Comparison of yeast mitochondrial Phe-tRNA synthetase subunits to their cytoplasmic counterparts: isolation and determination of amino acid compositions

Evolution of aminoacyl-tRNA synthetase quaternary structure and activity: Saccharomyces cerevisiae mitochondrial phenylalanyl-tRNA synthetase

Phenylalanyl-tRNA synthetases [L-phenylalanine:tRNAPhe ligase (AMP-forming), EC 6.1.1.20] from Escherichia coli, yeast cytoplasm, and mammalian cytoplasm have an unusual conserved alpha 2 beta 2 quaternary structure that is shared by only one other aminoacyl-tRNA synthetase. Both subunits are required for activity. We show here that a single mitochondrial polypeptide from Saccharomyces cerevisiae is an active phenylalanyl-tRNA synthetase. This protein (the MSF1 gene product) is active as a monomer. It has all three characteristic sequence motifs of the class II aminoacyl-tRNA synthetases, and its activity may result from the recruitment of additional sequences into an alpha-subunit-like structure.

Purification of cytoplasmic precursors of yeast mitochondrial phenylalanyl-tRNA synthetase subunits

Two polypeptidic precursors of yeast mitochondrial phenylalanyl-tRNA synthetase subunits were purified from the cytoplasm by immunoprecipitation with an insolubilized glutaraldehyde-treated IgG fraction, followed by two chromatographies on Sephadex G-200 and on DEAE-cellulose. Methionine was found as the N-terminal residue in both precursors, which exhibited N-terminal extensions.

Purification and reversible subunit dissociation of overproduced Escherichia coli phenylalanyl-tRNA synthetase

Phenylalanyl-tRNA synthetase (EC 6.1.1.20) has been purified to homogeneity from a 100-fold overproducing Escherichia coli strain carrying a hybrid pBR322 plasmid containing the pheS-pheT locus. The purified enzyme is identical to the phenylalanyl-tRNA synthetase isolated form an haploid strain. The enzyme was found to dissociate in the presence of 0.5 M NaSCN and the alpha- and beta-subunits composing the native alpha 2 beta 2 enzyme were separated by gel filtration. Neither isolated subunit showed significant catalytic activity. A complex indistinguishable from the native enzyme with full catalytic activity is recovered upon mixing the subunits. The N- and C-terminal sequences and the amino acid composition of each subunit were determined. They are compared to the available data concerning the primary structure of the subunits, as deduced from nucleotide sequencing of the pheS-pheT operon.

Mitochondrial phenylalanyl t-RNA synthetase from yeast: formation of enzyme-substrate complexes shown by heat or SH reagent inactivation

The binding of substrates to purified mitochondrial phenylalanyl-tRNA synthetase from yeast was examined using the kinetics of heat or p-hydroxymercurybenzoate inactivation. Individually magnesium chloride and each of the substrates protect the enzyme against thermal denaturation and p-hydroxymercurybenzoate inhibition. No enzyme protection is observed with ATP alone against p-hydroxymercurybenzoate inhibition. The combinations of the various substrates induce a synergistic protection effect. Protection constants of 31 microM and 0.3 microM were found for L-Phe and mt tRNAPhe respectively, from heat inactivation studies. The inhibition of the enzyme activity by p-hydroxymercurybenzoate can be reverted by 2-mercaptoethanol or dithiothreitol.

Phenylalanyl-tRNA synthetases from yeast cytoplasm and mitochondria. The presence of a carbohydrate moiety in the mitochondrial enzyme and immunological evidence for structural relationship

Homogeneous yeast cytoplasmic and mitochondrial phenylalanyl-tRNA synthetases (L-phenylalanine:tRNAPhe ligase (AMP-forming), EC 6.1.1.20) are analysed for structural differences. Only the large subunit of the mitochondrial enzyme is a glycoprotein with nearly 3% carbohydrate by weight. The carbohydrates present are: glucose, N-acetylglucosamine, mannose, galactose and N-acetylneuraminic acid. Removal of the sugar moieties yields an activity increase, but no significant change of sensitivity to proteolytic degradation. Antibodies to both homogeneous enzymes demonstrate a structural similarity for both types of subunit using the highly sensitive immunoblotting technique.

Phenylalanyl-tRNA synthetases from cytoplasm and mitochondria of yeast and hen liver: comparison of their structural and catalytic properties

Amino acid compositions and tryptic maps of the cytoplasmic and mitochondrial phenylalanyl-tRNA synthetases from yeast and hen liver, respectively, demonstrate the similarity of these enzymes, although they are clearly not identical. Moreover, similarity is noted for catalytic properties like stoichiometries of complex formation with tRNAPhe and negative cooperativity of tRNAPhe binding, triggered by substrates. Analysis of the kinetics at saturating and subsaturating substrate concentrations indicates the contribution of the transfer of phenylalanine from the adenylate to tRNAPhe to the rate-determining step in aminoacylation and subunit interactions in the tetrameric enzymes. Furthermore, the locations of substrate-binding sites appear rather constant within species and in interspecies comparison. Subtle differences at certain sites, although homology exists, are exemplified by a special regulatory effect on activity by other amino acids only in the case of the cytoplasmic enzyme from hen liver. The results support the idea of a common ancestry by gene duplication for the cytoplasmic and mitochondrial phenylalanyl-tRNA synthetases in fungi and animals, respectively.