Enzymic characterization of fission yeast farnesyl transferase: recognition of the -CAAL motif at the C-terminus

Protein farnesylation is critical for maintaining normal cell morphology and canavanine resistance in Schizosaccharomyces pombe

Protein farnesyltransferase (FTase) plays important roles in the growth and differentiation of eukaryotic cells. In this paper, we report the identification of the Schizosaccharomyces pombe gene cpp1(+) encoding the beta-subunit of FTase. The predicted amino acid sequence of the cpp1(+) gene product shares significant similarity with FTase beta-subunits from a variety of organisms. S. pombe FTase purified from E. coli exhibits high enzymatic activity toward the CAAX farnesylation motif substrates (where C represents cysteine, A represents aliphatic amino acid, and X is preferentially methionine, cysteine, serine, alanine, or glutamine) while showing little preference for CAAL geranylgeranylation motif substrates (where L represents leucine or phenylalanine). cpp1(+) is not essential for growth as shown by gene disruption; however, mutant cells exhibit rounded or irregular cell morphology. Expression of a geranylgeranylated mutant form, Ras1-CVIL, which can bypass farnesylation, rescues these morphological defects. We also identify a novel phenotype of cpp1(-) mutants, hypersensitivity to canavanine. This appears to be due to a 3-4-fold increase in the rate of arginine uptake as compared with wild-type cells. Expression of the geranylgeranylated mutant form of a novel farnesylated small GTPase, SpRheb, is able to suppress the elevated arginine uptake rate. These results demonstrate that protein farnesylation is critical for maintaining normal cell morphology through Ras1 and canavanine resistance through SpRheb.

Subcellular distribution of farnesyl protein transferase in rat liver

Farnesyl protein transferase (FPT) activity was measured in rat liver subcellular fractions by using an unspecific acceptor for the farnesyl groups. The highest specific activity was found in mitochondria and it exceeded that of the microsomes three-fold. Considerably lower specific activities were found in the nuclei and cytosol. Further subfractionation revealed that the mitochondrial FPT activity is located in the matrix. The beta-subunit of the mitochondrial enzyme has an apparent molecular mass of 46 kDa, which is similar to its cytosolic counterpart. The results suggest that protein farnesylation can take place in a number of subcellular organelles.

In vivo prenylation of rat proteins: modification of proteins with penta- and hexaprenyl groups

In vivo protein prenylation was studied in newborn rats by repeated injections of [3H]mevalonate. The highest level of protein-bound mevalonate metabolites was found in the kidney, but incorporation was observed in all organs studied. After fluorography of SDS-polyacrylamide gel electrophoresis-separated polypeptides, labeling was found in the 21- to 28-kDa molecular mass region and, after prolonged exposure of the film, additional bands at both higher and lower molecular masses could be detected. Protein prenylation in the kidney increased during the first 5 days after birth, whereas that in the liver reached a maximum on the fourth day. After methyliodide treatment of the prenylated proteins, farnesol, geranylgeraniol, and two larger isoprenoids, pentaprenol and hexaprenol, were released. In the liver, the ratio of protein-bound geranylgeraniol to farnesol increased from 2 to 4.5 during the first 5 days after birth. Upon subfractionation of the kidney, the highest level of labeling was found in mitochondria and microsomes. When the mitochondria were subfractionated into outer membranes, intermembrane space and an inner membrane/matrix fraction, the labeling pattern of prenylated polypeptides differed in all fractions. The results demonstrate that in vivo labeling of rats can be performed to study the extent, type, and distribution of protein prenylation.

Chromatographic assay and peptide substrate characterization of partially purified farnesyl- and geranylgeranyltransferases from rat brain cytosol

A simple method for partially purifying both farnesyltransferase and geranylgeranyltransferase from rat brain cytosol is presented. Each of the final protein preparations contains one single transferase activity. A common method of measurement of both activities is described. The assay, which follows substrate prenylation, is also convenient for the measurement of the concomitant decrease in cosubstrates during the two transfer reactions. The quantitative HPLC detection of the prenylated substrates and of the cosubstrate consumption is used here to follow the purification processes. The same method is also used for substrate-specificity studies of the two enzymes performed on 18 synthetic hexapeptides derived from the C-terminus of proteins known to be prenylated in vivo. These studies partially confirm the reported differences in the substrate specificities of the two prenyltransferases. However, the observed recognition of overlapping sequences by the two enzymes might have important consequences for the inhibition of either of the enzymes in vivo and for the design of specific inhibitors.

Prenylation of proteins in Trypanosoma brucei

Prenyl modification of proteins by farnesyl and geranylgeranyl isoprenoids occurs in a variety of eukaryotic cells. Culturing of Trypanosoma brucei in the presence of [3H]mevalonolactone (which is hydrolyzed in cells to give mevalonic acid, the precursor of protein prenyl groups) and an inhibitor of mevalonic acid biosynthesis leads to the radiolabeling of a specific set of proteins when analyzed by gel electrophoresis. T. brucei proteins were also labeled when cells were cultured in the presence of [3H]farnesol or [3H]geranylgeraniol, and each prenol labels a distinct set of proteins. Unlike mammalian cells, only a few T. brucei proteins of molecular weights similar to those of the mammalian Ras superfamily of GTPase (20-30 kDa) were labeled with [3H]farnesol or [3H]geranylgeraniol. When the 0-55% ammonium sulfate fraction of T. brucei cytosol was fractionated on anion exchange chromatography, protein farnesyltransferase (PFT) and protein geranylgeranyltransferase-I (PGGT-I) activities were detected and elute as two distinct peaks. Partially purified T. brucei PFT and PGGT-I display partly different specificities toward prenyl acceptor substrates from those of mammalian protein prenyltransferases. As shown previously, rat PFT utilizes proteins ending in CVLS and CVIM as efficient prenyl acceptors and rat PGGT-I utilizes proteins ending in CVLL and CVIM in vitro. On the contrary, T. brucei PFT farnesylates a protein ending in CVIM but not CVLS or CVLL, and T. brucei PGGT-I preferentially geranylgeranylates a protein ending in CVLL.

Genes encoding farnesyl cysteine carboxyl methyltransferase in Schizosaccharomyces pombe and Xenopus laevis

The mam4 mutation of Schizosaccharomyces pombe causes mating deficiency in h- cells but not in h+ cells. h- cells defective in mam4 do not secrete active mating pheromone M-factor. We cloned mam4 by complementation. The mam4 gene encodes a protein of 236 amino acids, with several potential membrane-spanning domains, which is 44% identical with farnesyl cysteine carboxyl methyltransferase encoded by STE14 and required for the modification of a-factor in Saccharomyces cerevisiae. Analysis of membrane fractions revealed that mam4 is responsible for the methyltransferase activity in S. pombe. Cells defective in mam4 produced farnesylated but unmethylated cysteine and small peptides but no intact M-factor. These observations strongly suggest that the mam4 gene product is farnesyl cysteine carboxyl methyltransferase that modifies M-factor. Furthermore, transcomplementation of S. pombe mam4 allowed us to isolate an apparent homolog of mam4 from Xenopus laevis (Xmam4). In addition to its sequence similarity to S. pombe mam4, the product of Xmam4 was shown to have a farnesyl cysteine carboxyl methyltransferase activity in S. pombe cells. The isolation of a vertebrate gene encoding farnesyl cysteine carboxyl methyltransferase opens the way to in-depth studies of the role of methylation in a large body of proteins, including Ras superfamily proteins.