Mutant G protein α subunit activated by Gβγ: A model for receptor activation?

How receptors catalyze exchange of GTP for GDP bound to the Gα subunit of trimeric G proteins is not known. One proposal is that the receptor uses the G protein's βγ heterodimer as a lever, tilting it to pull open the guanine nucleotide binding pocket of Gα. To test this possibility, we designed a mutant Gα that would bind to βγ in the tilted conformation. To do so, we excised a helical turn (four residues) from the N-terminal region of α(s), the α subunit of G(S), the stimulatory regulator of adenylyl cyclase. In the presence, but not in the absence, of transiently expressed β(1) and γ(2), this mutant (α(s)Δ), markedly stimulated cAMP accumulation. This effect depended on the ability of the coexpressed β protein to interact normally with the lip of the nucleotide binding pocket of α(s)Δ. We substituted alanine for an aspartate in β(1) that binds to a lysine (K206) in the lip of the α subunit's nucleotide binding pocket. Coexpressed with α(s)Δ and γ(2), this mutant, β(1)-D228A, elevated cAMP much less than did β(1)-wild type; it did bind to α(s)Δ normally, however, as indicated by its unimpaired ability to target α(s)Δ to the plasma membrane. We conclude that βγ can activate α(s) and that this effect probably involves both a tilt of βγ relative to α(s) and interaction of β with the lip of the nucleotide binding pocket. We speculate that receptors use a similar mechanism to activate trimeric G proteins.