Phosphorylation of the insulin receptor by casein kinase I

Insulin receptor was examined as a substrate for the multipotential protein kinase casein kinase I. Casein kinase I phosphorylated partially purified insulin receptor from human placenta as shown by immunoprecipitation of the complex with antiserum to the insulin receptor. Analysis of the phosphorylated complex by polyacrylamide gel electrophoresis under nonreducing conditions showed a major phosphorylated band at the position of the alpha 2 beta 2 complex. When the phosphorylated receptor was analyzed on polyacrylamide gels under reducing conditions, two phosphorylated bands, Mr 95,000 and Mr 135,000, were observed which corresponded to the alpha and beta subunits. The majority of the phosphate was associated with the beta subunit with minor phosphorylation of the alpha subunit. Phosphoamino acid analysis revealed that casein kinase I phosphorylated only seryl residues. The autophosphorylated alpha 2 beta 2 receptor purified by affinity chromatography on immobilized O-phosphotyrosyl binding antibody was also a substrate for casein kinase I. Reduction of the phosphorylated alpha 2 beta 2 receptor indicated that casein kinase I incorporated phosphate into seryl residues only in the beta subunit.