Isoforms of Mammalian Adenylyl Cyclase: Multiplicities of Signaling

G proteins mediate the effects of hormone signals on adenylyl cyclase.

A. Hormones that affect intracellular adenylyl cyclase activity bind to protein receptors that contain seven transmembrane domains (blue) and are thus anchored at the cell surface (brown double bar). The intracellular portion of these receptors interacts with GDP-bound heterotrimeric G proteins, resulting in the displacement of the bound GDP by GTP and concomitant dissociation of the GTP-bound Gα-subunit from the Gβγ-dimer.

B. Two families of Gα exist: GTP-bound Gα_s stimulates adenylyl cyclase, whereas GTP-bound Gα_i inhibits adenyly cyclase. Gα subunits possess an intrinsic GTPase activity, so that their dissociation from Gβγ and their effect on AC activity are transitory. (Yellow triangle represents GDP; red triangle represents GTP.)