We have studied the relationship between ras-GAP and G protein coupled receptors in a proliferative setting comprised of NIH3T3 expressing transfected muscarinic receptors (mAChRs). GAP expression plasmids were engineered to encode wild-type GAP, its carboxyl-terminal catalytic domain, a mutant lacking a portion of the catalytic domain, and an amino-terminal domain which contained the hydrophobic region as well as SH2-SH3 domains. Cotransfection of each GAP expression plasmid into NIH3T3 cells did not affect the transforming ability of the v-mos oncogene, but plasmids encoding wild-type GAP or the mutant consisting of an intact catalytic domain inhibited transformation induced by normal c-ras. Wild-type GAP also prevented transformation by m1 mAChRs, whereas the mutant consisting of only its catalytic domain lacked any demonstrable effect. In contrast, the N-terminal non-catalytic domain of GAP effectively prevented m1-induced focus-formation. Cell lines coexpressing m1 receptors and each of the GAP constructs revealed that GAP proteins do not affect m1 receptor density, receptor ligand binding characteristics or coupling to the PI-PLC signal transduction pathway. Thus, our findings suggest a role for the N-terminal non-catalytic domain of GAP in regulating biological functions mediated by G protein-coupled receptors.