Morphogenesis checkpoint kinase Swe1 is the executor of lipolysis-dependent cell-cycle progression

Abstract

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 ...

New role for ceramide in the pheromone response

“…Interestingly, sphingolipids have also been recently implicated in regulating the cell cycle block seen in a lipolysis defective tgl3tgl4 double mutant. 5 From this study as well as a previous study of ours, CDC55 was implicated as a critical mediator of the action of ceramide on cell cycle regulation (Ref. # 29 in Villasmil et al).…”

Regulation of Sphingolipid Biosynthesis by the Morphogenesis Checkpoint Kinase Swe1

“…Thus, it is apparent that Swe1 kinase, although not essential for cell survival, plays regulatory roles in numerous cell cycle-related processes. Indeed, in a recent study, we have shown that Swe1 phosphorylates and inhibits Cdc28 in response to defective lipolysis, which is a source of lipid precursors for sphingolipid synthesis ( 75 , 76 ). Here we show that the Swe1 checkpoint kinase positively regulates SPT, presumably by phosphorylating Orm2, independently of Ypk1.…”

Establishing the lipid droplet proteome: Mechanisms of lipid droplet protein targeting and degradation

“…The mature cytoplasmic LDs may remain in contact with the ER and/or associate with other organelles in the cell [4]. LDs serve diverse cellular functions, including sequestering toxic lipids [5,6] and acting as dynamic lipid storage depots that enable rapid mobilization of fatty acids for energy [7,8], membrane biosynthesis [9–11], and lipid signaling pathways [12,13]. Notably, dysregulation of LD homeostasis has been implicated in the pathogenesis of numerous diseases [14,15], including diseases associated with an excess of LDs (e.g.…”