Solution to the double discrimination problem by the class II threonyl-tRNA synthetase

Aminoacyl-tRNA synthetases play a crucial role in the translation of the ge netic code by attaching amino acids to their cognate tRNAs through a two-st ep aminoacylation reaction. Threonyl-tRNA synthetase, a class II enzyme, fa ces a double discrimination problem against similar amino acids valine and s(1) erine. The structure of the enzyme in complex with its tRNA showed for the first time the presence of a zinc ion in the active site of an aminoac yl-tRNA synthetase. The zinc ion is directly involved in amino acid recogni tion by forming a pentacoordinate intermediate with threonine, thus making valine binding impossible. We showed recently using a cocrystal structure o f the enzyme and a seryl-adenylate analog that serine could bind in the act ive site. However, the enzyme uses a separate and specific editing site in the N-terminal module for hydrolyzing the incorrectly formed ser-tRNA(Thr). Thus, both classes of synthetases use effectively the ability of the CCA-e nd of tRNA to switch between a hairpin and a helical conformation for amino acylation and editing. The study shows that the RNA molecule is a key playe r in the evolution of this family of enzymes. Since the editing modules app eared much later compared to the catalytic domains, they could serve as new targets for drug design.