Membrane-Binding Domain of the Small G Protein G25K Contains an S-(all- Trans-Geranylgeranyl)Cysteine Methyl Ester at its Carboxyl Terminus

We showed previously that a 23-kDa guanine nucleotide-binding protein (G protein) purified from bovine brain membranes is carboxyl methylated and that this modification occurs at or near the membrane-binding domain. In the present study, we identified this small G protein as G25K (formerly termed Gp). We demonstrated that proteolytic digests of3H-methylated G25K contained radiolabeled material that coeluted with synthetic S-(geranylgeranyl)cysteine methyl ester on reversed-phase HPLC. Further treatment by performic acid oxidation yielded radiolabeled material that coeluted with L-cysteic acid methyl ester, verifying that the isoprenoid moiety and carboxyl methyl ester are localized on a C-terminal cysteine residue. Analysis by gas chromatography-coupled mass spectrometry of material released from purified G25K by Raney nickel treatment positively identified the covalently bound lipid as an all-trans-geranylgeranyl (C20) isoprenoid moiety. These results suggest that geranylgeranyl modification and perhaps methyl esterification function in the membrane localization of this small G protein.