Geranylgeranylated Rab proteins usually terminate in either Cys-Cys or Cys-Xaa-Cys, where Xaa is Ala, Ser, or Gly. In both classes of proteins, the two cysteines are geranylgeranylated, but only the Cys-Xaa-Cys class has been shown to be carboxyl-methylated on the terminal cysteine in vivo. In the current study, we used recombinant Rab geranylgeranyltransferase and a Rab escort protein (REP-1) to attach geranylgeranyl residues to the two cysteines at the carboxyl terminus of Rab3A (Cys-Ala-Cys) and Rab1A (Cys-Cys). The geranylgeranylated proteins were then incubated with bovine cerebellar membranes that contain an enzyme that transfers [3H]methyl from S-[methyl-3H]adenosyl-L-methionine to geranylgeranylated cysteine. The enzyme transferred [3H]methyl to geranylgeranylated Rab3A but not to geranylgeranylated Rab1A. Replacement of the Cys-Ala-Cys terminus of Rab3A with Cys-Cys abolished methylation, and the opposite result was obtained when the Cys-Cys of Rab1A was replaced with Cys-Ala-Cys. When the Cys-Cys terminus of Rab1A was changed to Ser-Cys, methylation was restored. These studies suggest that the carboxyl-terminal cysteine of Rab proteins terminating in Cys-Xaa-Cys but not Cys-Cys is methylated and that the resistance of Cys-Cys proteins to methylation is attributable to the vicinal geranylgeranylated cysteines.
