Functional interaction between G alpha(z) and Rap1GAP suggests a novel form of cellular cross-talk

G(z) is a member of the G(i) family of trimeric G proteins whose primary ro le in cell physiology is still unknown. In an ongoing effort to elucidate t he cellular functions of G(z), the yeast two-hybrid system was employed to identify proteins that specifically interact with a mutationally activated form of G alpha(z). One of the molecules uncovered in this screen was Rap1G AP, a previously identified protein that specifically stimulates GTP hydrol ytic activity of the monomeric G protein Rap1 and thus is believed to funct ion as a down-regulator of Rap1 signaling. Like G(z), the precise role of R ap1 in cell physiology is poorly understood. Biochemical analysis using pur ified recombinant proteins revealed that the physical interaction between G alpha(z) and Rap1GAP blocks the ability of RGSs (regulators of G protein s ignaling) to stimulate GTP hydrolysis of the alpha subunit, and also attenu ates the ability of activated G alpha(z) to inhibit adenylyl cyclase, Struc ture-function analyses indicate that the first 74 amino-terminal residues o f Rap1GAP, a region distinct from the catalytic core domain responsible for the GAP activity toward Rap1, is required for this interaction. Co-precipi tation assays revealed that G alpha(z), Rap1GAP, and Rap1 can form a stable complex. These data suggest that Rap1GAP acts as a signal integrator to so mehow coordinate and/or integrate G(z) signaling and Rap1 signaling in cell s.