Mammalian high molecular weight and monomeric forms of valyl-tRNA synthetase

Global Effects of Mistranslation from an Editing Defect in Mammalian Cells

Aminoacyl-tRNA synthetases prevent mistranslation, or genetic code ambiguity, through specialized editing reactions. Mutations that disrupt editing in bacteria adversely affect cell growth and viability, and recent work in the mouse supports the idea that translational errors caused by an editing defect lead to a neurological disease-like phenotype. To further investigate the connection of mistranslation to cell pathology, we introduced an inducible transgene expressing an editing-deficient valyl-tRNA synthetase into mammalian cells. Introducing mistranslation precipitated a disruption of cell morphology and membrane blebbing, accompanied by activation of caspase-3, consistent with an apoptotic response. Addition of a noncanonical amino acid that is misactivated, but not cleared, by the editing-defective enzyme exacerbated these effects. A special ambiguity-detecting sensor provided direct readout of mistranslation in vivo, supporting the possibility that decreased translational fidelity could be associated with disease.

Glutamylsulfamoyladenosine and pyroglutamylsulfamoyladenosine are competitive inhibitors of E. coli glutamyl-tRNA synthetase

5'-O-[N-(L-glutamyl)-sulfamoyl] adenosine is a potent competitive inhibitor of E. coli glutamyl-tRNA synthetase with respect to glutamic acid (K(i) = 2.8 nM) and is the best inhibitor of this enzyme. It is a weaker inhibitor of mammalian glutamyl-tRNA synthetase (K(i) = 70 nM). The corresponding 5'-O-[N-(L-pyroglutamyl)-sulfamoyl] adenosine is a weak inhibitor (K(i) = 15 microM) of the E. coli enzyme.

Human acyl-CoA:cholesterol acyltransferase-1 is a homotetrameric enzyme in intact cells and in vitro

Acyl-CoA:cholesterol acyltransferase (ACAT) is a key enzyme in cellular cholesterol homeostasis and in atherosclerosis. ACAT-1 may function as an allosteric enzyme. We took a multifaceted approach to investigate the subunit composition of ACAT-1. When ACAT-1 with two different tags were co-expressed in the same Chinese hamster ovary cells, antibody specific to one tag caused co-immunoprecipitation of both types of ACAT-1 proteins. Radioimmunoprecipitations of cells expressing the untagged ACAT-1 or the 6-histidine-tagged ACAT-1 yielded a single radiolabeled band of predicted size on SDS-polyacrylamide gel electrophoresis. These results show that ACAT-1 exists as homo-oligomers in intact Chinese hamster ovary cells. We solubilized HisACAT-1 with the detergent deoxycholate or CHAPS (3-[(3-cholamidopropyl)-dimethylammonio]-1-propanesulfonic acid), performed gel filtration chromatography and sucrose density gradient centrifugations in H(2)O and D(2)O, and determined the Stokes radii and sedimentation coefficients of the HisACAT1-detergent complexes. The estimated molecular mass of HisACAT-1 is 263 kDa, which is 4 times that of the HisACAT-1 monomer (69 kDa). Finally, cross-linking experiments in intact cells and in vitro show that the increase in cross-linker concentrations causes an increase in size of the HisACAT-1-positive signals, forming material(s) 4 times the size of the monomer, supporting the conclusion that ACAT-1 is a homotetrameric enzyme.

Cloning, sequencing and expression of a cDNA encoding mammalian valyl-tRNA synthetase

A fragment of the cDNA encoding a rat valyl-tRNA synthetase (TrsVal)-like protein was cloned from a rat cDNA library in lambda gt11 using an oligodeoxyribonucleotide (oligo) probe. Three independent plaque clones containing the human TrsVal cDNA were then isolated from a lambda gt10 human erythroleukemia cDNA library using the rat cDNA fragment as the hybridization probe. Sequence analyses of the cDNA fragments provided a 3.2-kb sequence with an open reading frame that contained the 'HIGH' synthetase signature sequence and the tRNA 3'-end-binding motif, KMSKS, and putative Val-binding motif, EWCISRQ. The sequence was extended to the 3' end of the cDNA by the polymerase chain reaction using an internal primer and an oligo(dT) adapter. The deduced 1051-amino-acid sequence shares 65% identity with yeast TrsVal, and contains a highly basic N-terminal region, a newly evolved protease-sensitive region in sequence close to the C terminus, and several sites for protein kinase C phosphorylation. A 3-kb cDNA fragment was sub-cloned into plasmid pSVL and expressed in COS-7 cells; up to a sevenfold increase in TrsVal activity was obtained. These results confirm the cloning and sequencing of a human TrsVal-encoding cDNA.

