Enrichment and characterization of the mRNAs of four aminoacyl-tRNA synthetases from yeast

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.

Tryptophanyl-tRNA synthetase is a major soluble protein species in bovine pancreas

Besides their central role in protein synthesis, aminoacyl-tRNA synthetases have been found or thought to be involved in other processes. We present here a study showing that tryptophanyl-tRNA synthetase has a surprising tissular distribution. Indeed, immunochemical determinations showed that in several bovine organs such as liver, kidney and heart, tryptophanyl-tRNA synthetase constitutes, as expected, about 0.02% of soluble proteins. In spleen, brain cortex, stomach, cerebellum or duodenum, this amount is about 10-times higher, and in pancreas it is 100-fold. There is no correlation between these amounts and the RNA content of the organs. Moreover, the concentration of another aminoacyl-tRNA synthetase (methionyl-tRNA synthetase) is higher in liver than in pancreas, while the amount of tRNATrp is not higher in pancreas than in liver as compared to other tRNAs. Among several interpretations, it is possible that tryptophanyl-tRNA synthetase is involved in a function other than tRNA aminoacylation. This unknown function would be specific to the differentiated organs, since fetal cerebellum and fetal pancreas contain the same amount of tryptophanyl-tRNA synthetase as adult liver.

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.

Isolation and characterization of the yeast aspartyl-tRNA synthetase gene

A yeast genomic library in Escherichia coli, constructed by insertion of Sau3A restriction fragments into the hybrid Saccharomyces cerevisiae-E. coli plasmid pFL1, was screened by a radioimmunoassay (RIA) for colonies expressing yeast aspartyl-tRNA synthetase (AspRS). Four clones were isolated by this technique. Data obtained by Southern and restriction analysis of the inserts showed a common 3.8-kb BamHI restriction fragment which, when inserted into the plasmid pFL1, gave a positive RIA. Several controls showed that this 3.8-kb insert codes for the entire AspRS: (i) S. cerevisiae transformed by the PFL1 plasmid carrying the 3.8-kb fragment overproduces AspRS activity by a factor of ten compared to the wild-type yeast strain; and (ii) a new protein with electrophoretic behaviour similar to AspRS and immuno-reactive toward anti-AspRS appears in crude extracts of transformed yeast and E. coli.