The Pyrrolysyl-tRNA Synthetase Activity can be Improved by a P188 Mutation that Stabilizes the Full-Length Enzyme

The amber suppression-based noncanonical amino acid (ncAA) mutagenesis technique has been widely used in both basic and applied research. So far more than two hundred ncAAs have been genetically encoded by amber codon in both prokaryotes and eukaryotes using wild-type and engineered pyrrolysyl-tRNA synthetase (PylRS)-tRNA^Pyl (PylT) pairs. Methanosarcina mazei PylRS (MmPylRS) is arguably one of two most used PylRS variants. However, it contains an unstable N-terminal domain that is usually cleaved from the full-length protein during expression and therefore leads to a low enzyme activity. We discovered that the cleavage takes place after A189 and this cleavage is inhibited when MmPylRS is co-expressed with Ca. Methanomethylophilus alvus tRNA^Pyl (CmaPylT). In the presence of CmaPylT, MmPylRS is cleaved after an alternative site K110. MmPylRS is active toward CmaPylT. Its combined use with CmaPylT leads to enhanced incorporation of N^ε-Boc-lysine (BocK) at amber codon. To prevent MmPylRS from cleavage after A189 in the presence of its cognate M. mazei tRNA^Pyl (MmPylT), we introduced mutations at P188. Our results indicated that the P188G mutation stabilizes MmPylRS. We showed that the P188G mutation in wild-type MmPylRS or its engineered variants allows enhanced incorporation of BocK and other noncanonical amino acids including N^ε-acetyl-lysine when they are co-expressed with MmPylT.