Purification and some properties of the histidyl-tRNA synthetase from the cytosol of rabbit reticulocytes.

The histidyl-tRNA synthetase of rabbit reticulocyte cytosol has been purified 84 000-fold to apparent homogeneity with a specific activity of 687 nmol of histidyl-tRNA formed per min per mg of protein. Ten to 15% of the enzyme activity is sedimented with the ribosomes while the remainder is in the cytosol. The purified enzyme has a molecular weight of 122 000 as determined by sucrose density gradient centrifugation. Gel electrophoresis in the presence of 0.1% sodium dodecyl sulfate suggests that it is composed of two similar subunits with a molecular weight of approximately 64 000. The enzyme has a magnesium optimum of 45 mM; however, this is reduced to 5 mM in the presence of an intracellular potassium concentration (160 nM). The enzyme acylates the two histidine tRNA isoacceptors of rabbit reticulocytes with similar Km values and at similar rates.     

Immunochemical studies on phenylalanyl-tRNA synthetase from Escherichia coli.

Antibodies against the alpha and beta subunits of phenylalanyl-tRNA synthetase were fractionated by ion exchange chromatography into different classes and then digested with papain to yield the respective Fab fragments. The preparations obtained were used to investigate (i) whether the alpha and beta polypeptides share any common antigenic determinants and (ii) whether immunological methods are able to resolve the catalytic function of the subunits of this enzyme (or principally of oligomeric enzymes). As to the first problem, immunodiffusion and complement fixation experiments showed that there is no immunological relatedness between the subunits which argues against the existence of sequence homoligies. As to the second question investigated, it was found that any binding of immunoglobulins of Fab fragments to the alpha or the the beta subunit affects enzyme activity either in the direction of activation or inhibition. These results therefore show that the immunological approach is not appropriate for resolving subunit-specific funcitons, possibly as a consequence of conformational changes induced in the enzyme by the binding of the immunoglobulins of Fab fragments.     

Electron microscopy study of the aminoacyl-tRNA synthetase multienzymatic complex purified from rabbit reticulocytes

The morphology of the high molecular weight complex of aminoacyl-tRNA synthetases purified from rabbit reticulocytes has been investigated by electron microscopy. To stabilize it against dissociation, the complex was also studied after chemical crosslinking. Freeze fracture, drying shadowing and negative staining were used. The reticulocyte complex appears as a moderately elongated object with no simple compact shape. Upon rapid drying, the native complex dissociates and shows the presence of approximately 8 globular components, the individual size of which is 80–100 Å. The surface of the cross-linked complex shows several distinct globules which appear to extend out of a central core. The irregularly shaped crosslinked complex has a maximal dimension of 350±50 Å. The morphology of the synthetase complex is discussed with respect to some of the properties of this type of multienzymatic system.     

Biosynthesis and transport of yeast mitochondrial phenylalanyl-tRNA synthetase.

The biosynthesis of yeast mitochondrial Phe-tRNA synthetase is studied in vivo. Antibodies against the enzyme are raised in rabbits. They precipitate two proteins in the post-ribosomal supernatant of the yeast cell homogenate. Immunoprecipitate analysis on SDS - gel electrophoresis shows that the two types of mitochondrial enzyme subunits with molecular weights of 57,000 and 72,000, respectively, are cytoplasmically synthesized as larger, individual precursors. Terminal extensions of the precursors prevent enzyme activity. Mitochondrial membranes linked protease(s) play(s) an active role in maturation.     

Interference of ligands on the phenylalanyl-tRNA synthetase from yeast

The present paper reports a study of the mutual interactions between the substrates, the intermediate, and the products of the aminoacylation reaction, when bound to the phenylalanyl-tRNA synthetase from yeast. The following conclusions can be drawn. a) tRNAPhe displaces Phe-tRNAPhe from the synthetase by lowering the affinity of the enzyme for the aminoacylated tRNA. b) Phe-tRNAPhe and Phe-AMP compete for the catalytically active site of the enzyme. c) Chemically synthesized Phe-AMP, when added to the synthetase, primarily forms a low-affinity complex with the enzyme. The transformation of this complex into the high-affinity catalytic complex is a very slow process. These findings confirm a previous study, based on steady-state kinetics. A schematic representation of the aminoacylation process is given. It summarizes the present and previous results and illustrates a rather complex 'flip-flop' mechanism.     

