Functional analysis of a <i>S</i>-adenosylmethionine-insensitive methylenetetrahydrofolate reductase identified in methionine-accumulating yeast mutants

Essential amino acids (EAAs) are important for the maintenance of brain functions. Therefore, the yeast Saccharomyces cerevisiae that accumulates EAAs would help elderly people ingest appropriate levels of EAAs, which in turn could slow neurodegeneration, extend the healthy lifespan, and improve quality of life. Here, we isolated 2 mutant strains, ETH-80 and ETH-129, that accumulate the EAA methionine. Both strains were derived from a diploid laboratory yeast by conventional mutagenesis and carry a novel mutation in the MET13 gene, which encodes the Ser443Phe variant of methylenetetrahydrofolate reductase. Enzymatic analysis revealed that the Ser443Phe substitution abolished the sensitivity to S-adenosyl methionine (SAM)-mediated inhibition even in the presence of 2 m m SAM, while increasing the activity for NADPH-dependent reduction. Furthermore, yeast cells expressing the Ser443Phe variant showed a 4-fold increase in intracellular methionine content compared to the wild-type Met13. These findings will be useful for the future development of methionine-accumulating yeast strains.