The leucyl/phenylalanyl-tRNA-protein transferase. Overexpression and characterization of substrate recognition,...

Previous work has shown that, in the bacterium Escherichia coli, the aat gene is essential for the degradation of proteins bearing amino-terminal Arg and Lys residues via the N-end rule pathway of protein degradation. We now show that the aat gene encodes directly the leucyl/phenylalanyl-tRNA-protein transferase (L/F-transferase). This enzyme catalyzes the transfer of Leu, Phe, and, less efficiently, Met and Trp, from aminoacyl-tRNAs, to the amino terminus of acceptor proteins. We have used the cloned aat gene to overexpress and purify an affinity tagged L/F-transferase. The recombinant L/F-transferase is as active as the previously purified wild type enzyme and contains no detectable RNA component. We have used the recombinant enzyme to demonstrate that both the solubility and substrate specificity, for aminoacyl-tRNA substrates, of the L/F-transferase are dependent on ionic strength conditions and that the modified nucleotides found in natural tRNAs are not essential for recognition by the enzyme. Limited digestion of the L/F-transferase with trypsin removes the proline rich NH2 terminus of the enzyme identifying a globular core, and circular dichroism demonstrates that the L/F-transferase is predominantly alpha-helical. Finally, a region of sequence conservation between the L/F-transferase and the NH2-terminal protein acetylases has been identified.