THE 60-KDA PHOSPHOTYROSINE PROTEIN IN INSULIN-TREATED ADIPOCYTES IS ANEW MEMBER OF THE INSULIN-RECEPTOR SUBSTRATE FAMILY

A 60-kDa protein that undergoes rapid tyrosine phosphorylation in resp onse to insulin and then binds phosphatidylinositol 3-kinase has been previously described in adipocytes and hepatoma cells. We have isolate d this protein, referred to as pp60, from rat adipocytes, obtained the sequences of tryptic peptides, and cloned its cDNA. The predicted ami no acid sequence of pp60 reveals that it contains an N-terminal plecks trin homology domain, followed by a phosphotyrosine binding domain, fo llowed by a group of likely tyrosine phosphorylation sites, four of wh ich are in the YXXM motif that binds to the SH2 domains of phosphatidy linositol 3-kinase. The overall architecture of pp60 is thus the same as that of insulin receptor substrates 1 and 2 (IRS-1 and IRS-2), and furthermore both the pleckstrin homology and phosphotyrosine binding d omains are highly homologous (about 50% identical amino acids) to thes e domains in both IRS-1 and IRS-2. Thus, pp60 is a new member of the I RS family, which we have designated IRS-3.