Regulation of c-SRC Activity and Function by the Adapter Protein CAS

Abstract

SRC family kinases play essential roles in a variety of cellular functions, including proliferation, survival, differentiation, and apoptosis. The activities of these kinases are regulated by intramolecular interactions and by heterologous binding partners that modulate the transition between active and inactive structural conformations. p130 CAS (CAS) binds directly to both the SH2 and SH3 domains of c-SRC and therefore has the potential to structurally alter and activate this kinase. In this report, we demonstrate that overexpression of full-length CAS in COS-1 cells induces c-SRC-dependent tyrosine phosphorylation of multiple endogenous cellular proteins. A carboxy-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 located in the binding site for the SRC SH3 domain of CAS-CT disrupted CAS-CT's interaction with c-SRC and inhibited its ability to induce tyrosine phosphorylation of cortactin and paxillin. Murine C3H10T1/2 fibroblasts that expressed elevated levels of tyrosine phosphorylated CAS and c-SRC-CAS complexes exhibited an enhanced ability to form colonies in soft agar and to proliferate in the absence of serum or growth factors. CAS-CT fully substituted for CAS in mediating growth in soft agar but was less effective in promoting 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 activity of c-SRC.Homeostasis in multicellular organisms is maintained through the integration of diverse environmental signals for survival, proliferation, differentiation, and apoptosis. These signals are sensed by a variety of cell surface receptors that are coupled to complex networks of cytoplasmic regulatory proteins. SRC family nonreceptor protein tyrosine kinases (PTKs) are important components of many of these signaling networks, including those originating from integrin receptors, receptor PTKs, G-protein-coupled receptors, and cytokine receptors (for reviews, see references 1, 45, 54, 70 and 77). The activities of SRC kinases are tightly regulated and repressed under most circumstances. The importance of this negative regulation is highlighted by the fact that expression of constitutively activated forms of c-SRC results in cellular transformation, characterized by uncontrolled cell proliferation and deregulated cell survival (56, 61).The unique structure of SRC family kinases allows them to be regulated by substrate availability, as well as by the presence of other interacting proteins (31,45,54,55,70,72,74,77,86,87,91). Activity is down-modulated by a series of intramolecular interactions that impose conformational constraints on the catalytic domain, making it inaccessible to the substrate. This inactive confo...

Conversion of Mechanical Force into Biochemical Signaling

“…Second, it has high affinity binding abilities to Src SH2 and SH3 domains, which allows it to bind competitively to c-Src and other Src family members and activate these protein tyrosine kinases. pp125FAK (37) and pp130CAS (38) are also potential Src activators with Src SH2 and SH3 binding motifs. Their distribution is also associated with cytoskeleton.…”

The substrate domain of BCAR1 is essential for anti-estrogen-resistant proliferation of human breast cancer cells

“…SRC was shown to bind to the Src-binding site and to phosphorylate several YXXP motifs of BCAR1 [30][31][32][33]. These experiments have shown that BCAR1 phosphorylation may be important for some processes (cell migration), but not essential for anchorage-independent growth of SRC transformed fibroblasts [7,23,24,[34][35][36].…”

CAS promotes invasiveness of Src-transformed cells

“…Furthermore, overexpression of the CAS C-terminal region including the SBD promoted soft agar colony formation in murine C3H10T1/2 fibroblasts overexpressing c-Src (Burnham et al, 2000). Under these conditions, the interaction of the CAS SBD with the Src SH3 domain appears to relieve c-Src autoinhibitory interactions and thereby stimulate Src kinase activity (Burnham et al, 2000;Pellicena and Miller, 2001), essentially converting c-Src into an activated oncogenic form.…”