The active site His-460 of human acyl-coenzyme A:cholesterol acyltransferase 1 resides in a hitherto undisclosed transmembrane domain

Acetyl-coenzyme A acetyltransferase 1 promotes brown adipogenesis by activating the AMPK-PGC1α signaling pathway

Brown adipose tissue (BAT) is thermogenic, expressing high levels of uncoupling protein-1 to convert nutrient energy to heat energy, bypassing ATP synthesis. BAT is a promising therapeutic target for treatment of obesity and type 2 diabetes since it converts fatty acids into heat but mechanisms controlling brown adipogenesis remain unclear. Knockdown of acetyl-Coenzyme A acetyltransferase 1 (ACAT1) in C3H10T1/2 cells suppressed brown adipocyte maturation during the current study and ACAT1 overexpression promoted brown adipocyte maturation. The downstream target of AMP-activated protein kinase (AMPK), peroxisome proliferator-activated receptor gamma coactivator-1-α (PGC1α), was involved in the action of ACAT1 on brown adipocyte maturation. ACAT1 overexpression enhanced AMPK phosphorylation and promoted PGC1α expression. It is suggested that ACAT1 promotes brown adipocyte maturation by activating the AMPK-PGC1α signaling pathway.

Cellular Uptake of a Fluorescent Ligand Reveals Ghrelin O-Acyltransferase Interacts with Extracellular Peptides and Exhibits Unexpected Localization for a Secretory Pathway Enzyme

Ghrelin O-acyltransferase (GOAT) plays a central role in the maturation and activation of the peptide hormone ghrelin, which performs a wide range of endocrinological signaling roles. Using a tight-binding fluorescent ghrelin-derived peptide designed for high selectivity for GOAT over the ghrelin receptor GHSR, we demonstrate that GOAT interacts with extracellular ghrelin and facilitates ligand cell internalization in both transfected cells and prostate cancer cells endogenously expressing GOAT. Coupled with enzyme mutagenesis, ligand uptake studies support the interaction of the putative histidine general base within GOAT with the ghrelin peptide acylation site. Our work provides a new understanding of GOAT's catalytic mechanism, establishes that GOAT can interact with ghrelin and other peptides located outside the cell, and raises the possibility that other peptide hormones may exhibit similar complexity in their intercellular and organismal-level signaling pathways.

Rocking the MBOAT: Structural insights into the membrane bound O-acyltransferase family

The membrane-bound O-acyltransferase (MBOAT) superfamily catalyses the transfer of acyl chains to substrates implicated in essential cellular functions. Aberrant function of MBOATs is associated with various diseases and MBOATs are promising drug targets. There has been recent progress in structural characterisation of MBOATs, advancing our understanding of their functional mechanism. Integrating information across the MBOAT family, we characterise a common MBOAT fold and provide a blueprint for substrate and inhibitor engagement. This work provides context for the diverse substrates, mechanisms, and evolutionary relationships of protein and small-molecule MBOATs. Further work should aim to characterise MBOATs, as inherently lipid-associated proteins, within their membrane environment.

Acute ACAT1/SOAT1 Blockade Increases MAM Cholesterol and Strengthens ER-Mitochondria Connectivity

Cholesterol is a key component of all mammalian cell membranes. Disruptions in cholesterol metabolism have been observed in the context of various diseases, including neurodegenerative disorders such as Alzheimer's disease (AD). The genetic and pharmacological blockade of acyl-CoA:cholesterol acyltransferase 1/sterol O-acyltransferase 1 (ACAT1/SOAT1), a cholesterol storage enzyme found on the endoplasmic reticulum (ER) and enriched at the mitochondria-associated ER membrane (MAM), has been shown to reduce amyloid pathology and rescue cognitive deficits in mouse models of AD. Additionally, blocking ACAT1/SOAT1 activity stimulates autophagy and lysosomal biogenesis; however, the exact molecular connection between the ACAT1/SOAT1 blockade and these observed benefits remain unknown. Here, using biochemical fractionation techniques, we observe cholesterol accumulation at the MAM which leads to ACAT1/SOAT1 enrichment in this domain. MAM proteomics data suggests that ACAT1/SOAT1 inhibition strengthens the ER-mitochondria connection. Confocal and electron microscopy confirms that ACAT1/SOAT1 inhibition increases the number of ER-mitochondria contact sites and strengthens this connection by shortening the distance between these two organelles. This work demonstrates how directly manipulating local cholesterol levels at the MAM can alter inter-organellar contact sites and suggests that cholesterol buildup at the MAM is the impetus behind the therapeutic benefits of ACAT1/SOAT1 inhibition.

