Adding l-lysine derivatives to the genetic code of mammalian cells with engineered pyrrolysyl-tRNA synthetases

We report a method for site-specifically incorporating l-lysine derivatives into proteins in mammalian cells, based on the expression of the pyrrolysyl-tRNA synthetase (PylRS)-tRNA(Pyl) pair from Methanosarcina mazei. Different types of external promoters were tested for the expression of tRNA(Pyl) in Chinese hamster ovary cells. When tRNA(Pyl) was expressed from a gene cluster under the control of the U6 promoter, the wild-type PylRS-tRNA(Pyl) pair facilitated the most efficient incorporation of a pyrrolysine analog, N(epsilon)-tert-butyloxycarbonyl-l-lysine (Boc-lysine), into proteins at the amber position. This PylRS-tRNA(Pyl) system yielded the Boc-lysine-containing protein in an amount accounting for 1% of the total protein in human embryonic kidney (HEK) 293 cells. We also created a PylRS variant specific to N(epsilon)-benzyloxycarbonyl-l-lysine, to incorporate this long, bulky, non-natural lysine derivative into proteins in HEK293. The recently reported variant specific to N(epsilon)-acetyllysine was also expressed, resulting in the genetic encoding of this naturally-occurring lysine modification in mammalian cells.