p130(Cas) 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-te rminal domain homologous to Ras family guanine nucleotide exchange factors (GEFs), which bound to the focal adhesion docking protein p130(Cas). Work b y others has implicated both the human homologue of AND-34, BCAR3, and huma n p130(Cas) BCAR1, in the resistance of breast cancer cells to the anti-est rogen 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 ot her Ra1-GEFs, the Ra1 GEF activity of AND-34 is not augmented by constituti vely 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 d omain. Overexpression of p130(Cas), but not an AND-34-binding mutant of p13 0(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 path way involved in the control of Ra1, Rap, and R-Ras GTPases that may partici pate in the progression of breast cancer cells to tamoxifen resistance.