Targeting Sterol O-Acyltransferase/Acyl-CoA:Cholesterol Acyltransferase (ACAT): A Perspective on Small-Molecule Inhibitors and Their Therapeutic Potential

Sterol O-acyltransferase (SOAT) is a membrane-bound enzyme that aids the esterification of cholesterol and fatty acids to cholesterol esters. SOAT has been studied extensively as a potential drug target, since its inhibition can serve as an alternative to statin therapy. Two SOAT isozymes that have discrete functions in the human body, namely, SOAT1 and SOAT2, have been characterized. Over three decades of research has focused on candidate SOAT1 inhibitors with unsatisfactory results in clinical trials. Recent research has focused on targeting SOAT2 selectively. In this perspective, we summarize the literature covering various SOAT inhibitory agents and discuss the design, structural requirements, and mode of action of SOAT inhibitors.

Acyl-CoA:diacylglycerol acyltransferase: Properties, physiological roles, metabolic engineering and intentional control

Acyl-CoA:diacylglycerol acyltransferase (DGAT, EC 2.3.1.20) catalyzes the last reaction in the acyl-CoA-dependent biosynthesis of triacylglycerol (TAG). DGAT activity resides mainly in membrane-bound DGAT1 and DGAT2 in eukaryotes and bifunctional wax ester synthase-diacylglycerol acyltransferase (WSD) in bacteria, which are all membrane-bound proteins but exhibit no sequence homology to each other. Recent studies also identified other DGAT enzymes such as the soluble DGAT3 and diacylglycerol acetyltransferase (EaDAcT), as well as enzymes with DGAT activities including defective in cuticular ridges (DCR) and steryl and phytyl ester synthases (PESs). This review comprehensively discusses research advances on DGATs in prokaryotes and eukaryotes with a focus on their biochemical properties, physiological roles, and biotechnological and therapeutic applications. The review begins with a discussion of DGAT assay methods, followed by a systematic discussion of TAG biosynthesis and the properties and physiological role of DGATs. Thereafter, the review discusses the three-dimensional structure and insights into mechanism of action of human DGAT1, and the modeled DGAT1 from Brassica napus. The review then examines metabolic engineering strategies involving manipulation of DGAT, followed by a discussion of its therapeutic applications. DGAT in relation to improvement of livestock traits is also discussed along with DGATs in various other eukaryotic organisms.

SOAT1 is a new prognostic factor of colorectal cancer

Colorectal cancer (CRC) is one of the most common malignant gastrointestinal cancers. Metastasis is the major leading cause of death in patients with CRC, and many patients treated with radical surgery were diagnosed with metastasis during follow-up. However, the underlying molecular mechanisms regulating CRC metastasis are still elusive. Sterol o-acyltransferase 1 (SOAT1) is a critical participant in maintaining intracellular cholesterol balance. Here, by analyzing the clinical specimens and in vitro cell line experiments, we evaluated the clinical relevance and role of SOAT1 in regulating CRC metastasis. The results revealed that SOAT1 was overexpressed in colon cancer tissues compared to peritumor tissues at mRNA and protein levels. High intratumor SOAT1 expression correlates to lymph node metastasis and indicates poor patient disease-free survival and overall survival. The silencing of SOAT1 strongly inhibited the migration and invasion ability of CRC tumor cells. These results demonstrated that SOAT1 was upregulated in colon cancer. Upregulation of SOAT1 expression may promote CRC progression by enhancing the migration and invasion ability of CRC. Our results indicate that targeting SOAT1 activity may be applied as a promising therapeutic strategy for preventing the metastasis of CRC after radical surgical treatment.

