The free yeast aspartyl-tRNA synthetase differs from the tRNA(Asp)-complexed enzyme by structural changes in the catalytic site, hinge region, and anticodon-binding domain.

Aminoacyl-tRNA synthetases catalyze the specific charging of amino acid residues on tRNAs. Accurate recognition of a tRNA by its synthetase is achieved through sequence and structural signalling. It has been shown that tRNAs undergo large conformational changes upon binding to enzymes, but little is known about the conformational rearrangements in tRNA-bound synthetases. To address this issue the crystal structure of the dimeric class II aspartyl-tRNA synthetase (AspRS) from yeast was solved in its free form and compared to that of the protein associated to the cognate tRNA(Asp). The use of an enzyme truncated in N terminus improved the crystal quality and allowed us to solve and refine the structure of free AspRS at 2.3 A resolution. For the first time, snapshots are available for the different macromolecular states belonging to the same tRNA aminoacylation system, comprising the free forms for tRNA and enzyme, and their complex. Overall, the synthetase is less affected by the association than the tRNA, although significant local changes occur. They concern a rotation of the anticodon binding domain and a movement in the hinge region which connects the anticodon binding and active-site domains in the AspRS subunit. The most dramatic differences are observed in two evolutionary conserved loops. Both are in the neighborhood of the catalytic site and are of importance for ligand binding. The combination of this structural analysis with mutagenesis and enzymology data points to a tRNA binding process that starts by a recognition event between the tRNA anticodon loop and the synthetase anticodon binding module.