A Noncanonical Function of Sortase Enables Site‐Specific Conjugation of Small Molecules to Lysine Residues in Proteins

We provide the first demonstration that isopeptide ligation, a noncanonical activity of the enzyme sortase A, can be used to modify recombinant proteins. This reaction was used in vitro to conjugate small molecules to a peptide, an engineered targeting protein, and a full‐length monoclonal antibody with an exquisite level of control over the site of conjugation. Attachment to the protein substrate occurred exclusively through isopeptide bonds at a lysine ε‐amino group within a specific amino acid sequence. This reaction allows more than one molecule to be site‐specifically conjugated to a protein at internal sites, thereby overcoming significant limitations of the canonical native peptide ligation reaction catalyzed by sortase A. Our method provides a unique chemical ligation procedure that is orthogonal to existing methods, supplying a new method to site‐specifically modify lysine residues that will be a valuable addition to the protein conjugation toolbox.