GUANOSINE 5'-(GAMMA-THIO) TRIPHOSPHATE (GTP-GAMMA-S) INHIBITS PHOSPHORYLATION OF INSULIN-RECEPTOR AND A NOVEL GTP-BINDING PROTEIN, G(IR) FROM HUMAN PLACENTA

A novel 66 kDa GTP-binding protein, designated G(ir), has been partial ly purified along with insulin receptor (IR) from human placenta. This protein binds 8-azido-GTP, is ADP-ribosylated by pertussis toxin, pho sphorylated by IR tyrosine kinase and cross-reacts with antibodies aga inst synthetic peptides from the GTP-binding domain of G(2 alpha) (P96 0). Phosphorylation of IR-beta subunit and G(ir) by IR tyrosine kinase was almost completely inhibited by 100 mu M GTP gamma S, > 75% by 50 mu M and 20-30% by 1 mu M, while GDP at these concentrations had no si gnificant effect on the phosphorylation. IR tyrosine kinase phosphoryl ated G(ir) at the tyrosine residues. These studies indicate regulation of IR tyrosine kinase activity by guanosine phosphates and involvemen t of G(ir) in insulin action.