A complete inventory of all enzymes in the eukaryotic methionine salvage pathway.

The methionine salvage pathway is universally used to regenerate methionine from 5'-methylthioadenosine, a byproduct of certain reactions involving S-adenosylmethionine. We identified and verified the genes encoding the enzymes of all steps in this cycle in a commonly used eukaryotic model system: the yeast Saccharomyces cerevisiae. The genes encoding 5'-methylthioribose-1-phosphate isomerase and 5'-methylthioribulose-1-phosphate dehydratase are herein named MRI1 and MDE1, respectively. The 5'-methylthioadenosine phosphorylase was verified as Meu1p, the 2,3-dioxomethiopentane-1-phosphate enolase/phosphatase as Utr4p and the aci-reductone dioxygenase as Adi1p. The homologue of the enolase/phosphatase gene, YNL010w, was excluded from its candidate role in the cycle. The methodology used involved auxotrophic growth tests and analysis of intracellular 5'-methylthioadenosine in deletion mutants. The last step, a transamination of 4-methylthio-2-oxobutyrate to yield methionine, was found to be a highly redundant step. It was catalysed by amino acid transaminases, mainly coupled with aromatic and branched chain amino acids as amino donors, but also with proline, lysine and glutamate/glutamine. The aromatic amino acid transaminases, Aro8p and Aro9p, and the branched chain amino acid transaminases, Bat1p and Bat2p, seemed to be the main enzymes exhibiting 4-methylthio-2-oxobutyrate transaminase activity. Bat2p was found to be less specific and used proline, lysine, tyrosine and glutamate as amino donors in addition to the branched chain amino acids. Thus, for the first time, all enzymes of the methionine salvage pathway were identified in a eukaryote.