Selenolysine: A new tool for traceless isopeptide bond formation

Despite their biological importance, post-translationally modified proteins are notoriously difficult to produce in a homogeneous fashion using conventional expression systems. Chemical protein synthesis or semi-synthesis offers a solution to this problem; however, traditional strategies often rely on sulfur-based chemistry that is incompatible with the presence of multiple cysteine residues in the target protein. To overcome these limitations, we present the design and synthesis of γ-selenolysine, a selenol-containing form of the commonly modified proteinogenic amino acid, lysine. The utility of γ-selenolysine is demonstrated with the traceless ligation of the small ubiquitin-like modifier protein, SUMO-1, to a peptide fragment of human glucokinase. The resulting polypeptide is poised for native chemical ligation and selective deselenization in the presence of unprotected cysteine residues. Selenolysine’s straightforward synthesis and incorporation into synthetic peptides marks it as a universal handle for conjugating all ubiquitin-like modifying proteins to their targets.