Genetic Code Expansion of Mammalian Cells with Unnatural Amino Acids

The expansion of the genetic code of mammalian cells enables the incorporation of unnatural amino acids into proteins. This is achieved by adding components to the protein biosynthetic machinery, specifically an engineered aminoacyl‐tRNA synthetase/tRNA pair. The unnatural amino acids are chemically synthesized and supplemented to the growth medium. Using this methodology, fundamental new chemistries can be added to the functional repertoire of the genetic code of mammalian cells. This protocol outlines the steps necessary to incorporate a photocaged lysine into proteins and showcases its application in the optical triggering of protein translocation to the nucleus. © 2015 by John Wiley & Sons, Inc.