G-PROTEIN-GAMMA SUBUNITS CONTAIN A 20-CARBON ISOPRENOID

A small subset of cellular proteins are covalently modified by the addition of isoprenoid groups. These include p21ras, fungal mating factors, and nuclear lamins, which are isoprenylated at carboxyl-terminal cysteine residues with a 15-carbon farnesyl group. The similarity of the carboxyl-terminal sequences of these proteins with the α and γ subunits of signal-transducing guanine nucleotide-binding regulatory proteins (G proteins) prompted examination of isoprenylation of G protein subunits. PC-12 cells were incubated with the isoprenoid precursor [3H]mevalonolactone. The β and γ subunits were isolated by specific association with an affinity column of immobilized α subunits. The γ subunits were radiolabeled, and the tritiated lipid released from them by treatment with methyl iodide comigrated chromatographically with the 20-carbon isoprenoid geranylgeraniol. Label was not detected in G protein α or β subunits. Isoprenylation of γ subunits by the geranylgeranyl group is presumed to contribute to the association of G proteins with membranes.