Regulation of c-SRC activity and function by the adapter protein CAS

SRC family kinases play essential roles in a variety of cellular functions, including proliferation, survival, differentiation, and apoptosis, The act ivities of these kinases are regulated by intramolecular interactions and b y heterologous binding partners that modulate the transition between active and inactive structural conformations. p130(CAS) (CAS) binds directly to b oth the SH2 and SH3 domains of c-SRC and therefore has the potential to str ucturally alter and activate this kinase. In this report, we demonstrate th at overexpression of full-length CAS in COS-1 cells induces c-SRC-dependent tyrosine phosphorylation of multiple endogenous cellular proteins, A carbo xy-terminal fragment of CAS (CAS-CT), which contains the c-SRC binding site , was sufficient to induce c-SRC-dependent protein tyrosine kinase activity , as measured by tyrosine phosphorylation of cortactin, paxillin, and, to a lesser extent, focal adhesion kinase. A single amino acid substitution loc ated in the binding site for the SRC SH3 domain of GAS-CT disrupted CAS-CT' s interaction with c-SRC and inhibited its ability to induce tyrosine phosp horylation of cortactin and paxillin. Murine C3H10T1/2 fibroblasts that exp ressed elevated levels of tyrosine phosphorylated CAS and c-SRC-CAS complex es exhibited an enhanced ability to form colonies in soft agar and to proli ferate in the absence of serum or growth factors, GAS-CT fully substituted for CAS in mediating growth in soft agar but was less effective in promotin g serum-independent growth. These data suggest that CAS plays an important role in regulating specific signaling pathways governing cell growth and/or survival, in part through its ability to interact with and modulate the ac tivity of c-SRC.