Magnesium ion-mediated binding to tRNA by an amino-terminal peptide of a class II tRNA synthetase

Aspartyl-tRNA synthetase is a class II tRNA synthetase and occurs in a mult isynthetase complex in mammalian cells. Human Asp-tRNA synthetase contains a short 32-residue amino-terminal extension that can control the release of charged tRNA and its direct transfer to elongation factor 1 alpha; however , whether the extension binds to tRNA directly or interacts with the synthe tase active site is not known. Full-length human AspRS, but not amino-termi nal 32 residue-deleted, fully active As-pRS, was found to bind to noncognat e tRNA(fMet) in the presence of Mg2+. Synthetic amino-terminal peptides bou nd similarly to tRNA(fMet), whereas little or no binding of polynucleotides , poly(dA-dT), or polyphosphate to the peptides was found. The apparent bin ding constants to tRNA by the peptide increased with increasing concentrati ons of Mg2+, suggesting Mg2+ mediates the binding as a new mode of RNA pept ide interactions. The binding of tRNA(fMet), amino-terminal peptides was al so observed using fluorescence-labeled tRNAs and circular dichroism. These results suggest that a small peptide can bind to tRNA selectively and that evolution of class II tRNA synthetases may involve structural changes of am ino-terminal extensions for enhanced selective binding of tRNA.