Elongation factor-1 messenger-RNA levels in cultured cells are high compared to tissue and are not drastically affected further by oncogenic transformation

Copy-DNA clones covering the complete coding sequence of human Elongation Factor-1 gamma mRNA have been isolated and characterized. The expression of Elongation Factor-1 in a variety of cell lines and a number of tissues shows a large increase in Elongation Factor-1 mRNA going from tissue to cultured cells (20-fold). Messenger-RNA levels for Elongation Factor-1 alpha, -1 beta and -1 gamma increase in parallel suggesting coordinate regulation of the expression of these genes. Oncogenic transformation in vitro does not strongly affect Elongation Factor-1 mRNA levels.

Purification and properties of a high-molecular-mass complex between Val-tRNA synthetase and the heavy form of elongation factor 1 from mammalian cells

In extracts of various mammalian tissues obtained in the presence of protease inhibitors Val-tRNA synthetase exists exclusively as a complex with a molecular mass of about 800 kDa. This complex was purified by gel filtration and two HPLC steps and contained five different polypeptides with molecular masses of 140, 50, 50, 40 and 30 kDa. The complex seems to have no tissue or species specificity, because preparations with identical polypeptide composition were obtained by the same method from rabbit liver and reticulocytes, and rat and beef liver. Four low-molecular-mass polypeptides were identified by two-dimensional electrophoresis as subunits of the heavy form of elongation factor 1 (EF-1H). The complex possesses the activity of EF-1 in the poly(U)-directed translation system, indicating that EF-1H is an integral part of the complex. Gel filtration of the tissue extracts reveals three different peaks of EF-1 activity, corresponding to EF-1 alpha, EF-1H and the high-molecular-mass complex of Val-tRNA synthetase and EF-1H. All activity of Val-tRNA synthetase and about 25% of EF-1 activity are associated with the complex. Different forms of EF-1 revealed no significant differences in the nucleotide-binding properties, but the complex of Val-tRNA synthetase with EF-1H was 10 times more active in the poly(U)-directed binding of Phe-tRNAPhe to ribosomes than EF-1H. These results strongly suggest that the complex of Val-tRNA synthetase with EF-1H is a novel functionally active individual form of EF-1.

Co-purification of the aminoacyl-tRNA synthetase complex with the elongation factor eEF1

A multi-enzyme complex of mammalian aminoacyl-tRNA synthetases was isolated from rabbit reticulocytes, and purified by polyethylene glycol fractionation and gel filtration on Biogel A15m and affinity chromatography on tRNA-Sepharose. The synthetase complex contains nine synthetase activities, and the corresponding proteins as analyzed by SDS polyacrylamide gel electrophoresis. Three of the proteins showed the identical subunit molecular weights to those of the reticulocyte's elongation factor eEF1H. The eEF1 alpha protein could not be removed by second tRNA-Sepharose column chromatography, or gel filtration on Biogel A5m or Biogel A15m. Antibodies against eEF1 alpha react with the purified synthetase complex on the basis of dot blot analysis. This finding should provide new clues for elucidating the structural organization of the mammalian protein biosynthetic machinery.

Mammalian valyl-tRNA synthetase forms a complex with the first elongation factor

The high-molecular-mass form of valyl-tRNA synthetase is associated with the first elongation factor activity. It includes two polypeptides of about 50 kDa and two others of 40 and 30 kDa, identified as alpha, beta, gamma and delta subunits of eEF-1H. The complex of valyl-tRNA synthetase with eEF-1H is suggested to be a novel form of the first elongation factor.