[Phosphorylation of the receptor insulin and insulin-like growth factors]

We have found that the beta-subunit of the insulin receptor is a phosphoprotein and that the degree of phosphorylation is increased by the binding of insulin to its receptor. Furthermore we have found that the beta-subunit of the insulin receptor itself is a tyrosine-specific protein kinase. Tyrosine-specific protein kinases have been reported to be associated with the transforming gene product of RNA tumor viruses, the EGF receptor, the IGF-I receptor and a protein believed to be the receptor for PDGF. These findings suggest that determination of the endogenous substrated for these tyrosine-specific protein kinases may yield a sequence of regulatory proteins for cell growth and transformation. From this point of view, our recent findings that the purified insulin receptor-kinase can phosphorylate purified microtubule protein (tubulin, MAP2, tau) are interesting.