Determination of the membrane topology of PORCN, an O-acyl transferase that modifies Wnt signalling proteins

Wnt gradients elicit distinct cellular responses, such as proliferation, specification, differentiation and survival in a dose-dependent manner. Porcupine (PORCN), a membrane-bound O-acyl transferase (MBOAT) that resides in the endoplasmic reticulum, catalyses the addition of monounsaturated palmitate to Wnt proteins and is required for Wnt gradient formation and signalling. In humans, PORCN mutations are causal for focal dermal hypoplasia (FDH), an X-linked dominant syndrome characterized by defects in mesodermal and endodermal tissues. PORCN is also an emerging target for cancer therapeutics. Despite the importance of this enzyme, its structure remains poorly understood. Recently, the crystal structure of DltB, an MBOAT family member from bacteria, was solved. In this report, we use experimental data along with homology modelling to DltB to determine the membrane topology of PORCN. Our studies reveal that PORCN has 11 membrane domains, comprising nine transmembrane spanning domains and two reentrant domains. The N-terminus is oriented towards the lumen while the C-terminus is oriented towards the cytosol. Like DltB, PORCN has a funnel-like structure that is encapsulated by multiple membrane-spanning helices. This new model for PORCN topology allows us to map residues that are important for biological activity (and implicated in FDH) onto its three-dimensional structure.

Structural insights into the inhibition mechanism of human sterol O-acyltransferase 1 by a competitive inhibitor

Sterol O-acyltransferase 1 (SOAT1) is an endoplasmic reticulum (ER) resident, multi-transmembrane enzyme that belongs to the membrane-bound O-acyltransferase (MBOAT) family. It catalyzes the esterification of cholesterol to generate cholesteryl esters for cholesterol storage. SOAT1 is a target to treat several human diseases. However, its structure and mechanism remain elusive since its discovery. Here, we report the structure of human SOAT1 (hSOAT1) determined by cryo-EM. hSOAT1 is a tetramer consisted of a dimer of dimer. The structure of hSOAT1 dimer at 3.5 Å resolution reveals that a small molecule inhibitor CI-976 binds inside the catalytic chamber and blocks the accessibility of the active site residues H460, N421 and W420. Our results pave the way for future mechanistic study and rational drug design targeting hSOAT1 and other mammalian MBOAT family members.

Structure of nevanimibe-bound tetrameric human ACAT1

Cholesterol is an essential component of mammalian cell membranes, constituting up to 50% of plasma membrane lipids. By contrast, it accounts for only 5% of lipids in the endoplasmic reticulum (ER)¹. The ER enzyme sterol O-acyltransferase 1 (also named acyl-coenzyme A:cholesterol acyltransferase, ACAT1) transfers a long-chain fatty acid to cholesterol to form cholesteryl esters that coalesce into cytosolic lipid droplets. Under conditions of cholesterol overload, ACAT1 maintains the low cholesterol concentration of the ER and thereby has an essential role in cholesterol homeostasis^2,3. ACAT1 has also been implicated in Alzheimer's disease⁴, atherosclerosis⁵ and cancers⁶. Here we report a cryo-electron microscopy structure of human ACAT1 in complex with nevanimibe⁷, an inhibitor that is in clinical trials for the treatment of congenital adrenal hyperplasia. The ACAT1 holoenzyme is a tetramer that consists of two homodimers. Each monomer contains nine transmembrane helices (TMs), six of which (TM4-TM9) form a cavity that accommodates nevanimibe and an endogenous acyl-coenzyme A. This cavity also contains a histidine that has previously been identified as essential for catalytic activity⁸. Our structural data and biochemical analyses provide a physical model to explain the process of cholesterol esterification, as well as details of the interaction between nevanimibe and ACAT1, which may help to accelerate the development of ACAT1 inhibitors to treat related diseases.