Particulate forms of phenylalanyl-tRNA synthetase from Ehrlich ascites cells

Over 80% of the phenylalanyl-tRNA synthetase activity in Ehrlich ascites cell homogenates was found to be associated with the high speed particulate fraction. This enzyme activity occurred in two principle forms: activity bound to the ribosomes, and activity as part of a complex sedimenting at approximately 25S in a sucrose density gradient. The ribosome-associated enzyme was shown to be bound to the 60S ribosomal subunit. Exposure of the ribosomes to RNA resulted in removal of synthetase activity from the ribosomes and the concomitant appearance of activity in a complex sedimenting at 25S.     

Expression and characterization of a human mitochondrial phenylalanyl-tRNA synthetase.

Human mitochondrial phenylalanyl-tRNA synthetase (mtPheRS) has been identified from the human EST database. Using consensus sequences derived from conserved regions of the alpha and beta-subunits from bacterial PheRS, two partially sequenced cDNA clones were identified. Unexpectedly, sequence analysis indicated that one of these clones was a truncated form of the other. Detailed analysis indicates that unlike the (alphabeta)2 structure of the prokaryotic and eukaryotic cytoplasmic forms of PheRS, the human mtPheRS consists of a single polypeptide chain. This protein has been cloned and expressed in Escherichia coli. Gel filtration and analytical velocity sedimentation centrifugation indicate that the human mtPheRS is active in a monomeric form. The N-terminal 314 amino acid residues appear to be analogous to the alpha-subunit of the prokaryotic PheRS, while the C-terminal 100 amino acid residues correspond to a region of the beta-subunit known to interact with the anticodon of tRNAPhe. Comparisons with the sequences of PheRS from yeast and Drosophila mitochondria indicate they are 42 % and 51 % identical with the human mtPheRS, respectively. Sequence analysis confirms the presence of motifs characteristic of class II aminoacyl-tRNA synthetases. KM and kcat values for ATP:PPi exchange and for the aminoacylation reaction carried out by human mtPheRS have been determined. Evolutionary origins of this small monomeric human mtPheRS are unknown, however, implications are that this enzyme is a result of the simplification of the more complex (alphabeta)2 bacterial PheRS in which specific functional regions were retained.     

Yeast phenylalanyl-tRNA synthetase. Properties of the histidyl residues.

Reactivity of the histidyl groups of yeast phenylalanyl-tRNA synthetase was studied in the absence or presence of substrates. In the absence of substrates about 10 histidine residues were found to react with similar kinetic constants. Phenylalanine at 10(-3) M was found to protect two histidyl residues; increasing the amino acid concentration to 5 . 10(-3) M resulted in the protection of two more histidyl groups. tRNAPhe did not afford any protection to histidine residues, but acylated phenylalanyl-tRNA (Phe-tRNAPhe) protected two of the four histidyl groups already protected by phenylalanine. These results suggest the existence of two different sets of accepting sites for phenylalanine: one specific for the free amino acid, the other one specific for the amino acid linked to the tRNA, but being accessible to free phenylalanine, with a somewhat lower binding constant, ATP was found to mask around four histidyl residues against diethylpyrocarbonate modification. By photoirradiation of enzyme-phenylalanine complex in the presence of rose bengale, a significant amount of amino acid was bound to the alpha subunit (Mr = 73 000) of phenylalanyl-tRNA synthetase, confirming that the amino acid binding site is located on this subunit, as previously suggested by modification of thiol groups. Upon irradiation of an enzyme-tRNA complex, almost no covalent binding of tRNA occurred during enzyme inactivation, suggesting that the histidyl residues involved in the enzymic activity are not required for tRNA binding.     

Neurospora crassa glutamine synthetase. Translation of specific messenger ribonucleic acid in a cell-free system derived from rabbit reticulocytes.

The total reticulocyte lysate cell-free protein-synthesizing system was incubated in the presence of Neurospora crassa RNA. With the aid of an antibody directed against purified N. crassa glutamine synthetase, the synthesis of a specific protein was detected. This protein precipitates with antiglutamine synthetase using both direct and indirect procedures, migrates with the same molecular weight as the monomer of N. crassa glutamine synthetase when subjected to acrylamide gel electrophoresis in the presence of sodium dodecyl sulfate, and chromatographs as N. crassa glutamine synthetase on anthranilate-bound Sepharose. These data indicate the translation of the mRNA that codes for N. crassa glutamine synthetase. This RNA behaves as poly(A)-containing material when fractionated on oly(U)-Sepha-rose.     

