No abstract

The aflylic thioether bond of the prenylcysteines of prenylated proteins has been shown to be cleaved by 2-naphthol under alkaline conditions to yield substituted naphthopyrans. These products are readily resolved from interfering materials by HPLC and have a strongly absorbing chromophore. Thus, this reaction is suitable for quantitative analysis of prenyl substituents of proteins, and we have examined a number of tissues for their content of prenylcysteines. These amino acids are present in mammalian tissues at a concentration of 0.36-1.4 nmol/mg of protein, with a ratio of geranylgeranylcysteine to farnesylcysteine in the range of 4 to 10. Prenylcysteines were also found in the cytosolic fraction of two mouse tissues at about one-third the concentration of the whole organ. The level of these modified amino acids was found to be sigicantly less in a yeast, a fungus, a brown alga, a higher plant, and an insect. Again, geranylgeranylcysteine is predominant. Prenylcysteines were absent from Eschenchia coil but present in an archaebacterium. The prenylcysteine content of mmalan tissue is about 1% of that of cholesterol and about equal to that of ubiquinones and dolichols. Calculations indicate that about 0.5% of all proteins are prenylated.Prenylated proteins contain isoprenoid-modified cysteines in the carboxyl-terminal region (1-3). This posttranslational modification has usually been identified by metabolic labeling of proteins of cells in culture with radioisotopic mevalonate. Metabolic labeling is limiting because many organisms-e.g., yeasts and plants-do not incorporate this precursor readily. In addition, this technique may be impractical, since large doses of isotope would be required for studies with animals. The specific isoprenoid involved with this amino acid (4, 5) has been established by protein hydrolysis and isolation of the prenylcysteine (6) or by cleavage of the prenylcysteine with iodomethane (7) or Raney nickel (4, 5) followed by isolation of the cleavage product. Recently, fast atom bombardment mass spectrometry has been reported as a convenient method for qualitative analysis of prenylated peptides (8). Quantitative analysis with these techniques by direct determination of mass would be difficult if not impossible. Consequently, an alternative method for identifying prenyl modifications would be useful. In prenylated proteins the allylic thioether bond of the prenylcysteines is unique and provides an avenue for selective reactions for identification of prenyl groups. A good nucleophile should react preferentially with and cleave the isoprenoid from cysteine. Prenyl groups linked through oxygen or nitrogen would be expected to be unreactive. For example, a model study using radioactive farnesyl pyrophosphate and our naphthoxide conditions showed about 0.1% of a radioactive product. If the reagent possessed a strongly absorbing chromophoric group, then the derivative would provide a means for detection of the product. We have allowed prenylated proteins to react with 2-naphthol and hav...

The mevalonate-derived portion of a prenylated protein from Chinese hamster ovary cells has been established as diterpenoid (C20). This group is linked to a carboxyl-terminal cysteine as a thioether. It was removed from the protein by hydrazinolysis followed by Raney nickel desulfurization, and the resulting hydrocarbon fraction was analyzed by gas chromatography-mass spectrometry.

Prenyltransferase (EC 2.5.1.1) has been obtained from chicken liver in a stable crystalline form. The enzyme has been shown to be homogeneous by polyacrylamide gel electrophoresis at pH 8.4, and by electrophoresis in sodium dodecyl sulfate containing gels. Electrofocusing of the crystalline enzyme results in a single sharp protein peak with a pI of 5.72. The protein is a dimer of molecular weight 86,000 whose subunits were not resolved by gel electrophoresis in sodium dodecyl sulfate. Michaelis constants of 0.5 muM for both isopentenyl pyrophosphate and geranyl pyrophosphate are 3-20-fold lower than those found for prenyltransferase from yeast or pig liver (Eberhardt, N., and Rilling, H. C. (1974), J. Biol. Chem. (in press); Dorsey, J. K., Dorsey, J. A., and Porter, J. W. (1966), J. Biol. Chem. 241, 5353; Holloway, P. W., and Popjak, G. (1967), Biochem. J. 104, 57). The enzyme primarily synthesizes farnesyl pyrophosphosphate from dimethylallyl or geranyl pyrophosphate although some geranylgeranyl pyrophosphate is formed under certain conditons. This is the first preparation of a stable crystalline enzyme of sterol and terpene biosynthesis.

Prenyltransferase (farnesyl pyrophosphate synthetase) was purified from avian liver and characterized by Sephadex and sodium dodecyl sulfate gel chromatography, peptide mapping, and end-group analysis. The enzyme is 85 800 +/- 4280 daltons and consists of two identical subunits as judged by sodium dodecyl sulfate gel electrophoresis, peptide mapping, and end-group analysis. Chemical analysis of the protein revealed no lipid or carbohydrate components. Avian prenyltransferase synthesizes farnesyl pyrophosphate from either dimethylallyl or geranyl pyrophosphate and isopentenyl pyrophosphate. A lower rate of geranylgeranyl pyrophosphate synthesis from farnesyl pyrophosphate and isopentenyl pyrophosphate was also demonstrated. Michaelis constants for farnesyl pyrophosphate synthesis are 0.5 muM for both isopentenyl pyrophosphate and geranyl pyrophosphate. The V max for the reaction is 1990 nmol min-1 mg-1 (170 mol min-1 mol-1 enzyme). Substrate inhibition by isopentenyl pyrophosphate is evident at high isopentenyl pyrophosphate and low geranyl pyrophosphate concentrations. Michaelis constants for geranylgeranyl pyrophosphate synthesis are 9 muM for farnesyl pyrophosphate and 20 muM for isopentenyl pyrophosphate. The Vmax is 16 nmol min-1 mg-1 (1.4 mol min-1 mol-1 enzyme). Two moles of each of the allylic substrates is bound per mol of enzyme. The apparent dissociation constants for dimethylallyl, geranyl, and farnesyl pyrophosphates are 1.8, 0.17, and 0.73 muM, respectively. Dimethylallyl and geranyl pyrophosphates bound competitively to prenyltransferase with one-for-one displacement. Four moles of isopentenyl pyrophosphate was bound per mole of enzyme. Citronellyl pyrophosphate, an analogue of geranyl pyrophosphate, was competitive with the binding of 2 of the 4 mol of isopentenyl pyrophosphate bound. The data are interpreted to indicate that each subunit of avian liver prenyltransferase has a single allylic binding site accommodating dimethylallyl, geranyl, and farnesyl pyrophosphates, and one binding site for isopentenyl pyrophosphate. In the absence of an allylic pyrophosphate or analogue, isopentenyl pyrophosphate also can bind to the allylic site.