p130Cas regulates the activity of AND-34, a novel Ra1, Rap1, and R-Ras guanine nucleotide exchange factor

We previously identified a novel murine protein, AND-34, with a carboxyl-terminal domain homologous to Ras family guanine nucleotide exchange factors (GEFs), which bound to the focal adhesion docking protein p130(Cas). Work by others has implicated both the human homologue of AND-34, BCAR3, and human p130(Cas) BCAR1, in the resistance of breast cancer cells to the anti-estrogen tamoxifen. Here we report that AND-34 displays GEF activity on Ra1A, Rap1A, and R-Ras but not Ha-Ras GTPases in cells. In contrast to several other Ra1-GEFs, the Ra1 GEF activity of AND-34 is not augmented by constitutively active Ha-Ras(Val-12), consistent with the absence of a detectable Ras-binding domain. Efficient binding to AND-34 required both the Src-binding domain and a flanking carboxyl-terminal region of p130(Cas). The p130(Cas)-binding site mapped to a carboxyl-terminal sequence within the AND-34 GEF domain. Overexpression of p130(Cas), but not an AND-34-binding mutant of p130(Cas), inhibited the Ra1 GEF activity of co-transfected AND-34. This work identifies a new potential function for p130(Cas) and a new regulatory pathway involved in the control of Ra1, Rap, and R-Ras GTPases that may participate in the progression of breast cancer cells to tamoxifen resistance.