Some properties of the lipoxygenase from rabbit reticulocytes.

A lipoxygenase was enriched from the stoma-free supernatant of rabbit reticulocytes. The enzyme causes drastic deterioration of mitochondrial membranes. The release of matrix enzymes is paralleled by formation of products of lipid peroxidation. The enzyme reacts with isolated phospholipds and free cis-unsaturated fatty acids. Some properties were determined: molecular weight, isoelectric point, temperature and pH-dependence and Km value for linoleic acid. The enzyme is inhibited by reaction products and a variety of inhibitors, especially antioxidants and chelating agents.     

Mechanism of discrimination between cognate and non-cognate tRNAs by phenylalanyl-tRNA synthetase from yeast.

The interaction between phenylalanyl-tRNA synthetase from yeast and Escherichia coli and tRNAPhe (yeast), tRNASer (yeast), tRNA1Val (E. coli) has been investigated by ultracentrifugation analysis, fluorescence titrations and fast kinetic techniques. The fluorescence of the Y-base of tRNAPhe and the intrinsic fluorescence of the synthetases have been used as optical indicators. 1. Specific complexes between phenylalanyl-tRNA synthetase and tRNAPhe from yeast are formed in a two-step mechanism: a nearly diffusion-controlled recombination is followed by a fast conformational transition. Binding constants, rate constants and changes in the quantum yield of the Y-base fluorescence upon binding are given under a variety of conditions with respect to pH, added salt, concentration of Mg2+ ions and temperature. 2. Heterologous complexes between phenylalanyl-tRNA synthetase (E. coli) and tRNAPhe (yeast) are formed in a similar two-step mechanism as the specific complexes; the conformational transition, however, is slower by a factor 4-5. 3. Formation of non-specific complexes between phenylalanyl-tRNA synthetase (yeast) and tRNATyr (E. coli) proceeds in a one-step mechanism. Phenylalanyl-tRNA synthetase (yeast) binds either two molecules of tRNAPhe (yeast) or only one molecule of tRNATyr (E. coli); tRNA1Val (E. coli) or tRNASer (yeast) are also bound in a 1:1 stoichiometry. Binding constants for complexes of phenylalanyl-tRNA synthetase (yeast) and tRNATyr (E. coli) are determined under a variety of conditions. In contrast to specific complex formation, non-specific binding is disfavoured by the presence of Mg2+ ions, and is not affected by pH and the presence of pyrophosphate. The difference in the stabilities of specific and non-specific complexes can be varied by a factor of 2--100 depending on the ionic conditions. Discrimination of cognate and non-cognate tRNA by phenylalanyl-tRNA synthetase (yeast) is discussed in terms of the binding mechanism, the topology of the binding sites, the nature of interacting forces and the relation between specificity and ionic conditions.     

Comparison of cytoplasmic informosome proteins with RNA-binding proteins and translation initiation factors from rabbit reticulocytes.

Using one-and two-dimensional electrophoresis, the free and polyribosomal informosome proteins and a preparation of total RNA-binding proteins from rabbit reticulocytes were compared. It was shown that the major proteins of free and polyribosomal informosomes are similar only to the minor components of RNA-binding proteins. On the other hand, the major RNA-binding proteins, two of which are elongation translation factors EF-1L and EF2, can be present in informosome preparations only as minor components. The major proteins of polyribosomal informosomes do not coincide in terms of electrophoretic mobility with initiation factors eIF-2, eIF-2A, eIF-3, eIF-4A and eIF-4B. The major proteins of free informosomes differ in their electrophoretic mobility from initiation factors eIF-2A, eIF-4A and eIF-4B as well as from the alpha- and beta-subunits of initiation factor eIF-2.     

Removal of iron from diferric rabbit serum transferrin by rabbit reticulocytes.

When radioiron-labelled transferrin with 55Fe located predominantly in the N-terminal iron-binding site and 59Fe predominantly in the C-terminal iron-binding site was incubated with rabbit reticulocytes, both radioisotopes of iron were removed at similar rates. Electrophoresis of transferrin samples taken during the course of an incubation, in polyacrylamide gels containing 6 M-urea, showed that iron was removed in a pairwise fashion, giving rise to iron-free transferrin.