Carboxyl methylation and COOH-terminal processing of the brain G-protein γ-subunit

The enzymatic methylation of the guanine nucleotide-binding proteins (G-proteins) γ-subunit was investigated in brain membranes. Brain membranes were methylated in vitro using [³H-methyl]S-adenosylmethionine, and the G-protein βγ-complex was purified using an anti-β antibody to assay for the protein during purification. The isolated G-protein βγ-complex was found to be carboxyl methylated on the γ-subunit. The methyl group was localized by tryptic digestion to the carboxyl-terminal of the protein. The methylated tryptic peptides contained a modified cysteine and were very hydrophobic, suggesting additional modification by lipidation. The evidence suggests that the COOH-terminal of G-γ is modified in a manner similar to the processing that occurs with the ras proteins.