Catalyzes the formation of the signaling molecule cAMP in response to G-protein signaling. ATP = 3',5'-cyclic AMP + diphosphate Binds 2 magnesium ions per subunit. Is also active with manganese (in vitro). The protein contains two modules with six transmembrane helices each; both are required for catalytic activity. Isolated N-terminal or C-terminal guanylate cyclase domains have no catalytic activity, but when they are brought together, enzyme activity is restored. The active site is at the interface of the two domains. Both contribute substrate-binding residues, but the catalytic metal ions are bound exclusively via the N-terminal guanylate cyclase domain. Belongs to the adenylyl cyclase class-4/guanylyl cyclase family.