Triton X-100 or octyl glucoside inactivates acyl-CoA:cholesterol acyltransferase 1 by dissociating it from a two-fold dimer to a two-fold monomer

Cholesterol is an important lipid molecule and is needed for all mammalian cells. In various cell types, excess cholesterol is stored as cholesteryl esters; acyl-CoA:cholesterol acyltransferase 1 (ACAT1) plays an essential role in this storage process. ACAT1 is located at the endoplasmic reticulum and has nine transmembrane domains (TMDs). It is a member of the membrane-bound O-acyltransferase (MBOAT) family, in which members contain multiple TMDs and participate in a variety of biological functions. When solubilized in the zwitterionic detergent CHAPS, ACAT1 can be purified to homogeneity with full enzyme activity and behaves as a homotetrameric protein. ACAT1 contains two dimerization motifs. The first motif is located near the N-terminus and is not conserved in MBOATs. Deletion of the N-terminal dimerization domain converts ACAT1 to a dimer with full catalytic activity; therefore, ACAT1 is a two-fold dimer. The second dimerization domain, located near the C-terminus, is conserved in MBOATs; however, it was not known whether the C-terminal dimerization domain is required for enzyme activity. Here we show that treating ACAT1 with non-ionic detergent, Triton X-100 or octyl glucoside, causes the enzyme to become a two-fold monomer without any enzymatic activity. Detergent exchange of Triton X-100 with CHAPS restores ACAT1 to a two-fold dimer but fails to restore its enzymatic activity. These results implicate that ACAT1 requires hydrophobic subunit interactions near the C-terminus in order to remain active as a two-fold dimer. Our results also caution the use of Triton X-100 or octyl glucoside to purify other MBOATs.

The ER membrane protein complex promotes biogenesis of sterol-related enzymes maintaining cholesterol homeostasis

The eukaryotic endoplasmic reticulum (ER) membrane contains essential complexes that oversee protein biogenesis and lipid metabolism, impacting nearly all aspects of cell physiology. The ER membrane protein complex (EMC) is a newly described transmembrane domain (TMD) insertase linked with various phenotypes, but whose clients and cellular responsibilities remain incompletely understood. We report that EMC deficiency limits the cellular boundaries defining cholesterol tolerance, reflected by diminished viability with limiting or excessive extracellular cholesterol. Lipidomic and proteomic analyses revealed defective biogenesis and concomitant loss of the TMD-containing ER-resident enzymes sterol-O-acyltransferase 1 (SOAT1) and squalene synthase (SQS, also known as FDFT1), which serve strategic roles in the adaptation of cells to changes in cholesterol availability. Insertion of the weakly hydrophobic tail-anchor (TA) of SQS into the ER membrane by the EMC ensures sufficient flux through the sterol biosynthetic pathway while biogenesis of polytopic SOAT1 promoted by the EMC provides cells with the ability to store free cholesterol as inert cholesteryl esters. By facilitating insertion of TMDs that permit essential mammalian sterol-regulating enzymes to mature accurately, the EMC is an important biogenic determinant of cellular robustness to fluctuations in cholesterol availability.

This article has an associated First Person interview with the first author of the paper.

Highlighted Article: The ER membrane protein complex promotes biogenesis of key membrane-bound enzymes responsible for regulation of cholesterol biosynthesis and storage, an important determinant of mammalian cell viability.

Newly Identified Essential Amino Acids Affecting Chlorella ellipsoidea DGAT1 Function Revealed by Site-Directed Mutagenesis

Diacylglycerol acyltransferase (DGAT) is a rate-limiting enzyme in the synthesis of triacylglycerol (TAG), the most important form of energy storage in plants. Some residues have previously been proven to be crucial for DGAT1 activity. In this study, we used site-directed mutagenesis of the CeDGAT1 gene from Chlorella ellipsoidea to alter 16 amino acids to investigate effects on DGAT1 function. Of the 16 residues (L482R, E542R, Y553A, G577R, R579D, Y582R, R596D, H603D, H609D, A624R, F629R, S632A, W650R, A651R, Q658H, and P660R), we newly identified 5 (L482, R579, H603, A651, and P660) as being essential for DGAT1 function and 7 (E542, G577, R596, H609, A624, S632, and Q658) that significantly affect DGAT1 function to different degrees, as revealed by heterologous expression of the mutants in yeast strain INVSc1. Importantly, compared with CeDGAT1, expression of the mutant CeDGAT1Y553A significantly increased the total fatty acid and TAG contents of INVSc1. Comparison among CeDGAT1Y553A, GmDGAT1Y341A, AtDGAT1Y364A, BnDGAT1Y347A, and BoDGAT1Y352A, in which tyrosine at the position corresponding to the 553rd residue in CeDGAT1 is changed into alanine, indicated that the impact of changing Y to A at position 553 is specific for CeDGAT1. Overall, the results provide novel insight into the structure and function of DGAT1, as well as a mutant gene with high potential for lipid improvement in microalgae and plants.

