CRK PROTEIN BINDS TO PDGF RECEPTOR AND INSULIN-RECEPTOR SUBSTRATE-1 WITH DIFFERENT MODULATING EFFECTS ON PDGF-DEPENDENT AND INSULIN-DEPENDENT SIGNALING PATHWAYS

We have studied the involvement of murine c-Crk, an SH2/SH3 containing adaptor protein, in signaling pathways stimulated by different recept or tyrosine kinases. We show here that c-Crk is associated with compon ents of insulin-and PDGF-dependent signaling pathways. Insulin treatme nt of murine myoblast cells induces the formation of stable complex of endogenous c-Crk with insulin receptor substrate-1 (IRS-1) mediated v ia the SH2 domain of Crk. The ligand dependent physical association of c-Crk with IRS-1 is direct. However IRS-1 is also co-precipitated wit h c-Crk from quiescent L6 cells. The association of IRS-1 with c-Crk i n quiescent cells is probably not direct since Far Western blot analys is did not reveal the binding of neither SH2 domain nor amino-terminal SH3 domain of c-Crk to IRS-1 from unstimulated cells. We also show th at PDGF treatment of murine myoblast cells induces association of c-Cr k with the PDGF receptor and tyrosine phosphorylation of c-Crk. Overex pression of c-Crk enhanced insulin-but not PDGF-induced activation of MAP kinases when compared to parental cell lines. Thus, the formation of the direct IRS-1/Crk complex appears to be crucial for Crk-mediated insulin-induced activation of MAP kinase, whereas Crk is probably inv olved in other PDGF-induced responses. These data provide support to t he hypothesis that insulin and PDGF employ different mechanisms for ac tivation of MAP kinase cascade.