NOVEL METHYLTRANSFERASE ACTIVITY MODIFYING THE CARBOXY-TERMINAL BIS(GERANYLGERANYL)-CYS-ALA-CYS STRUCTURE OF SMALL GTP-BINDING PROTEINS

Proteins containing CX(3), CXC, and CC (where C is cysteine and X is u ndefined) undergo posttranslational isoprenylation at their cysteine r esidues. In the case of proteins which terminate in CX(3), proteolytic removal of X(3) is followed by the carboxymethylation of the isopreny lated cysteine residue. CXC proteins also undergo C-terminal methylati on. The present study addresses the question of whether this methylati on is catalyzed by a different isoprenylated protein methyltransferase than that previously described for CX(3) proteins. The S-adenosylmeth ionine (AdoMet) dependent methylation of a small l-L-cysteinyl-L-alany l-S-geranylgeranyl-L-cysteine (Ac(GG)CysAla(GG)Cys)-was investigated u sing membranes from a variety of bovine tissues as sources of enzyme. Ac(GG)CysAla(GG)Cys was a substrate for methylation, while Ac(GG)Cys(G G)Cys was not. Reciprocal inhibition studies on the methylation reacti ons of the CXC peptide and of N-acetyl-S-farnesyl-L-cysteine (AFC), a previously described methyltransferase substrate, suggested that these reactions are catalyzed by distinct enzymatic activities. Farnesylthi oacetic acid (FTA), a potent competitive inhibitor of the methylation of AFC, did not inhibit the methylation of the CXC peptide. Moreover t he K-I values for S-adenosylhomocysteine and S-adenosylethionine inhib ition differed for the two enzymatic activities. These data indicate t hat more than one AdoMet-dependent methyltransferase is involved in th e carboxymethylation of isoprenylated proteins.