FUNCTIONAL-SIGNIFICANCE OF BETA-GAMMA-SUBUNIT CARBOXYMETHYLATION FOR THE ACTIVATION OF PHOSPHOLIPASE-C AND PHOSPHOINOSITIDE 3-KINASE

The gamma subunits of heterotrimeric G proteins are isoprenylated and methylated at their carboxyl-terminal cysteine residues. Since methylation is the only reversible reaction in the isoprenylation pathway, it could be a site of regulation of G protein activity. beta subunits have been shown to activate a number of effecters involved in signal transduction 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 demethylated T-beta gamma. The abilities of methylated and demethylated T-beta gamma to stimulate G protein regulated phosphatidylinositol-specific phospholipase C (PIPLC) and phosphoinositide 3-kinase (PI3K) were determined. It is reported here that there is a strong dependence on methylation for activating both PIPLC and PI3K. Demethylated T-beta gamma is at least 10-fold less active than its methylated counterpart. Therefore, methylation may play an important role in the regulation of these effecters and of signal transduction processes in general.