Prenylated protein methyltransferases do not distinguish between farnesylated and geranylgeranylated substrates

Proteins that are post-translationally modified by prenylation can be either farnesylated (C-15) or geranylgeranylated (C-20) by separate prenyltransferase enzymes. Prenylated proteins are also methylated at their C-terminal residue by S-adenosylmethionine-linked methylation. In this paper we show that the methylation of farnesylated and geranyl-geranylated substrates can be accounted for by the presence of a single enzyme. It is demonstrated that the Km and Vmax. values for the retinal rod outer segment methyltransferase, measured with small molecule farnesylated and geranylgeranylated substrates, are identical. These substrates mutually inhibit each other's methylation, with KI values being equal to their Km values. The Km for S-adenosylmethionine was measured to be the same with either farnesylated or geranylgeranylated substrates. Competitive inhibitors of the methyltransferase containing either a geranylgeranyl or a farnesyl group equally block the methylation of synthetic geranylgeranylated and farnesylated substrates of the enzyme. Importantly, these inhibitors are also equipotent at inhibiting the methylation of the physiological substrates of the rod outer segment methyltransferase. These substrates are both farnesylated and geranylgeranylated. One of these substrates had previously been identified as the farnesylated gamma subunit of transducin. Therefore it appears that the same enzymic activity can methylate both farnesylated and geranylgeranylated substrates.