Association of Both Inhibitory and Stimulatory G alpha Subunits Implies Adenylyl Cyclase 5 Deactivation

Adenylyl cyclase (AC) generates cyclic AMP required for a variety of cellular functions, and its regulation plays a major role in cellular signal transduction in eukaryotes and prokaryotes. All membrane-bound AC isoforms in eukaryotes can be activated by stimulatory G-proteins, but only AC1, AC5, and AC6 can be both stimulated and inhibited by active G alpha subunits, G alpha(s) and G(alpha)i, respectively. In principle, these G alpha(r) sensitive AC isoforms could form both binary and ternary complexes with G alpha subunits due to the noncompetitive association of inhibitory and stimulatory G alpha. However, the formation and possible catalytic activity of a putative ternary complex have not yet been experimentally confirmed due to its proposed short-lived nature. Here, the catalytic activity of such a ternary complex consisting of apo AC5, stimulatory G alpha(olf), and inhibitory G alpha(il) is investigated via classical molecular dynamics simulations. Trajectories of inhibited and stimulated binary complexes, ACS:G alpha(il) and ACS:G alpha(olf), respectively, as well as Ga-free ACS were also obtained to compare the sampled ACS conformation in the ternary complex to -those sampled under different G alpha conditions. This comparison suggests that association of both Ga subunits results in an ACS conformation similar to that sampled by the ACS:G alpha(il) complex, indicating that the ternary complex mainly samples an inactive conformation.