Site-Specific Sequential Protein Labeling Catalyzed by a Single Recombinant Ligase.

Protein ligases of defined substrate specificity are versatile tools for protein engineering. Upon completion of the reaction, the products of currently reported protein ligases contain the amino acid sequence that is recognized by that same ligase, resulting in repeated cycles of ligation and hydrolysis as competing reactions. Thus, previous efforts to sequentially label proteins at distinct positions required ligases of orthogonal specificity. A recombinant asparaginyl endopeptidase, AEP1, is promiscuous for incoming nucleophiles. This promiscuity enabled us to define a nucleophile composed of natural amino acids that is ligated efficiently to the substrate yet yields a product that is poorly recognized by AEP1. Proteins modified with an efficient recognition module could be readily modified to yield a defined product bearing a cleavage-resistant motif, whereas proteins containing this inferior recognition motif remained essentially unmodified. We demonstrate the versatility of the N- or C-terminal protein modifications obtainable with this approach and modify the N- and C-termini of a single substrate protein in a sequential, site-specific manner in excellent yield.