Abstract
Lipid droplets (LDs) are conserved specialized organelles that are surrounded by a single phospholipid layer, enclosing neutral lipids in form of triacylglycerol (TAG) and/or sterol ester. They are formed at the endoplasmic reticulum (ER), and stay connected to it in the yeast cell. These LDs mainly play major roles in lipid storage and homeostasis. Moreover, they have several functions in the yeast cell including; protein degradation, protein glycosylation, response to ER stress, in addition to providing substrate for energy metabolism and for membrane synthesis. In general, TAG and phosphatidylinositol (PI) synthesis share the same precursor; phosphatidic acid (PA), which controls their level. By the actions of lipases, phosphatases and/or kinases; one can act as a precursor for another, thus explaining the reason of their inverse correlation. TAG lipolysis is catalyzed by TAG lipases producing fatty acids and diacylglycerol (DAG). The DAG produced from lipolysis is phosphorylated by diacylglycerol kinase (Dgk1) to form PA. On the other hand, PA is a cell signaling molecule and once increased, it upregulates cytidine diphosphate diacylglycerol (CDP-DAG) pathway for the PI synthesis. Major TAG lipases are mainly localized in LD, whereas minor amounts of them are localized in ER. Once DAG is produced inside the LD, it is transferred to the ER by inheritance cortical ER protein, Ice2, for further reactions. The TAG and PI levels are also controlled by transcriptional regulation in response to inositol and zinc. The aim of this study was to clarify how LDs regulate lipid homeostasis in the yeast cell, and particularly focuses on the balance between TAG and PI.
Highlights
Lipids are involved in several biological processes in the cells including: formation of the cell membrane; which insulates the cell interior protecting it from the surrounding environment, storage and production of energy within the cell, cell signaling, facilitating cell division by providing membrane flexibility and mediating the cell fusion and apoptosis (Kimura et al, 2016)
In this review we focused on TAG synthesis, lipid droplets (LDs) biogenesis, TAG and PI inverse correlation, and discussed the mechanism of regulation of PI by TAG synthesis, storage and degradation, to clarify how phospholipids and storage lipids are regulated
Previous works conducted by Jain et al, (2007); Riekhof et al, (2007) revealed that the second reaction is the acylation of lyso-Phosphatidic acid (PA) by Slc1 and Ale1 acyltransferases to form sn1, 2diacylgycerol-3-phosphate called PA
Summary
Lipids are involved in several biological processes in the cells including: formation of the cell membrane; which insulates the cell interior protecting it from the surrounding environment, storage and production of energy within the cell, cell signaling, facilitating cell division by providing membrane flexibility and mediating the cell fusion and apoptosis (Kimura et al, 2016). According to the recent study of Yeagle, (2016), phosphatidylinositol (PI) is a major phospholipid in addition to phosphatidyl choline (PC), phosphatidyl ethanolamine (PE) and phosphatidyl serine (PS); and composes up to 10 % of the total phospholipid in the eukaryotic cells. Both TAG and PI biosynthesis takes places in ER; the last step of synthesis of some amounts of TAG is completed in LDs. TAG is built up of three fatty acids and a glycerol, while PI is built up of two fatty acids, a glycerol and a phosphate group with an inositol ring (Fig. 1). In this review we focused on TAG synthesis, LD biogenesis, TAG and PI inverse correlation, and discussed the mechanism of regulation of PI by TAG synthesis, storage and degradation, to clarify how phospholipids and storage lipids are regulated
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