A ROLE FOR CAP, A NOVEL, MULTIFUNCTIONAL SRC HOMOLOGY-3 DOMAIN-CONTAINING PROTEIN IN FORMATION OF ACTIN STRESS FIBERS AND FOCAL ADHESIONS

c-Cbl-associated protein, CAP, was originally cloned from a 3T3-L1 adi pocyte cDNA expression library using full-length c-Cbl as a bait, CAP contains a unique structure, with three adjacent Src homology-3 (SH3) domains in the COOH terminus and a region sharing significant sequence similarity with the peptide hormone sorbin, Expression of CAP in NIH- 3T3 cells overexpressing the insulin receptor induced the formation of stress fibers and focal adhesions, This effect of CAP expression on t he organization of the actin-based cytoskeleton was independent of the type of integrin receptors engaged with extracellular matrix, whereas membrane ruffling and decreased actin stress fibers induced by insuli n were not affected by expression of CAP. Immunofluorescence microscop y demonstrated that CAP colocalized with actin stress fibers. Moreover , CAP interacted with the focal adhesion kinase, p125FAK, both in vitr o and in vivo through one of the SH3 domains of CAP, The increased for mation of stress fibers and focal adhesions in CAP-expressing cells wa s correlated with decreased tyrosine phosphorylation of p125FAK in gro wing cells or upon integrin-mediated cell adhesion, These results sugg est that CAP may mediate signals for the formation of stress fibers an d focal adhesions.