Identification of novel lysophospholipid acyltransferases in yeast (LPT1) and humans (LPCAT3) and regulation of triglyceride synthesis in yeast

Abstract

Cells require membranes for survival and need energy to meet the costs of living. Phospholipids and triglyceride together fulfill both the requirements. Phospholipids are predominant membrane lipids and triglycerides allow for storage of surplus energy therby enhancing survival in energy poor conditions. Therefore, an understanding of how cell regulates triglyceride synthesis and maintains membrane composition by acyl chain remodeling of phospholipids is important.Saccharomyces cerevisiae was used as a model system to study the intracellular regulation of triglyceride synthesis at the step of diglyceride esterification. Using metabolic labeling with [3H] oleate, we found that the inability to hydrolyze fatty acids, significantly (p<0.05) increased triglyceride synthesis by 75%. Disabling glycogen synthesis (alternative energy reserve) did not alter triglyceride synthesis. Additionally, inability to store energy as triglyceride did not result in a compensatory increase in glycogen synthesis. To address the possibility of regulation at the level of enzyme activity, microsomal diacylglycerol acyltransferase assays were performed in the presence of a variety of metabolites (AMP, ADP, ATP, NAD+, NADH, acetyl-CoA and malonyl-CoA). DGAT activity was not regulated at physiological concentrations of these metabolites.Acyl chains in the sn-2 position of phospholipids are introduced by de novo synthetic pathway involving 1-acylglycerol-3-phosphate acyltransferase (AGPAT) or by acyl chain remodeling that requires lysophospholipid acyltransferases (LPLAT). LPT1 (YOR175c) an acyltransferase with unknown function was identified by its synthetic lethal interaction with Slc1, the known AGPAT in yeast. LPT1 was shown to encode for a novel AGPAT/ LPLAT in yeast with high apparent affinity for saturated acyl-CoAs but higher apparent Vmax for monounsaturated acyl-CoAs. Lpt1 does not share sequence similarity with known LPLAT/AGPAT family members and belongs to membranebound-O-acyltransferase (MBOAT) super family. Homology search with Lpt1 identified the human MBOAT5, a protein with unknown function. Cell lysates from insect cells expressing human MBOAT5 (renamed as LPCAT3) showed a 10 fold increase in LPCAT activity over control. In summary, the identification of a novel LPLAT in yeast has proved to be the founding member of LPLAT family in humans. Future characterization of this family is likely to shed light on how membrane composition is adjusted and controlled in humans.%%%%Ph.D., Biological Sciences – Drexel University, 2008