Modulation of guanine nucleotide effects on the insulin receptor by MgCl2

Insulin binding to partially purified rat adipocyte insulin receptors is inhibited approximately 40-60 percent by 1 mM GTP-gamma-S in the presence of 2 mM MgCl2. However, in the presence of 10 mM MgCl2, GTP-gamma-S does not inhibit binding. Increasing MgCl2 from 0.5 to 10 mM enhances the phosphorylation of calmodulin catalyzed by the insulin receptor but also reduces the inhibition seen with 500 microM GTP-gamma-S. The reversal of the GTP-gamma-S-induced inhibition of calmodulin phosphorylation by high concentrations of MgCl2 appears to be due to an effect on the calmodulin molecule since MgCl2 has little effect on the inhibition of phosphorylation of histone Hf2b or poly (Glu4, Tyr1) induced by GTP-gamma-S. Our data suggest that there are at least two GTP-binding proteins associated with the insulin receptor, one that regulates insulin binding and is modulated by MgCl2 and one that regulates substrate phosphorylation and/or receptor-substrate coupling and is not altered by MgCl2.