The CRISPR/Cas9 system has emerged as an important tool in biomedical research for a wide range of applications, with significant potential for genome engineering and gene therapy. In order to achieve conditional control of the CRISPR/Cas9 system, a genetically encoded light-activated Cas9 was engineered through the site-specific installation of a caged lysine amino acid. Several potential lysine residues were identified as viable caging sites that can be modified to optically control Cas9 function, as demonstrated through optical activation and deactivation of both exogenous and endogenous gene function.

Rapamycin-induced protein heterodimerization of FKBP12 and FRB is one of the most commonly employed switches to conditionally control biological processes. We developed an optically activated rapamycin dimer that does not induce FKBP12-FRB dimerization until exposed to light, and applied it to control kinase, protease, and recombinase function.

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.

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