How receptors catalyze exchange of CTP for GDP bound to the G alpha subunit
of trimeric G proteins is not known. One proposal is that the receptor use
s the C protein's beta gamma heterodimer as a lever, tilting it to pull ope
n the guanine nucleotide binding pocket of G alpha. To test this possibilit
y, we designed a mutant G alpha that would bind to beta gamma in the tilted
conformation. To do so, we excised a helical turn (four residues) from the
N-terminal region of alpha (s), the a subunit of G(s), the stimulatory reg
ulator of adenylyl cyclase. In the presence, but not in the absence, of tra
nsiently expressed beta (1) and gamma (2), this mutant (alpha (s)Delta), ma
rkedly stimulated cAMP accumulation. This effect depended on the ability of
the coexpressed beta protein to interact normally with the lip of the nucl
eotide binding pocket of alpha (2)Delta. We substituted alanine for an aspa
rtate in beta (1) that binds to a lysine (K206) in the lip of the a subunit
's nucleotide binding pocket. Coexpressed with alpha (2)Delta and gamma (2)
, this mutant, beta1-D228A, elevated cAMP much less than did beta (1)-wild
type; it did bind to alpha (2)Delta normally, however, as indicated by its
unimpaired ability to target alpha (2)Delta to the plasma membrane. We conc
lude that beta gamma can activate alpha (s) and that this effect probably i
nvolves both a tilt of beta gamma relative to alpha (s) and interaction of
beta with the lip of the nucleotide binding pocket. We speculate that recep
tors use a similar mechanism to activate trimeric G proteins.