Membrane topology and identification of key residues of EaDAcT, a plant MBOAT with unusual substrate specificity

Euonymus alatus diacylglycerol acetyltransferase (EaDAcT) catalyzes the transfer of an acetyl group from acetyl-CoA to the sn-3 position of diacylglycerol to form 3-acetyl-1,2-diacyl-sn-glycerol (acetyl-TAG). EaDAcT belongs to a small, plant-specific subfamily of the membrane bound O-acyltransferases (MBOAT) that acylate different lipid substrates. Sucrose gradient density centrifugation revealed that EaDAcT colocalizes to the same fractions as an endoplasmic reticulum (ER)-specific marker. By mapping the membrane topology of EaDAcT, we obtained an experimentally determined topology model for a plant MBOAT. The EaDAcT model contains four transmembrane domains (TMDs), with both the N- and C-termini orientated toward the lumen of the ER. In addition, there is a large cytoplasmic loop between the first and second TMDs, with the MBOAT signature region of the protein embedded in the third TMD close to the interface between the membrane and the cytoplasm. During topology mapping, we discovered two cysteine residues (C187 and C293) located on opposite sides of the membrane that are important for enzyme activity. In order to identify additional amino acid residues important for acetyltransferase activity, we isolated and characterized acetyltransferases from other acetyl-TAG-producing plants. Among them, the acetyltransferase from Euonymus fortunei possessed the highest activity in vivo and in vitro. Mutagenesis of conserved amino acids revealed that S253, H257, D258 and V263 are essential for EaDAcT activity. Alteration of residues unique to the acetyltransferases did not alter the unique acyl donor specificity of EaDAcT, suggesting that multiple amino acids are important for substrate recognition.

Selective inhibition of sterolO-acyltransferase 1 isozyme by beauveriolide III in intact cells

Beauveriolide III (BeauIII) inhibited sterol O-acyltransferases 1 and 2 (SOAT1 and SOAT2), which are endoplasmic reticulum (ER) membrane proteins, in an enzyme-based assay, and selectively inhibited SOAT1 in a cell-based assay using SOAT1-/SOAT2-CHO cells. This discrepancy in SOAT inhibition by BeauIII was investigated. In the enzyme-based assay, BeauIII inhibited SOAT1 and SOAT2 to a similar extent using microsomes prepared from cells disrupted under the strongest sonication condition. In semi-intact SOAT1-/SOAT2-CHO cells prepared by a treatment with digitonin (plasma membrane permeabilized), BeauIII selectively inhibited SOAT1 (IC₅₀; 5.0 µM (SOAT1) vs >90 µM (SOAT2)), while in those treated with saponin (plasma membrane and ER membrane permeabilized), BeauIII inhibited SOAT1 (IC₅₀, 1.8 µM) and SOAT2 (5.9 µM). SOAT1-selective inhibition by BeauIII was reproduced in intact ER fractions prepared from SOAT1/SOAT2-CHO cells. A Western blotting analysis revealed that biotin-labeled beauveriolide bound to the SOAT1 protein prepared from SOAT1-CHO cells. We concluded that BeauIII binds to a putative active site responsible for SOAT1 that is located on the cytosolic side of the ER, while BeauIII is not accessible to the corresponding active site for SOAT2 located on the luminal side.

Determining the Topology of Membrane-Bound Proteins Using PEGylation

Biochemical methods can help elucidate the membrane topology of hydrophobic membrane proteins where X-ray crystallography is difficult or impractical, providing important structural data. Here, we describe the method of PEGylation, which uses a cysteine-reactive molecule, maleimide polyethylene glycol (mPEG), to determine the cytosolic accessibility of introduced cysteine residues. This accessibility is visualized using Western blotting to detect a band shift that indicates cysteine labeling by mPEG. Using scanning cysteine mutagenesis, followed by PEGylation, one can map the accessibility of the introduced cysteines, hence inferring the membrane topology of the protein.We used PEGylation to determine the membrane topology of the sterol regulatory domain of a cholesterol synthesis enzyme, squalene monooxygenase, identifying that it is anchored to the membrane via a re-entrant loop.

