smg p21s in addition to smg p21 GAP. It is likely that the conversion from the GDP-bound inactive form of smg p21s to the GTP-bound active form is regulated by smg p21 GDS and that its reverse reaction is regulated by smg p21 GAP.