Expression of selenoproteins via genetic code expansion in mammalian cells.

Selenoproteins, which contain the 21st amino acid selenocysteine, play roles in maintaining cellular redox homeostasis. Many open questions remain in the field of selenoprotein biology, including the functions of a number of uncharacterized human selenoproteins, and the properties of selenocysteine compared to its analogous amino acid cysteine. The mechanism of selenocysteine incorporation involves an intricate machinery that deviates from the mechanism of incorporation for the canonical 20 amino acids. As a result, recombinant expression of selenoproteins has been historically challenging, and has hindered a deeper evaluation of selenoprotein biology. Genetic code expansion methods, which incorporate protected analogs of selenocysteine, allow the endogenous selenocysteine incorporation mechanism to be bypassed entirely to facilitate selenoprotein expression. Here we present a method for incorporating a photocaged selenocysteine amino acid (DMNB-Sec) into human selenoproteins directly in mammalian cells. This approach offers the opportunity to study human selenoproteins in their native cellular environment and should advance our understanding of selenoprotein biology.