EXPRESSION OF THE INSULIN-RECEPTOR WITH A RECOMBINANT VACCINIA VIRUS - BIOCHEMICAL-EVIDENCE THAT THE INSULIN-RECEPTOR HAS INTRINSIC SERINE KINASE-ACTIVITY

We have previously reported the tight association of a serine kinase a ctivity with the human insulin receptor (Lewis, R. E., Wu, G. P., MacD onald, R. G., and Czech, M. P. (1990) J. Biol. Chem. 265, 947-954). We tested the possibility that the associated serine kinase activity was intrinsic to the receptor catalytic domain, The ratio of phosphoserin e to phosphotyrosine on insulin receptors phosphorylated in vitro was used as an index of the associated serine kinase activity. Phosphoryla tion and phosphoamino acid analysis of insulin proreceptors revealed a ssociated serine kinase activity early in receptor synthesis, Insulin receptors were expressed in HeLa cells using a recombinant vaccinia vi rus. The ratio of phosphoserine to phosphotyrosine on insulin receptor s expressed by the recombinant vaccinia virus was determined relative to endogenous insulin receptors in cells treated with alpha-amanitin t o block host cell mRNA synthesis, alpha-Amanitin treatment had no effe ct on the ratio of phosphoserine to phosphotyrosine on insulin recepto rs expressed from the recombinant virus even though they were present in a 4000-fold excess above endogenous receptors, We conclude that the serine kinase activity associated with the insulin receptor is intrin sic to the receptor catalytic domain. Receptor-catalyzed autophosphory lation of serine may play an important role in modulating insulin rece ptor signaling.