Accumulation and Storage of Age-Damaged (R,S)AdoMet in S. cerevisiae and its Subsequent Utilization by Two Methyltransferase Enzymes

C.R. Vinci, S.G. Clarke
University of California, Los Angeles, Los Angeles, CA

Cells employ numerous pathways to combat the aging process, some of which involves either the disposal or repair of damaged small molecules. The biological methyl donor S-adenosyl-L-methionine (AdoMet) is one of the most significant of these damageable small molecules due to its role as methyl donor in almost all methyltransferase reactions. AdoMet can exist in two diastereoisomeric states with respect to its sulfonium ion. The S configuration, (S,S)-AdoMet, is the only form that is produced enzymatically as well as the only form used in almost all biological methylation reactions. Under physiological conditions, however, the sulfonium ion can spontaneously racemize to the R form, producing (R,S)-AdoMet. As of yet, (R,S)-AdoMet has no known physiological function and may inhibit cellular reactions.

Previously, we found two Saccharomyces cerevisiae enzymes that are capable of recognizing (R,S)-AdoMet and using it to methylate homocysteine to form methionine. These enzymes are the products of the SAM4 and MHT1 genes, identified earlier as homocysteine methyltransferases dependent upon AdoMet and S-methylmethionine, respectively. We found that Sam4 recognizes both (S,S)- and (R,S)-AdoMet, but that its activity is much higher with the R,S form. Mht1 reacts with only the R,S form of AdoMet, whereas no activity is seen with the S,S form. We have further shown that R,S-Specific homocysteine methyltransferase activity is not specific to yeast, occurring in the extracts of Arabidopsis thaliana, Drosophila melanogaster, and Caenorhabditis elegans, though curiously not in Mus musculus.

Currently, we have found that (R,S)AdoMet can build up in yeast kept in stationary phase and that this accumulation is significantly greater in cells in which both Sam4 and Mht1 have been knocked out. We have also found that (R,S)AdoMet can be transported into yeast cells from the external media via the transport protein Sam3 and that it can be stored in its R,S form, presumably in the vacuole. These findings suggest that (R,S)AdoMet could accumulate in yeast and be stored in the vacuole, but then be converted to methionine and eventually (S,S)AdoMet as needed through the action of the Sam4 and Mht1 enzymes.

Keywords: Age-damaged AdoMet, Yeast, Methyltransferases, Diastereoisomers