THE ROLE OF G-PROTEIN METHYLATION IN THE FUNCTION OF A GERANYLGERANYLATED BETA-GAMMA ISOFORM

The gamma subunit of heterotrimeric G proteins is isoprenylated and me thylated on its carboxyl terminal cysteine residue. While retinal tran sducin is farnesylated, all other gamma subunits are modified by geran ylgeranylation. An immobilized form of pig liver esterase (iPLE) is ab le to hydrolyze the methyl ester of a geranylgeranylated Beta gamma is oform (beta(1) gamma(2)). Since methylation is the only reversible rea ction in the isoprenylation pathway, it could be a site of regulation of G protein activity. With both the methylated and demethylated beta( 1) gamma(2) now available, the role of methylation for a geranylgerany lated heterotrimeric G protein may be addressed. Here, it is reported that methylation has no effect on the ability of beta gamma to interac t with an alpha subunit, as probed by ADP-ribosylation studies with pe rtussis toxin, and has a small effect (less than 2-fold) on the abilit y of geranylgeranylated beta gamma to activate phosphatidylinositol-sp ecific phospholipase C (PIPLC) and phosphoinositide 3 kinase (PI3K). I n binding studies, demethylation only slightly decreased the ability o f beta(1) gamma(2) to adhere to azolectin vesicles. Therefore, methyla tion of heterotrimeric G proteins appears to have only a minor effect in signal transduction processes which can be correlated to a decrease in hydrophobicity of the beta gamma subunit.