Regulation of S-adenosylmethionine levels in Saccharomyces cerevisiae

Methylenetetrahydrofolate reductase (MTHFR) catalyzes the reduction of 5,10-methylenetetrahydrofolate to 5-methyltetrahydrofolate, used to methylate homocysteine in methionine biosynthesis. Methionine can be activated by ATP to give rise to the universal methyl donor, S-adenosylmethionine (AdoMet). Previously, a chimeric MTHFR (Chimera-1) comprised of the yeast Met13p N-terminal catalytic domain and the Arabidopsis thaliana MTHFR (AtMTHFR-1) C-terminal regulatory domain was constructed (Roje, S., Chan, S. Y., Kaplan, F., Raymond, R. K., Horne, D. W., Appling, D. R., and Hanson, A. D. (2002) J. Biol. Chem. 277, 4056-4061). Engineered yeast (SCY4) expressing Chimera-1 accumulated more than 100-fold more AdoMet and 7-fold more methionine than the wild type. Surprisingly, SCY4 showed no appreciable growth defect. The ability of yeast to hyperaccumulate AdoMet was investigated by studying the intracellular compartmentation of AdoMet as well as the mode of hyperaccumulation. Previous studies have established that AdoMet is distributed between the cytosol and the vacuole. A strain expressing Chimera-1 and lacking either vacuoles (vps33 mutant) or vacuolar polyphosphate (vtc1 mutant) was not viable when grown under conditions that favored AdoMet hyperaccumulation. The hyperaccumulation of AdoMet was a robust phenomenon when these cells were grown in medium containing glycine and formate but did not occur when these supplements were replaced by serine. The basis of the nutrient-dependent AdoMet hyperaccumulation effect is discussed in relation to homocysteine biosynthesis and sulfur metabolism.