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Genetic code expansion via amber suppression allows cotranslational, site-specific introduction of nonnatural chemical groups into proteins in the living cell. The archaeal pyrrolysine-tRNA/pyrrolysine-tRNA synthetase (PylT/RS) pair from Methanosarcina mazei (Mma) has been established for incorporation of a wide range of noncanonical amino acids (ncAAs) in mammalian cells. Once integrated in an engineered protein, ncAAs allow for simple click-chemistry derivatization, photo-cage control of enzyme activity, and site-specific placement of posttranslational modifications. We previously described a modular amber suppression plasmid system for generating stable cell lines via piggyBac transposition in a range of mammalian cells. Here we detail a general protocol for the generation of CRISPR-Cas9 knock-in cell lines using the same plasmid system. The knock-in strategy relies on CRISPR-Cas9-induced double-strand breaks (DSBs) and nonhomologous end joining (NHEJ) repair to target the PylT/RS expression cassette to the AAVS1 safe harbor locus in human cells. MmaPylRS expression from this single locus is sufficient for efficient amber suppression when the cells are subsequently transfected transiently with a PylT/gene of interest plasmid., (© 2023. The Author(s), under exclusive license to Springer Science+Business Media, LLC, part of Springer Nature.)

Noncanonical amino acid mutagenesis via amber suppression provides the means to tailor proteins inside living cells. A wide range of noncanonical amino acids have been incorporated using the Methanococcus pyrrolysyl-tRNA synthetase/tRNACUA (PylRS/PylT) in mammalian cell systems in proof of principle experiments, for (1) minimal genetically encoded fluorescence or affinity tagging, (2) photo-control of enzymes, (3) genetically encoded posttranslational protein modifications. We have developed a general and efficient method to genomically integrate the PylRS/PylT amber suppression machinery using PiggyBac-mediated transposition. A general protocol for the generation of stable amber suppression cell lines is described here. Using the modular plasmid system, homogenous and highly efficient amber suppression in a wide range of cell lines can be achieved.