Low molecular weight aspartyl-tRNA synthetase from porcine thyroid. Purification, characterization, and heterogeneity

Nucleolar localization of human methionyl-tRNA synthetase and its role in ribosomal RNA synthesis

Human aminoacyl-tRNA synthetases (ARSs) are normally located in cytoplasm and are involved in protein synthesis. In the present work, we found that human methionyl-tRNA synthetase (MRS) was translocated to nucleolus in proliferative cells, but disappeared in quiescent cells. The nucleolar localization of MRS was triggered by various growth factors such as insulin, PDGF, and EGF. The presence of MRS in nucleoli depended on the integrity of RNA and the activity of RNA polymerase I in the nucleolus. The ribosomal RNA synthesis was specifically decreased by the treatment of anti-MRS antibody as determined by nuclear run-on assay and immunostaining with anti-Br antibody after incorporating Br-UTP into nascent RNA. Thus, human MRS plays a role in the biogenesis of rRNA in nucleoli, while it is catalytically involved in protein synthesis in cytoplasm.

Genomic organization of the rat aspartyl-tRNA synthetase gene family: a single active gene and several retropseudogenes

The genomic organization of the gene encoding rat aspartyl-tRNA synthetase (AspRS), a class II aminoacyl-tRNA synthetase (aaRS), was determined. A single active gene and several pseudogenes were isolated from a rat genomic DNA library and characterized. The active DRS1 gene encoding the rat AspRS spans approximately 60 kb and is divided into 16 exons. Exons 8-16, encoding the nt-binding domain of the synthetase, are clustered in the 3'-region of the gene, whereas exons 3, 4, and 5, encoding the anticodon-binding domain are separated by large introns (up to 15 kb) containing LINE sequences. One of the pseudogenes, psi DRS1, has a nt sequence 93% identical to that of the complete cDNA sequence of rat AspRS but several stop codons interrupt the coding sequence, thus identifying psi DRS1 to an inactive processed pseudogene. Two repetitive elements from the LINE family are inserted into psi DRS1. Calculation of nt substitution rates suggests that psi DRS1 sequences arose approximately 27 Myr ago. The other pseudogene, psi DRS2, should be more ancient. Taken together, these results clearly demonstrate that the AspRS gene family is composed of only one active gene. The availability of the gene structure of AspRS could help to clarify molecular evolution of class II aaRS.

The microheterogeneity of the crystallizable yeast cytoplasmic aspartyl-tRNA synthetase

Yeast aspartyl-tRNA synthetase is a dimeric enzyme (alpha 2, Mr 125,000) which can be crystallized either alone or complexed with tRNAAsp. When analyzed by electrophoretic methods, the pure enzyme presents structural heterogeneities even when recovered from crystals. Up to three enzyme populations could be identified by polyacrylamide gel electrophoresis and more than ten by isoelectric focusing. They have similar molecular masses and mainly differ in their charge. All are fully active. This microheterogeneity is also revealed by ion-exchange chromatography and chromatofocusing. Several levels of heterogeneity have been defined. A first type, which is reversible, is linked to redox effects and/or to conformational states of the protein. A second one, revealed by immunological methods, is generated by partial and differential proteolysis occurring during enzyme purification from yeast cells harvested in growth phase. As demonstrated by end-group analysis, the fragmentation concerns exclusively the N-terminal end of the enzyme. The main cleavage points are Gln-19, Val-20 and Gly-26. Six minor cuts are observed between positions 14 and 33. The present data are discussed in the perspective of the crystallographic studies on aspartyl-tRNA synthetase.

Pyrophosphate-caused inhibition of the aminoacylation of tRNA by the leucyl-tRNA synthetase from Neurospora crassa

Inorganic pyrophosphate inhibits the aminoacylation of tRNALeu by the leucyl-tRNA synthetase from Neurospora crassa giving very low Kapp.i, PPi values of 3-20 microM. The inhibition by pyrophosphate, together with earlier kinetic data, suggest a reaction mechanism where leucine, ATP and tRNA are bound to the enzyme in almost random order, and pyrophosphate is dissociated before the rate-limiting step. A kinetic analysis of this mechanism shows that the measured Kapp.i values do not give the real dissociation constant but it is about 0.4 mM. Other dissociation constants are 90 microM for leucine, 2.2 mM for ATP and 1 microM for tRNALeu. At the approximate conditions of the living cell (2 mM ATP, 100 microM leucine and 150 microM PPi) the leucyl-tRNA synthetase is about 85% inhibited by pyrophosphate.

