The C-terminal extension of yeast seryl-tRNA synthetase affects stability of the enzyme and its substrate affinity

Saccharomyces cerevisiae seryl-tRNA synthetase (SerRS) contains a 20-amino acid C-terminal extension, which is not found in prokaryotic SerRS enzymes. A truncated yeast SES1 gene, lacking the 60 base pairs that encode this C-terminal domain, is able to complement a yeast SES1 null allele strain; thus, the C-terminal extension in SerRS is dispensable for the viability of the cell. However, the removal of the C-terminal peptide affects both stability of the enzyme and its affinity for the substrates. The truncation mutant binds tRNA with 3.6-fold higher affinity, while the Km for serine is 4-fold increased relative to the wild-type SerRS. This indicates the importance of the C-terminal extension in maintaining the overall structure of SerRS.