Genetically Encoded ε-N-Methyl Lysine in Mammalian Cells

Newly evolved pyrrolysyl‐tRNA synthetases from M. barkeri and M. mazei tRNA${{{\rm Pyl}\hfill \atop {\rm CUA}\hfill}}}$ were used to site‐specifically incorporate photocaged methyl lysine into proteins in both E. coli and mammalian cells with high fidelity and efficiency. A GFP mutant containing photocaged methyl lysine at residue 40 was efficiently photoconverted to GFP40→methyl lysine with >365 nm light. This amino acid should facilitate studies of the function of protein methylation in vitro and in living cells.