INACTIVATION OF RAF-1 BY A PROTEIN-TYROSINE-PHOSPHATASE STIMULATED BYGTP AND RECONSTITUTED BY G(ALPHA-I O) SUBUNITS/

A membrane associated form of Raf-1 in v-Ras transformed NIH 3T3 cells can be inactivated by protein phosphatases regulated by GTP. Herein, a distinct protein-tyrosine phosphatase (PTPase) in membrane preparati ons from v-Ras transformed NiH 3T3 cells was found to be activated by guanyl 5'-yl imidodiphosphate (GMPPNP) and was identified as an effect or for pertussis toxin (PTx)-sensitive G-protein alpha subunits. PTPas e activation was blocked by prior treatment of cells with PTx. PTPase activation by GTP, but not GMPPNP, was transient. A GMPPNP-stimulated PTPase (PTPase-G) co-purified with G(alpha i/o) subunits during Supero se 6 and Mono Q chromatography. PTPase-G; activity in Superose 6 fract ions from GDP-treated membranes was reconstituted by activated G(alpha i/o) but not G(beta gamma), subunits. PTPase-G; may contribute to GMP PNP-stimulated inactivation of Raf-1 in v-Ras cell membranes because R af-1 inactivation was PTx-sensitive and PTPase-G inactivated exogenous Raf-1.