Department of Structural Biology, The Weizmann Institute of Science
In view of the notorious idiosyncrasies of each aminoacyl-tRNA synthetase and its functional complexes with cognate tRNA it is necessary to obtain detailed structural information about every tRNA-synthetase complex. Structural studies of PheRS-tRNAPhe complex are of special interest and importance because PheRS has the most complex subunit organization among 20 aaRSs (ab)2, demonstrates anomalous charging properties and contains a variety of structural domains with yet unknown functions. The crystal structure of the functional complex tRNAPhe - Phenylalanyl-tRNA synthetase from Thermus thermophilus (FRSTT) was solved at 3.3 resolution. The crystal structure of FRSTT complexed with the cognate tRNA revealed a novel mode of synthetase-tRNA interactions involving all the four subunits of the (ab)2 enzyme, which demonstrates the absolute functional necessity for a heterodimeric organization of the molecule. The function of anticodon recognition is performed by the B8 domain having the topology of the RBD fold of the small spliceosomal protein U1A. The conformation of the N-terminal domain of the a-subunit that is disordered in the native enzyme structure is stabilized in presence of tRNA. This domain represents a coiled-coil structure (helical arm) similar to the N-terminal domain of SRS. The overall orientation of tRNA with respect to the active site appeared to be similar to other tRNA-RS structures of class II. It means that the choice of the primary site of aminoacylation depends only on the local conformation of the CCA end of the tRNA substrate.