Chromatofocusing of aminoacyl-tRNA synthetases, extracted from NMRI mouse liver

Noncanonical function of glutamyl-prolyl-tRNA synthetase: gene-specific silencing of translation

Aminoacyl tRNA synthetases (ARS) catalyze the ligation of amino acids to cognate tRNAs. Chordate ARSs have evolved distinctive features absent from ancestral forms, including compartmentalization in a multisynthetase complex (MSC), noncatalytic peptide appendages, and ancillary functions unrelated to aminoacylation. Here, we show that glutamyl-prolyl-tRNA synthetase (GluProRS), a bifunctional ARS of the MSC, has a regulated, noncanonical activity that blocks synthesis of a specific protein. GluProRS was identified as a component of the interferon (IFN)-gamma-activated inhibitor of translation (GAIT) complex by RNA affinity chromatography using the ceruloplasmin (Cp) GAIT element as ligand. In response to IFN-gamma, GluProRS is phosphorylated and released from the MSC, binds the Cp 3'-untranslated region in an mRNP containing three additional proteins, and silences Cp mRNA translation. Thus, GluProRS has divergent functions in protein synthesis: in the MSC, its aminoacylation activity supports global translation, but translocation of GluProRS to an inflammation-responsive mRNP causes gene-specific translational silencing.

Histidyl-tRNA synthetase

Histidyl-tRNA synthetase (HisRS) is responsible for the synthesis of histidyl-transfer RNA, which is essential for the incorporation of histidine into proteins. This amino acid has uniquely moderate basic properties and is an important group in many catalytic functions of enzymes. A compilation of currently known primary structures of HisRS shows that the subunits of these homo-dimeric enzymes consist of 420-550 amino acid residues. This represents a relatively short chain length among aminoacyl-tRNA synthetases (aaRS), whose peptide chain sizes range from about 300 to 1100 amino acid residues. The crystal structures of HisRS from two organisms and their complexes with histidine, histidyl-adenylate and histidinol with ATP have been solved. HisRS from Escherichia coli and Thermus thermophilus are very similar dimeric enzymes consisting of three domains: the N-terminal catalytic domain containing the six-stranded antiparallel beta-sheet and the three motifs characteristic of class II aaRS, a HisRS-specific helical domain inserted between motifs 2 and 3 that may contact the acceptor stem of the tRNA, and a C-terminal alpha/beta domain that may be involved in the recognition of the anticodon stem and loop of tRNA(His). The aminoacylation reaction follows the standard two-step mechanism. HisRS also belongs to the group of aaRS that can rapidly synthesize diadenosine tetraphosphate, a compound that is suspected to be involved in several regulatory mechanisms of cell metabolism. Many analogs of histidine have been tested for their properties as substrates or inhibitors of HisRS, leading to the elucidation of structure-activity relationships concerning configuration, importance of the carboxy and amino group, and the nature of the side chain. HisRS has been found to act as a particularly important antigen in autoimmune diseases such as rheumatic arthritis or myositis. Successful attempts have been made to identify epitopes responsible for the complexation with such auto-antibodies.

A mammalian tryptophanyl-tRNA synthetase is associated with protein kinase activity

Bovine Trp-tRNA synthetase is a dimer with subunit molecular mass of 60 kDa (p60) which catalyzes ATP-dependent formation of tryptophanyl-tRNA. Evidence is presented that Trp-tRNA synthetase whose homogeneity had been proven by SDS/PAGE and silver staining of the gel is autophosphorylated in vitro. Anti-(Trp-tRNA synthetase) antibodies, whose specificity was verified by using a combination of different approaches, were able to effectively inhibit and immunoprecipitate the Trp-tRNA-synthetase-associated kinase activity. The two-dimensional tryptic phosphopeptide map of autophosphorylated p60 Trp-tRNA synthetase was found to be similar to that of its major 40-kDa degradation fragment bearing resemblance to previously demonstrated unlabeled peptide patterns of the Trp-tRNA synthetase forms. Trp-tRNA synthetase which had undergone denaturation during SDS/PAGE, regained serine/threonine specific protein kinase activity (PK 60) after guanidine treatment. Trp-tRNA synthetase induced phosphorylation of specific substrate such as 100-kDa protein in non-immune but not in anti-(Trp-tRNA synthetase) sera which distinguishes Trp-tRNA-synthetase-associated kinase from other protein kinases. Sequence analysis permitted the identification of regions of bovine Trp-tRNA synthetase sharing similarity with the catalytic domains of known protein kinases. These findings suggest that PK 60 and Trp-tRNA synthetase (p60) are either closely related or identical.

Lysyl-tRNA synthetase from yeast. Discrimination of amino acids by native and phosphorylated species

Discrimination factors (D) which are characteristic for discrimination between lysine and 19 naturally occurring non-cognate amino acids have been determined from kcat and Km values for native and phosphorylated lysyl-tRNA synthetases from yeast. Generally, both species of this class II aminoacyl-tRNA synthetase are considerably less specific than the class I synthetases specific for isoleucine, valine, tyrosine, and arginine. D values of the native enzyme are in the range 90-1700, D values of the phosphorylated species in the range 40-770. The phosphorylated enzyme acts faster and less accurately. In aminoacylation of tRNALys-C-C-A(2'NH2) discrimination factors D1 vary over 30-980 for the native and over 8-300 for the phosphorylated enzyme. From AMP formation stoichiometry and D1 values pretransfer proof-reading factors (II1) of 1.1-56 were calculated for for the native enzyme, factors of 1.0-44 for the phosphorylated species. Post-transfer proof-reading factors (II2) were calculated from D values and AMP formation stoichiometry in acylation of tRNALys-C-C-A. Pretransfer proof-reading is the main correction step, posttransfer proof-reading is less effective or negligible (II2 approximately 1-8). Initial discrimination factors (I), which are due to differences in Gibbs free energies of binding between lysine and noncognate substrates (delta delta GI), were calculated from discrimination and proof-reading factors. In contrast to class I synthetases, for lysyl-tRNA synthetase only one initial discrimination step can be assumed and amino acid recognition is reduced to a three-step process instead of the four-step recognition observed for the class I synthetases. Plots of delta delta GI values against accessible surface areas of amino acids show clearly that phosphorylation of the enzyme changes the structures of the amino acid binding sites. This is illustrated by a hypothetical 'stopper model' of these sites.

The activities of aminoacyl-tRNA synthetase phosphatase and aminoacyl-tRNA synthetases in MPC-11 and Krebs II ascites cells

The activity of aminoacyl-tRNA synthetase phosphatase as well as the activities of aminoacyl-tRNA synthetases in Krebs II ascites cells and MPC-11 cells have been investigated. The activity of the phosphatase was greater in the tumor cells than in normal tissues. The aminoacyl-tRNA synthetase activities were 100-300 times higher than the activities found in the uterus of ovariectomized mice, but not very different from the activities found in the liver. The influence of cyclic AMP. 2-deoxyadenosine 3-phosphate and 2-deoxyguanosine 3-phosphate on the growth of MPC-11 cells, grown in suspension culture, was also investigated.