Association of Both Inhibitory and Stimulatory Gα 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 Gα subunits, Gα _s and Gα _i , respectively. In principle, these Gα _i -sensitive AC isoforms could form both binary and ternary complexes with Gα subunits due to the noncompetitive association of inhibitory and stimulatory Gα. 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 α _olf , and inhibitory Gα _i1 is investigated via classical molecular dynamics simulations. Trajectories of inhibited and stimulated binary complexes, AC5:Gα _i1 and AC5:G α _olf , respectively, as well as Gα-free AC5 were also obtained to compare the sampled AC5 conformation in the ternary complex to those sampled under different Gα conditions. This comparison suggests that association of both Gα subunits results in an AC5 conformation similar to that sampled by the AC5:Gα _i1 complex, indicating that the ternary complex mainly samples an inactive conformation.