The first steps in insulin action are binding of insulin to its receptor and activation of the insulin receptor kinase. As there is indirect evidence that further signal transduction might involve a guanine-nucleotide-binding protein (G-protein), we studied whether insulin modulates GTP binding to plasma membrane proteins of fat cells and skeletal muscle. We found that insulin rapidly increased (30 s) binding of guanosine 5′-[gamma-thio]triphosphate (GTP[S]) in a dose dependent manner (0.03-2.0 nM). This effect was not altered by pertussis toxin, but it was abolished by cholera toxin treatment of fat cells. Scatchard analysis of the binding data showed that the increased GTP[S] binding is due to a decrease in the Kd for GTP from 100 nM to 50 nM. Furthermore, binding of GTP to these plasma membranes inhibited both the binding of 125I-insulin to the insulin receptor and the stimulation of the insulin receptor kinase, suggesting a feedback interaction between the insulin-stimulated GTP-binding site and the insulin receptor. In order to identify this insulin-stimulated GTP-binding site, plasma membranes were labelled with the photoreactive GTP analogue [alpha-32P]GTP gamma-azidoanilide. We found that insulin selectively stimulated GTP binding to a 40 kDa protein. In conclusion, in plasma membranes of fat cells and skeletal muscle, the insulin receptor interacts with a 40 kDa GTP-binding site. We speculate that this 40 kDa GTP-binding site might be a G-protein which is involved in insulin signal transmission.
