Limited tryptic digestion of leucyl-tRNA synthetase and characterization of its active fragment.

Leucyl-tRNA synthetase (LeuRS, EC 6.1.1.4) from E. coli underwent limited proteolysis by trypsin which cut off 6K peptide and converted the intact LeuRS into a 96K fragment. The truncated enzyme retained the PPi exchange activity with the same kinetic parameters as those of native LeuRS but lost the tRNALeu charging, binding and other tRNALeu-related activities. N-terminus analysis showed that the 6K peptide was located at the C-terminus of Leu-RS. This small part played a crucial role in tRNALeu binding. Our results suggest that the two activities, PPi exchange and tRNA charging are independent of each other and correspond to different structural regions of LeuRS. The C-terminal region might be the tRNALeu binding site of LeuRS.