Guanosine 5'-(γ-thio) triphosphate (GTPγS) 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 partially purified along with insulin receptor (IR) from human placenta. This protein binds 8-azido-GTP, is ADP-ribosylated by pertussis toxin, phosphorylated by IR tyrosine kinase and cross-reacts with antibodies against synthetic peptides from the GTP-binding domain of G_zα (P960). Phosphorylation of IR-β subunit and G_ir by IR tyrosine kinase was almost completely inhibited by 100 μM GTPγS, >75% by 50 μM and 20-30% by 1 μM, while GDP at these concentrations had no significant effect on the phosphorylation. IR tyrosine kinase phosphorylated G_ir at the tyrosine residues. These studies indicate regulation of IR tyrosine kinase activity by guanosine phosphates and involvement of G_ir in insulin action.