A complex from cultured Chinese hamster ovary cells containing nine aminoacyl-tRNA synthetases. Thermolabile leucyl-tRNA synthetase from the tsH1 mutant cell line is an integral component of this complex

The size distribution of the 20 aminoacyl-tRNA synthetases from wild-type Chinese hamster ovary (CHO) cells and from the mutant cell line tsH1, containing a temperature-sensitive leucyl-tRNA synthetase, was determined by gel filtration. Nine aminoacyl-tRNA synthetases, specific for arginine, aspartic acid, glutamic acid, glutamine, isoleucine, leucine, lysine, methionine and proline, which coeluted as high-Mr entities (Mr approximately 1.2 X 10(6)), were further co-purified to yield a multienzyme complex, the polypeptide composition of which was identical to that previously determined for the complex from rabbit liver. Immunoprecipitates obtained from crude extracts of wild-type and tsH1 mutant cells, using specific antibodies directed to the lysyl-tRNA or methionyl-tRNA synthetase components of the complex, displayed the same polypeptide compositions as that of the purified complex, thereby establishing the heterotypic nature of this complex. Although the activity of leucyl-tRNA synthetase from the mutant cells, grown at a permissive temperature, was low compared to that from the wild-type, the polypeptide of Mr 129 000, corresponding to this enzyme, was present in similar amounts and occurred exclusively as a component of the high-Mr complex. Finally, we report that attempts to demonstrate phosphorylation of the components of the complex from cultured CHO, HeLa and C3 cells were unsuccessful.

Is Ap4A an activator of eukaryotic DNA replication?

The most well established fact concerning Ap4A metabolism is that the concentration of this compound is cell cycle and cell proliferation dependent. An additional intriguing fact is that Ap4A can stimulate DNA synthesis in cell extracts, and when injected into living cells. In view of these facts, it is not surprising that Ap4A has been postulated to regulate the initiation of DNA replication. However, in our opinion, experimental efforts designed to test this hypothesis do not conclusively link Ap4A to DNA replication. Work on the mechanism of stimulation of DNA synthesis in vitro indicates that Ap4A and a variety of adenylated nucleotides increase DNA synthetic rates by acting as primers. Thus far there is no evidence that this primer function plays a role in the initiation of normal DNA replication in vivo, or that Ap4A is unique in this capacity to stimulate initiation processes. Additional experiments have shown an association of partially purified DNA alpha polymerase with both tryptophanyl-tRNA synthetase and a protein capable of binding Ap4A. The Ap4A-binding protein appears to be necessary for Ap4A to assume the correct conformation for priming, since physiological levels of Ap4A are not stimulatory for highly purified DNA alpha polymerase. The relevance of tRNA synthetases to the regulation hypothesis is their ability to produce Ap4A. Ironically, mammalian tryptophanyl-tRNA synthetase does not appear to have this capacity. Furthermore, the association of alpha polymerase with either Ap4A-binding protein or tryptophanyl-tRNA synthetase in vivo has not been conclusively demonstrated. Although Ap4A has been postulated to regulate many phenomena in eukaryotes and bacteria, such as entry into S phase and the response to oxygen deprivation, the links between Ap4A and these processes are still only circumstantial. It is tempting to extrapolate from the alarmone and stringent responses of bacteria to other systems, but these phenomena are not known to occur in eukaryotic cells. Similar deprivation and inhibition experiments in mammalian cells have been shown to stop growth at a synchronous position in cell cycle, and the Ap4A concentration has been found simply to vary accordingly. The addition or depletion of Ap4A from intracellular pools has not been shown to alter cell cycle. Therefore, while the speculation concerning the role of Ap4A in vivo is a good source of future experiments, at this point its role as an important regulatory compound is far from demonstrated.

Large scale purification and structural properties of yeast aspartyl-tRNA synthetase

A large scale purification procedure of baker's yeast aspartyl-tRNA synthetase is described which yields more than 200 mg pure protein starting from 30 Kg of wet commercial cells. The synthetase is an alpha 2 dimer of Mr = 125,000 +/- 5,000 which can be crystallized (J. Mol. Biol. 138, 1980, 129-135). The enzyme has an elongated shape with a Stokes radius of 50 A and a frictional ratio of 1.5. The synthetase has a tendency to aggregate but methods are described where this effect is overcome.