The posttranslationally modified C-terminal structure of bovine aortic smooth muscle rhoA p21

rhoA p21, a ras p21-like small GTP-binding protein, has the same C-terminal consensus motif of Cys-A-A-X (A is an aliphatic amino acid and X is any amino acid) as ras p21s, which is posttranslationally processed. We here determine the posttranslationally processed C-terminal structure of the rhoA p21 purified from bovine aortic smooth muscle. Incubation of rhoA p21-expressing insect cells with exogenous [³']mevalonolactone caused the labeling of rhoA p21, suggesting that rhoA p21 is prenylated. Consistently, Raney nickel treatment of rhoA p21 released a geranylgeranyl moiety as estimated by gas chromatography/mass spectrometry. No lipid moiety was released by KOH or NH₂OH treatment. Extensive digestion of rhoA p21 with Achromobacter protease I yielded a C-terminal peptide, Ser-Gly-Cys¹⁹⁰, that lacked the three C-terminal amino acids predicted from the cDNA but was geranylgeranylated and carboxyl methylated at the cysteine residue. Bovine brain cytosol geranylgeranylated the bacterial rhoA p21 having the three C-terminal amino acids predicted from the cDNA but not the protein lacking the three C-terminal amino acids. Bovine brain membranes methylated the synthetic C-terminal peptide with 10 amino acids of rhoA p21 which was geranylgeranylated at its C-terminal cysteine residue but not the peptide which was not geranylgeranylated. These results suggest that rhoA p21 is first geranylgeranylated followed by removal of the three C-terminal amino acids and the subsequent carboxyl methylation of the exposed cysteine residue.