IDENTIFICATION OF SIRM, A NOVEL INSULIN-REGULATED SH3 BINDING-PROTEINTHAT ASSOCIATES WITH GRB-2 AND FYN

We have previously developed a mouse model of insulin-resistant diabet es by targeted inactivation of the insulin receptor gene. During studi es of gene expression in livers of insulin receptor-deficient mice, we identified a novel cDNA, which me have termed sirm (Son of Insulin Re ceptor Mutant mice), sirm is largely, albeit not exclusively, expresse d in insulin-responsive tissues, Insulin is a potent modulator of sirm expression, and sirm mRNA levels correlate with tissue sensitivity to insulin. The product of the sirm gene is a serine/threonine-rich prot ein with several proline-rich motifs and an NPNY motif, conforming to the consensus sequence recognized by the phosphotyrosine binding domai ns of insulin receptor substrate and Shc proteins, However, Sirm bears no extended homologies with other known proteins. Based on the sequen ces of the proline-rich domains, we sought to determine whether Sirm b inds to the SH3 domains of FYN and Grb-8, We demonstrate here that Sir m binds to FYN and Grb-8 in 3TS-L1 adipocytes and that insulin treatme nt results in the dissociation of the Sirm FYN and Sirm-Grb-2 complexe s. We also show that Sirm is a substrate for the kinase activity of FY N in vitro, Based on the patterns of expression of sirm, its regulatio n by insulin, and the interactions with molecules in the insulin signa ling pathway, we surmise that Sirm plays a role in modulating tissue s ensitivity to insulin.