Structure-specific tRNA-binding protein from the extreme thermophile Aquifex aeolicus

The genome of the bacterium Aquifex aeolicus encodes a polypeptide which is related to a small portion of a sequence found in one prokaryotic and two eukaryotic tRNA synthetases. It also is related to a portion of Arc1p, a tR NA-binding protein believed to be important for nuclear trafficking of tRNA s, Here we cloned, expressed and purified the all amino acid polypeptide (d esignated Trbp111) and showed by ultracentrifugation analysis that it is a stable dimer in solution. The protein was also crystallized in a monoclinic lattice, X-ray diffraction analysis at 2.8 Angstrom resolution revealed a prominent non-crystallographic 2-fold axis, consistent with the presence of a symmetric homodimeric structure, Band-shift analysis with polyacrylamide gels showed that the dimer binds tRNAs, but not RNA duplexes, RNA hairpins , single-stranded RNA nor 5S rRNA, Complex formation with respect to tRNA i s non-specific, with a single tRNA bound per dimer. Thus, Trbp111 is a stru cture-specific tRNA-binding protein. These results and other considerations raise the possibility that Trbp111 is a tRNA-specific chaperone which stab ilizes the native L-shaped fold in the extreme thermophile and which has be en incorporated into much larger tRNA-binding proteins of higher organisms.