A 180,000 molecular weight glycoprotein substrate of the insulin receptor tyrosine kinase is present in human placenta and in rat liver, muscle, heart and brain plasma membrane preparations.

Cell signalling for insulin may include insulin receptor tyrosine kinase catalysing the phosphorylation of one or more cell proteins. Since temporally the insulin receptor will encounter plasma membrane proteins first, we have studied the in vitro phosphorylation of purified plasma membrane preparations. Two proteins were immunoprecipitated with anti-phosphotyrosine antibody from rat liver, muscle, heart and brain membranes and from human placenta membranes: the insulin receptor (detected as a phosphorylated-beta-subunit) and a 180,000 molecular weight protein (pp180). pp180 is a monomeric glycoprotein that in the absence of dithiothreitol migrated in denaturing gels like a 150,000 molecular weight protein. pp180 was a substrate for the insulin receptor: (i) receptor and pp180 phosphorylation followed a similar insulin dose-response, although fold-stimulation of autophosphorylation was greater; and (ii) removal of insulin receptors with monoclonal antibodies prevented subsequent pp180 phosphorylation. Insulin-activated receptors increased the extent, but not the rate, of pp180 phosphorylation; the increased phosphate was incorporated into tyrosine and appeared to do so in three or four of pp180's 12 tryptic phosphopeptides. Some data suggest that pp180 is the same protein in each of the tested tissues. The occurrence of pp180, an insulin receptor substrate, in plasma membranes of several insulin responsive tissues suggests that it has a role in insulin signalling.