An efficient opal-suppressor tryptophanyl pair creates new routes for simultaneously incorporating up to three distinct noncanonical amino acids into proteins in mammalian cells

The site-specific incorporation of multiple distinct noncanonical amino acids (ncAAs) into proteins in mammalian cells is an emergent technology with much potential. For each different ncAA to be incorporated, this technology requires a distinct orthogonal aminoacyl-tRNA synthetase (aaRS)/tRNA pair that recognizes a distinct nonsense codon. The aaRS/tRNA pairs currently available for ncAA mutagenesis in eukaryotes are all traditionally used to decode the TAG nonsense codon. Unfortunately, these pairs suppress the other two nonsense codons, TGA or TAA, at a significantly lower level, compromising the scope of multi-ncAA mutagenesis. Here we report that the bacteria-derived tryptophanyl (EcTrp) pair is an excellent TGA-suppressor in mammalian cells. Additionally, we show that this pair does not cross-react with any of the three previously established aaRS/tRNA pairs. Consequently, the TGA-suppressing EcTrp pair can be combined with TAG-suppressing pyrrolysyl (archaeal), tyrosyl (bacterial), or leucyl (bacterial) pairs to develop three new routes for dual-ncAA incorporation in mammalian cells. We show that all three platforms enable site-specific incorporation of two distinct ncAAs into proteins - including a full-length humanized antibody - with excellent fidelity and good efficiency. Finally, we combined the EcTrp pair with the bacterial Tyr pair and the archaeal pyrrolysyl pair to site-specifically incorporate different combinations of three distinct ncAAs into a reporter protein in mammalian cells.