Molecular identification of transmembrane protein 68 as an endoplasmic reticulum-anchored and brain-specific protein

Ping’an Chang,Christoph Heier,Quan Sun,Liping Han,Feifei Huang,Wenzhen Qin,Heping Cao

doi:10.1371/journal.pone.0176980

Abstract

Acyltransferases catalyze essential reactions in the buildup and remodeling of glycerophospholipids and contribute to the maintenance and diversity of cellular membranes. Transmembrane protein 68 (TMEM68) is an evolutionarily conserved protein of unknown function, that forms a distinct subgroup within the glycerophospholipid acyltransferase family. In the current study we expressed murine TMEM68 for the first time in mammalian cells to characterize its subcellular localization, topology, and possible biological function(s). We show that TMEM68 is an integral membrane protein and orients both, the N- and C-terminus towards the cytosol. Live cell imaging demonstrated that TMEM68 is localized mainly at the endoplasmic reticulum (ER), but not at cellular lipid droplets (LDs). The positioning of TMEM68 at the ER was dependent on its first transmembrane domain (TMD), which by itself was sufficient to target cytosolic green fluorescence protein (GFP) to the ER. In contrast, a second TMD was dispensable for ER localization of TMEM68. Finally, we found that among multiple murine tissues the expression level of TMEM68 transcripts was highest in brain. We conclude that TMEM68 is an integral ER membrane protein and a putative acyltransferase involved in brain glycerolipid metabolism.

Highlights

Glycerophospholipids are essential structural components of biological membranes, lipoproteins and pulmonary surfactant, and serve as precursors of bioactive signaling lipids, such as platelet-activating factor (PAF) and eicosanoids [1, 2]
De novo biosynthesis of glycerophospholipids is initiated by glycerol-3-phosphate acyltransferase (GPAT), which catalyzes the synthesis of lysophosphatidic acid (LPA) from glycerol 3-phosphate by transfer of an acyl moiety from acyl-CoAs
Phylogenetic alignments with glycerophospholipid and DAG acyltransferase family members of human, mouse and fruit fly revealed that murine Transmembrane protein 68 (TMEM68) and orthologous proteins form an evolutionarily conserved subgroup, which is distinct from other acyltransferases (Fig 1C)

Summary

Introduction

Glycerophospholipids are essential structural components of biological membranes, lipoproteins and pulmonary surfactant, and serve as precursors of bioactive signaling lipids, such as platelet-activating factor (PAF) and eicosanoids [1, 2]. De novo biosynthesis of glycerophospholipids is initiated by glycerol-3-phosphate acyltransferase (GPAT), which catalyzes the synthesis of lysophosphatidic acid (LPA) from glycerol 3-phosphate by transfer of an acyl moiety from acyl-CoAs. A second acyl moiety is transferred to LPA by 1-acylglycerol-3-phosphate acyltransferase (AGPAT) resulting in the formation of phosphatidic acid (PA). May 4, 2017 phospholipids, diacylglycerol (DAG), and triacylglycerol (TAG) and can be further metabolized via different enzymatic pathways [4, 5]. Dephosphorylation of PA by PA phosphatase yields DAG as a precursor for the synthesis of TAG, as well as for phosphatidylcholine (PC) and phosphatidylethanolamine (PE) via a metabolic route commonly referred to as the Kennedy pathway. A second metabolic route converts PA to cytidine-diphospho-DAG, the precursor of other glycerophospholipids, such as phosphatidylinositol (PI), phosphatidylglycerol (PG), cardiolipin (CL), and phosphatidylserine (PS) [4, 5]

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Results

Conclusion