Isoelectric focusing and isoelectric points of aminoacyl-tRNA synthetases

Handling mammalian mitochondrial tRNAs and aminoacyl-tRNA synthetases for functional and structural characterization

The mammalian mitochondrial (mt) genome codes for only 13 proteins, which are essential components in the process of oxidative phosphorylation of ADP into ATP. Synthesis of these proteins relies on a proper mt translation machinery. While 22 tRNAs and 2 rRNAs are also coded by the mt genome, all other factors including the set of aminoacyl-tRNA synthetases (aaRSs) are encoded in the nucleus and imported. Investigation of mammalian mt aminoacylation systems (and mt translation in general) gains more and more interest not only in regard of evolutionary considerations but also with respect to the growing number of diseases linked to mutations in the genes of either mt-tRNAs, synthetases or other factors. Here we report on methodological approaches for biochemical, functional, and structural characterization of human/mammalian mt-tRNAs and aaRSs. Procedures for preparation of native and in vitro transcribed tRNAs are accompanied by recommendations for specific handling of tRNAs incline to structural instability and chemical fragility. Large-scale preparation of mg amounts of highly soluble recombinant synthetases is a prerequisite for structural investigations that requires particular optimizations. Successful examples leading to crystallization of four mt-aaRSs and high-resolution structures are recalled and limitations discussed. Finally, the need for and the state-of-the-art in setting up an in vitro mt translation system are emphasized. Biochemical characterization of a subset of mammalian aminoacylation systems has already revealed a number of unprecedented peculiarities of interest for the study of evolution and forensic research. Further efforts in this field will certainly be rewarded by many exciting discoveries.

Interaction of eukaryotic threonyl-tRNA synthetase with high-Mr RNAs and tRNA(Thr)

Threonyl-tRNA synthetase of rabbit reticulocytes was purified to homogeneity. We have found that this enzyme can interact not only with cognate tRNA(Thr), but also with high-Mr RNAs. tRNA(Thr) removes rRNA from the complexes with threonyl-tRNA synthetase. On the other hand, rRNA is unable to dissociate tRNA(Thr) from the complexes with the enzyme. Despite its dimeric organization, threonyl-tRNA synthetase is unable to form stable ternary complexes with tRNA(Thr) and rRNA. In the extract of rabbit reticulocytes about one-third of the threonyl-tRNA synthetase molecules are in association with cognate tRNA(Thr) and thus are unable to interact with high-Mr RNAs.

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.

Purification and characterization of bolaffinine, a toxic protein from Boletus affinis Peck (Boletaceae)

A toxic protein of Mr 22,000, called bolaffinine, has been purified from the mushroom, Boletus affinis Peck (Boletaceae), with a 0.65% yield using a procedure involving four steps: ammonium sulfate precipitation, two chromatographies on ion-exchange columns and gel filtration on Sephadex G-75. It is thermolabile and can be inactivated by organic solvents. Its isoelectric point lies between pH 9 and 10. It contains 234 amino acids, no free SH groups and one disulfide bridge as evidenced by reactions with 5-5'-dithiobis-nitrobenzoate (DTNB) and 2-mercaptoethanol. When injected into mice, it provokes the death of animals within 16-24 h. The 24 h LD50 is 61 mg/kg of body weight. Bolaffinine inhibits protein synthesis in hepatoma tissue culture and in a rabbit reticulocyte lysate.

The complete amino acid sequence of cytoplasmic aspartyl-tRNA synthetase from Saccharomyces cerevisiae

The crystallizable cytoplasmic aspartyl-tRNA synthetase from Saccharomyces cerevisiae is a dimer made up of identical subunits (Mr 63 000). Its primary structure was established using peptide sequences from four different digests of the native and citraconylated enzyme with trypsin, cyanogen bromide and staphylococcal protease. The oligonucleotide sequence of the structural gene was used as a template for the final alignment of the various peptides in the correct order.

High-performance liquid chromatographic analysis of crystals of tRNA, aminoacyl-tRNA synthetase, and their complex

High-performance liquid chromatography on an ion exchanger column was successfully used for a rapid biochemical analysis of crystals of yeast tRNAAsp and aspartyl-tRNA synthetase as well as cocrystals formed by the synthetase and the tRNA.