Utilizing Selenocysteine for Expressed Protein Ligation and Bioconjugations.

Employing selenocysteine-containing protein fragments to form the amide bond between respective protein fragments significantly extends the current capabilities of the widely used protein engineering method, expressed protein ligation. Selenocysteine-mediated ligation is noteworthy for its high yield and efficiency. However, it has so far been restricted to solid-phase synthesized seleno-peptides and thus constrained by where the selenocysteine can be positioned. Here we employ heterologously expressed seleno-fragments to overcome the placement and size restrictions in selenocysteine-mediated chemical ligation. Following ligation, the selenocysteine can be deselenized into an alanine or serine, resulting in nonselenoproteins. This greatly extends the flexibility in selecting the conjugation site in expressed protein ligations with no influence on native cysteines. Furthermore, the selenocysteine can be used to selectively introduce site-specific protein modifications. Therefore, selenocysteine-mediated expressed protein ligation simplifies incorporation of post-translational modifications into the protein scaffold.