Precise photochemical control of protein function can be achieved through the site-specific introduction of caging groups. Chemical and enzymatic methods, including in vitro translation and chemical ligation, have been used to photocage proteins in vitro. These methods have been extended to allow the introduction of caged proteins into cells by permeabilization or microinjection, but cellular delivery remains challenging. Since lysine residues are key determinants for nuclear localization sequences, the target of key post-translational modifications (including ubiquitination, methylation, and acetylation), and key residues in many important enzyme active sites, we were interested in photocaging lysine to control protein localization, post-translational modification, and enzymatic activity. Photochemical control of these important functions mediated by lysine residues in proteins has not previously been demonstrated in living cells. Here we synthesized 1 and evolved a pyrrolysyl-tRNA synthetase/tRNA pair to genetically encode the incorporation of this amino acid in response to an amber codon in mammalian cells. To exemplify the utility of this amino acid, we caged the nuclear localization sequences (NLSs) of nucleoplasmin and the tumor suppressor p53 in human cells, thus mislocalizing the proteins in the cytosol. We triggered protein nuclear import with a pulse of light, allowing us to directly quantify the kinetics of nuclear import.