Transnational control of lipid homeostasis in the cell cycle of budding yeast

Abstract

Proper coordination between cell growth and division relies heavily on several metabolic processes such as lipid metabolism. Lipids play indispensable roles both as key signaling molecules and in membrane biogenesis. Moreover, de novo lipid synthesis is essential to executing the faithful progression of cell cycle events. Our lab discovered that the translational efficiency of the critical lipogenic enzymes acetyl coenzyme A carboxylase (ACC1) and fatty acid synthase (FAS) peaked in mitosis. We also reported that the abundance of significant lipid species was higher late in the cell cycle. Overall, all the evidence suggested that lipid synthesis impinged on the later phases of the cell cycle. However, it is not known if and how lipid biogenesis can promote any specific cell cycle event. In this thesis, I report that increased lipogenesis promotes nuclear division. I mutated inhibitory elements, upstream open reading frames (uORFs), in the 5' leaders of ACC1 and FAS1 of budding yeast. Translation of ACC1 and FAS1 is de-repressed in the absence of the uORFs. The protein level of Fas1p is elevated almost 4-fold, with no significant changes in the mRNA level. Furthermore, the uORF mutant produces more lipids and accelerates the nuclear division. Accurate lipid trafficking is also vital for the proper timing of mitotic events. I used a temperature-sensitive mutant of Sec14p (sec14-1), a phosphatidylinositol transfer protein (PITP), to investigate the roles of lipid trafficking in the cell cycle. Even at the permissive temperature, sec14-1 cells suffer from a perturbed cell cycle. The mutant cells are bigger, grow slowly, and are delayed in the G2/M phase. Nuclear division is severely delayed in sec14-1 cells. However, de-repressing the translation of ACC1 and FAS1 corrects the cell cycle phenotypes of sec14-1 cells. These results provide a context to probe connections among membrane trafficking, lipogenesis, and cell division. In summary, I have identified a unique gain-of-function mutant of fatty acid synthesis. My work shows that translational control of lipogenesis is a critical input in the cell cycle. It is not merely required for mitosis, but it can actively promote nuclear division, a key landmark of the eukaryotic cell cycle.