INVITRO INSULIN-RECEPTOR CATALYZES PHOSPHORYLATION OF CLATHRIN HEAVY-CHAIN AND A PLASMA-MEMBRANE 180,000 MOLECULAR-WEIGHT PROTEIN

Insulin receptor mutation studies indicate that the receptor tyrosine kinase activity is necessary for receptor endocytosis, and several insulin receptor-containing tissues have a plasma membrane-associated protein (M(r) congruent-to 180,000, p180) whose tyrosine phosphorylation is receptor catalysed. Since clathrin heavy chain (M(r) congruent-to 180,000 in dodecyl sulphate gel electrophoresis) is a major component of coated vesicles, the latter functioning in receptor endocytosis, we investigated whether insulin receptors can catalyse clathrin phosphorylation and whether p180 is clathrin. Bovine brain triskelion or coated vesicles and P-32-ATP were added to prephosphorylated insulin receptor preparations (wheat germ agglutinin-purified human placenta membrane proteins). Antiphosphotyrosine immunoprecipitated a phosphorylated 180,000 molecular weight protein. Insulin (10(-7)M) increased the rate of phosphorylation. Monoclonal anti-clathrin antibody immunoprecipitated the phosphorylated 180,000 molecular weight protein, whereas monoclonal anti-insulin receptor antibodies (alpha-IR1, MA10) immunoprecipitated both insulin receptors and the phosphorylated 180,000 molecular weight protein. In the absence of added clathrin, anticlathrin immunoprecipitated no proteins, and alpha-IR1 imunoprecipitated only the insulin receptor. Density gradient (glycerol 7.5-30%, w/v) centrifugation separated human placenta microsomal membrane proteins into endosomal, plasma membrane, cytoplasmic and coated vesicle fractions. Antiphosphotyrosine immunoprecipitated phosphorylated-microsomal proteins that centrifuged into endosomal and plasma membrane fractions. Addition of glycerol gradient fractions to a prephosphorylated insulin receptor preparation, however, gave a tyrosine-phosphorylated 180,000 molecular weight protein when cytoplasmic and coated vesicle fractions were added. Taken together these results suggest: (1) that, in vitro, human placenta insulin receptors can phosphorylate bovine brain and human placenta clathrin heavy chain; (2) that both assembled and unassembled clathrin can be phosphorylated; and (3) that p180, the plasma membrane-associated insulin receptor substrate, is not clathrin heavy chain.