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

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Abstract
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.