Insulin receptor function is inhibited by guanosine 5'-[gamma-thio]triphosphate (GTP[S]).

The regulatory role of GTP-binding proteins (G-proteins) in insulin receptor function was investigated using isolated insulin receptors and plasma membranes from rat adipocytes. Treatment of isolated insulin receptors with 1 mM-guanosine 5'-gamma-thio]triphosphate (GTPS]) inhibited insulin-stimulated phosphorylation of the beta-subunit, histone Hf2b and poly(GluNa4,Tyr1) by 22%, 65% and 65% respectively. Phosphorylation of calmodulin by the insulin receptor kinase was also inhibited by 1 mM-GTPS] both in the absence (by 88%) and in the presence (by 81%) of insulin. In the absence of insulin, 1 mM-GTP had the same effect on calmodulin phosphorylation as 1 mM-GTPS]. However, when insulin was present, GTP was less effective than GTPS] (41% versus 81% inhibition). Concentrations of GTPS] greater than 250 microM are necessary to inhibit phosphorylation. Although these concentrations are relatively high, the effect of GTPS] is not due to competition with 32P]ATP for the insulin receptor kinase since (1) other nucleotide triphosphates did not inhibit phosphorylation as much as did GTPS] (or GTP) and (2) the Vmax of the ATP-dependent kinase reaction was decreased in the presence of GTPS]. GTPS] (1 mM) also inhibited insulin binding to isolated receptors and plasma membranes, by 80% and 50% respectively. Finally, an antibody raised to a peptide sequence common to the alpha-subunits of G-proteins Gs, Gi, Go and transducin detected G-proteins in plasma membranes but failed to detect them in the insulin receptor preparation. These results indicate that GTP inhibits insulin receptor function, but does so through a mechanism that does not require a conventional GTP-binding protein.