S-adenosyl-L-methionine (AdoMet)-dependent methylation is central to the regulation of many biological processes: more than 50 AdoMet-dependent methyltransferases methylate a broad spectrum of cellular compounds including nucleic acids, proteins and lipids. Common to all AdoMet-dependent methyltransferase reactions is the release of the strong product inhibitor S-adenosyl-L-homocysteine (AdoHcy), as a by-product of the reaction. S-adenosyl-L-homocysteine hydrolase is the only eukaryotic enzyme capable of reversible AdoHcy hydrolysis to adenosine and homocysteine and, thus, relief from AdoHcy inhibition. Impaired S-adenosyl-L-homocysteine hydrolase activity in humans results in AdoHcy accumulation and severe pathological consequences. Hyperhomocysteinemia, which is characterized by elevated levels of homocysteine in blood, also exhibits a similar phenotype of AdoHcy accumulation due to the reversal of the direction of the S-adenosyl-L-homocysteine hydrolase reaction. Inhibition of S-adenosyl-L-homocysteine hydrolase is also linked to antiviral effects. In this review the advantages of yeast as an experimental system to understand pathologies associated with AdoHcy accumulation will be discussed.

Cell growth and division requires the precise duplication of cellular DNA content but also of membranes and organelles. Knowledge about the cell-cycle-dependent regulation of membrane and storage lipid homeostasis is only rudimentary. Previous work from our laboratory has shown that the breakdown of triacylglycerols (TGs) is regulated in a cell-cycle-dependent manner, by activation of the Tgl4 lipase by the major cyclin-dependent kinase Cdc28. The lipases Tgl3 and Tgl4 are required for efficient cell-cycle progression during the G1/S (Gap1/replication phase) transition, at the onset of bud formation, and their absence leads to a cell-cycle delay. We now show that defective lipolysis activates the Swe1 morphogenesis checkpoint kinase that halts cell-cycle progression by phosphorylation of Cdc28 at tyrosine residue 19. Saturated longchain fatty acids and phytosphingosine supplementation rescue the cell-cycle delay in the Tgl3/Tgl4 lipase-deficient strain, suggesting that Swe1 activity responds to imbalanced sphingolipid metabolism, in the absence of TG degradation. We propose a model by which TG-derived sphingolipids are required to activate the protein phosphatase 2A (PP2A Cdc55 ) to attenuate Swe1 phosphorylation and its inhibitory effect on Cdc28 at the G1/S transition of the cell cycle.T he eukaryotic cell cycle is a highly coordinated and conserved process. In addition to DNA replication, one of the major requirements for the cell to progress through the cell cycle is the precise duplication of membrane-enclosed organelles and other cellular components before cell division. Knowledge about the mechanisms regulating (membrane) lipid homeostasis during the cell cycle is scarce (1), however several levels of evidence suggest regulation of key enzymes of lipid metabolism in a cell-cycledependent manner. The PAH1-encoded phosphatidic acid (PA) phosphatase (Pah1), a key enzyme of triacylglycerol (TG) synthesis that provides the TG precursor diacylglycerol (DG), is phosphorylated and inactivated by the cyclin-dependent kinases Cdc28 and Pho85-Pho80 (2, 3). Kurat et al. showed that Tgl4, next to Tgl3, one of the two major TG lipases in yeast and the ortholog of mammalian ATGL (4, 5), is also phosphorylated by Cdc28. In contrast to Pah1, however, Tgl4 is activated by Cdc28 (6). This inverse regulation of Pah1 and Tgl4 by Cdc28-dependent phosphorylation led to the model by which the TG content oscillates during the cell cycle: On the one hand, TG synthesis serves as a buffer for excess de novo generated fatty acids (FAs), and on the other hand, in times of increased demand-that is, at the onset of bud formation and bud growth-Tgl4-catalyzed lipolysis becomes active to provide TG-derived precursors for membrane lipid synthesis (6).TG and membrane phospholipids share the same intermediates, PA and DG; PA is generated by sequential acylation of glycerol-3-phosphate, reactions that mostly take place in the endoplasmic reticulum (ER) membrane (7). The dephosphorylation of PA to DG by the PAH1-encoded PA phosphatase Pah1 is ...

S-Adenosyl-l-homocysteine hydrolase (AdoHcy hydrolase; Sah1 in yeast/AHCY in mammals) degrades AdoHcy, a by-product and strong product inhibitor of S-adenosyl-l-methionine (AdoMet)-dependent methylation reactions, to adenosine and homocysteine (Hcy). This reaction is reversible, so any elevation of Hcy levels, such as in hyperhomocysteinemia (HHcy), drives the formation of AdoHcy, with detrimental consequences for cellular methylation reactions. HHcy, a pathological condition linked to cardiovascular and neurological disorders, as well as fatty liver among others, is associated with a deregulation of lipid metabolism. Here, we developed a yeast model of HHcy to identify mechanisms that dysregulate lipid metabolism. Hcy supplementation to wildtype cells up-regulated cellular fatty acid and triacylglycerol content and induced a shift in fatty acid composition, similar to changes observed in mutants lacking Sah1. Expression of the irreversible bacterial pathway for AdoHcy degradation in yeast allowed us to dissect the impact of AdoHcy accumulation on lipid metabolism from the impact of elevated Hcy. Expression of this pathway fully suppressed the growth deficit of sah1 mutants as well as the deregulation of lipid metabolism in both the sah1 mutant and Hcy-exposed wildtype, showing that AdoHcy accumulation mediates the deregulation of lipid metabolism in response to elevated Hcy in yeast. Furthermore, Hcy supplementation in yeast led to increased resistance to cerulenin, an inhibitor of fatty acid synthase, as well as to a concomitant decline of condensing enzymes involved in very long-chain fatty acid synthesis, in line with the observed shift in fatty acid content and composition.