Protein farnesyltransferase from Trypanosoma brucei. A heterodimer of 61- and 65-kda subunits as a new target for antiparasite therapeutics.

We have previously shown that protein prenylation occurs in the Trypanosomatids Trypanosoma brucei (T. brucei), Trypanosoma cruzi, and Leishmania mexicana and that protein farnesyltransferase (PFT) activity can be detected in cytosolic extracts of insect (procyclic) form T. brucei. A PFT that transfers the farnesyl group from farnesyl pyrophosphate to a cysteine that is 4 residues upstream of the C terminus of the Ras GTP-binding protein RAS1-CVIM has now been purified 60,000-fold to near homogeneity from procyclic T. brucei. By screening a mixture of hexapeptides SSCALX (X is 20 different amino acids), it was found that SSCALM binds to T. brucei PFT with sub-micromolar affinity, and affinity chromatography using this peptide was a key step in the purification of this enzyme. On SDS-polyacrylamide gel electrophoresis, the enzyme migrates as a pair of bands with apparent molecular masses of 61 and 65 kDa, and thus its subunits are approximately 30% larger than those of the mammalian homolog. The 61-kDa band was identified as the putative beta-subunit by photoaffinity labeling with a 32P-labeled analog of farnesyl pyrophosphate. Mimetics of the C-terminal tetrapeptide of prenyl acceptors have been previously shown to inhibit mammalian PFT, and these compounds also inhibit T. brucei PFT with affinities in the nanomolar to micromolar range, although the structure-activity relationship is very different for parasite versus mammalian enzyme. Unlike mammalian cells, the growth of bloodstream T. brucei is completely inhibited by low micromolar concentrations of two of the PFT inhibitors, and these compounds also block protein farnesylation in cultured parasites. These compounds also potently block the growth of the intracellular (amastigote) form of T. cruzi grown in fibroblast host cells. The results suggest that protein farnesylation is a target for the development of anti-trypanosomatid chemotherapeutics.