Ca. Parish et al., FUNCTIONAL-SIGNIFICANCE OF BETA-GAMMA-SUBUNIT CARBOXYMETHYLATION FOR THE ACTIVATION OF PHOSPHOLIPASE-C AND PHOSPHOINOSITIDE 3-KINASE, Biochemistry, 34(23), 1995, pp. 7722-7727
The gamma subunits of heterotrimeric G proteins are isoprenylated and
methylated at their carboxyl-terminal cysteine residues. Since methyla
tion is the only reversible reaction in the isoprenylation pathway, it
could be a site of regulation of G protein activity. beta subunits ha
ve been shown to activate a number of effecters involved in signal tra
nsduction pathways. The methyl group of retinal transducin (T) can be
hydrolyzed by an immobilized form of pig liver esterase, allowing for
a direct determination of the activities of methylated and demethylate
d T-beta gamma. The abilities of methylated and demethylated T-beta ga
mma to stimulate G protein regulated phosphatidylinositol-specific pho
spholipase C (PIPLC) and phosphoinositide 3-kinase (PI3K) were determi
ned. It is reported here that there is a strong dependence on methylat
ion for activating both PIPLC and PI3K. Demethylated T-beta gamma is a
t least 10-fold less active than its methylated counterpart. Therefore
, methylation may play an important role in the regulation of these ef
fecters and of signal transduction processes in general.