A NOVEL, MULTIFUNCTIONAL C-CBL BINDING-PROTEIN IN INSULIN-RECEPTOR SIGNALING IN 3T3-L1 ADIPOCYTES

The protein product of the c-Cbl proto-oncogene is prominently tyrosin e phosphorylated in response to insulin in 3TS-L1 adipocytes and not i n 3T3-L1 fibroblasts. After insulin-dependent tyrosine phosphorylation , c-Cbl specifically associates with endogenous c-Crk and Fyn. These r esults suggest a role for tyrosine-phosphorylated c-Cbl in 3T3-L1 adip ocyte activation by insulin. A yeast two-hybrid cDNA library prepared from fully differentiated 3T3-L1 adipocytes was screened with full-len gth c-Cbl as the target protein in an attempt to identify adipose-spec ific signaling proteins that interact with c-Cbl and potentially are i nvolved in its tyrosine phosphorylation in 3T3-L1 adipocytes, Here we describe the isolation and the characterization of a novel protein tha t we termed CAP for c-Cbl-assosiated protein. CAP contains a unique st ructure with three adjacent Src homolog 3 (SH3) domains in the C termi nus and a region showing significant sequence similarity with the pept ide hormone sorbin, Both CAP mRNA and proteins are expressed predomina tely in 3T3-L1 adipocytes and not in 3T3-L1 fibroblasts. CAP associate s with c-Cbl in 3T3-L1 adipocytes independently of insulin stimulation in vivo and in vitro in an SH3-domain-mediated manner. Furthermore, w e detected the association of CAP with the insulin receptor. Insulin s timulation resulted in the dissociation of CAP from the insulin recept or. Taken together, these data suggest that CAP represents a novel c-C bl binding protein in 3T3-L1 adipocytes likely to participate in insul in signaling.