N γ ‐Hydroxyasparagine: A Multifunctional Unnatural Amino Acid That is a Good P1 Substrate of Asparaginyl Peptide Ligases

PAL-Mediated Ligation for Protein and Cell-Surface Modification

Peptidyl Asx-specific ligases (PALs) effect peptide ligation by catalyzing transpeptidation reactions at Asn/Asp-peptide bonds. Owing to their high efficiency and mild aqueous reaction conditions, these ligases have emerged as powerful biotechnological tools for protein manipulation in recent years. PALs are enzymes of the asparaginyl endopeptidase (AEP) superfamily but have predominant transpeptidase activity as opposed to typical AEPs which are predominantly hydrolases. Butelase-1 and VyPAL2, two PALs discovered by our teams, have been used successfully in a wide range of applications, including macrocyclization of synthetic peptides and recombinant proteins, protein N- or C-terminal modification, and cell-surface labeling. As shown in numerous reports, PAL-mediated ligation is highly efficient at Asn junctions. Although considerably less efficient, Asp-specific ligation has also been shown to be practically useful under suitable conditions. Herein, we describe the methods of using VyPAL2 for protein macrocyclization and labeling at an Asp residue as well as for protein dual labeling through orthogonal Asp- and Asn-directed ligations. We also describe a method for cell-surface protein modification using butelase-1, demonstrating its advantageous features over previous methods.