An essential residue in the flexible peptide linking the two idiosynchratic domains of bacterial tyrosyl-tRNA synthetases

Comparison of quercetin and dihydroquercetin: antioxidant-independent actions on erythrocyte and platelet membrane

We investigated the effects of two flavonoids quercetin and dihydroquercetin (DHQ), which have different solubilities and antioxidant capacities, on hemolysis and platelet aggregation in human blood. Exposure of human red blood cells (RBCs) to free radicals generated by 2,2'-azobis(2-amidinopropane) dihydrochloride (AAPH) for 2h resulted in 63.5+/-3.9% hemolysis (vehicle: 0.3+/-0.4%). Pre-incubation of RBCs with lipid-soluble quercetin and water-soluble DHQ for 30min significantly reduced the AAPH-induced hemolysis to 3.6+/-1.5% and 32.5+/-5.6% respectively. In contrast, quercetin and DHQ were similarly effective in reducing phospholipase C-induced hemolysis (37.2+/-9.1% and 45.4+/-10.0% versus vehicle 75.7+/-5.2%, P<0.001). Pre-incubation with quercetin, but not DHQ, inhibited the aggregation of platelets by adenosine diphosphate. DHQ was more potent than quercetin in inhibiting superoxide produced by xanthine oxidase. These results suggest that the antihemolytic effects of flavonoids may not be directly mediated by removal of free radicals and may likely be due to their interaction with cell membrane.

Expression, purification, and characterization of human tyrosyl-tRNA synthetase

Human tyrosyl-tRNA synthetase is a homodimeric enzyme and each subunit is near 58 KD. It catalyzes the aminoacylation of tRNA(Tyr) by L-tyrosine. The His(6)-tagged human TyrS gene was obtained by RT-PCR from total RNA of human lung giant-cell cancer strain 95 D. It was confirmed by sequencing and cloned into the expression vector pET-24 a (+) to yield pET-24 a (+)-HTyrRS, which was transfected into Escherichia coli BL21-CodonPlus-RIL. The induced-expression level of His(6)-tagged human TyrRS was about 24% of total cell proteins under IPTG inducing. The recombinant protein was conveniently purified in a single step by metal (Ni(2+)) chelate affinity chromatography. About 22.3mg purified enzyme could be obtained from 1L cell culture. The k(cat) value of His(6)-tagged human TyrRS in the second step of tRNA(Tyr) aminoacylation was 1.49 s(-1). The K(m) values of tyrosine and tRNA(Tyr) were 0.3 and 0.9 microM. Six His residues at the C terminus of human TyrRS have little effect on the activities of the enzyme compared with other eukaryotic TyrRSs.

Class I tyrosyl-tRNA synthetase has a class II mode of cognate tRNA recognition

Bacterial tyrosyl-tRNA synthetases (TyrRS) possess a flexibly linked C-terminal domain of approximately 80 residues, which has hitherto been disordered in crystal structures of the enzyme. We have determined the structure of Thermus thermophilus TyrRS at 2.0 A resolution in a crystal form in which the C-terminal domain is ordered, and confirm that the fold is similar to part of the C-terminal domain of ribosomal protein S4. We have also determined the structure at 2.9 A resolution of the complex of T.thermophilus TyrRS with cognate tRNA(tyr)(G Psi A). In this structure, the C-terminal domain binds between the characteristic long variable arm of the tRNA and the anti-codon stem, thus recognizing the unique shape of the tRNA. The anticodon bases have a novel conformation with A-36 stacked on G-34, and both G-34 and Psi-35 are base-specifically recognized. The tRNA binds across the two subunits of the dimeric enzyme and, remarkably, the mode of recognition of the class I TyrRS for its cognate tRNA resembles that of a class II synthetase in being from the major groove side of the acceptor stem.

tRNA-like recognition of group I introns by a tyrosyl-tRNA synthetase

The Neurospora crassa mitochondrial tyrosyl-tRNA synthetase (CYT-18 protein) functions in splicing group I introns by promoting the formation of the catalytically active RNA structure. Previous work suggested that CYT-18 recognizes a conserved tRNA-like structure of the group I intron catalytic core. Here, directed hydroxyl-radical cleavage assays show that the nucleotide-binding fold and C-terminal domains of CYT-18 interact with the expected group I intron cognates of the aminoacyl-acceptor stem and D-anticodon arms, respectively. Further, three-dimensional graphic modeling, supported by biochemical data, shows that conserved regions of group I introns can be superimposed over interacting regions of the tRNA in a Thermus thermophilus TyrRS/tRNA(Tyr) cocrystal structure. Our results support the hypothesis that CYT-18 and other aminoacyl-tRNA synthetases interact with group I introns by recognizing conserved tRNA-like structural features of the intron RNAs.

Structure and dynamics of the anticodon arm binding domain of Bacillus stearothermophilus Tyrosyl-tRNA synthetase

The structure of a recombinant protein, TyrRS(delta4), corresponding to the anticodon arm binding domain of Bacillus stearothermophilus tyrosyl-tRNA synthetase, has been solved, and its dynamics have been studied by nuclear magnetic resonance (NMR). It is the first structure described for such a domain of a tyrosyl-tRNA synthetase. It consists of a five-stranded beta sheet, packed against two alpha helices on one side and one alpha helix on the other side. A large part of the domain is structurally similar to other functionally unrelated RNA binding proteins. The basic residues known to be essential for tRNA binding and charging are exposed to the solvent on the same face of the molecule. The structure of TyrRS(delta4), together with previous mutagenesis data, allows one to delineate the region of interaction with tRNATyr.