Sterol O-Acyltransferase 2 Contributes to the Yolk Cholesterol Trafficking during Zebrafish Embryogenesis

To elucidate whether Sterol O-acyltransferase (Soat) mediates the absorption and transportation of yolk lipids to the developing embryo, zebrafish soat1 and soat2 were cloned and studied. In the adult zebrafish, soat1 was detected ubiquitously while soat2 mRNA was detected specifically in the liver, intestine, brain and testis. Whole mount in situ hybridization demonstrated that both soat1 and soat2 expressed in the yolk syncytial layer, hatching gland and developing cardiovascular as well as digestive systems, suggesting that Soats may play important roles in the lipid trafficking and utilization during embryonic development. The enzymatic activity of zebrafish Soat2 was confirmed by Oil Red O staining in the HEK293 cells overexpressing this gene, and could be quenched by Soat2 inhibitor Pyripyropene A (PPPA). The zebrafish embryos injected with PPPA or morpholino oligo against soat2 in the yolk showed significantly larger yolk when compared with wild-type embryos, especially at 72 hpf, indicating a slower rate of yolk consumption. Our result indicated that zebrafish Soat2 is catalytically active in synthesizing cholesteryl esters and contributes to the yolk cholesterol trafficking during zebrafish embryogenesis.

Chemogenetic E-MAP in Saccharomyces cerevisiae for Identification of Membrane Transporters Operating Lipid Flip Flop

While most yeast enzymes for the biosynthesis of glycerophospholipids, sphingolipids and ergosterol are known, genes for several postulated transporters allowing the flopping of biosynthetic intermediates and newly made lipids from the cytosolic to the lumenal side of the membrane are still not identified. An E-MAP measuring the growth of 142'108 double mutants generated by systematically crossing 543 hypomorphic or deletion alleles in genes encoding multispan membrane proteins, both on media with or without an inhibitor of fatty acid synthesis, was generated. Flc proteins, represented by 4 homologous genes encoding presumed FAD or calcium transporters of the ER, have a severe depression of sphingolipid biosynthesis and elevated detergent sensitivity of the ER. FLC1, FLC2 and FLC3 are redundant in granting a common function, which remains essential even when the severe cell wall defect of flc mutants is compensated by osmotic support. Biochemical characterization of some other genetic interactions shows that Cst26 is the enzyme mainly responsible for the introduction of saturated very long chain fatty acids into phosphatidylinositol and that the GPI lipid remodelase Cwh43, responsible for introducing ceramides into GPI anchors having a C26:0 fatty acid in sn-2 of the glycerol moiety can also use lyso-GPI protein anchors and various base resistant lipids as substrates. Furthermore, we observe that adjacent deletions in several chromosomal regions show strong negative genetic interactions with a single gene on another chromosome suggesting the presence of undeclared suppressor mutations in certain chromosomal regions that need to be identified in order to yield meaningful E-map data.

All living cells define their boundaries by lipid-containing membranes, which are impermeable to ions and water-soluble metabolic intermediates, and thus allow maintaining constant conditions inside the cells and stopping metabolic intermediates from diffusing away. Membranes are formed by amphiphilic lipids that have a hydrophilic and a hydrophobic component. Such lipids form flat double-layered sheets (bilayers) wherein the hydrophilic components of the constituent lipids are directed towards the aqueous surroundings, the hydrophobic ones populate the center of the bilayer. Membranes grow when enzymes resident in the bilayer synthesize new amphiphilic lipids. These enzymes have their active site on one side of the membrane and insert the newly made lipids in only one of the two layers. To ensure symmetric growth of membranes, cells need flippases catalyzing the transfer of lipids from one into the other layer. To identify unknown flippases we performed a chemogenetic interaction screen able to bring to light functions of unknown proteins through their genetic interaction with genes of known function. The data point to Flc proteins as potential lipid flippases of the endoplasmic reticulum, reveal novel lipid modifying activities of Cst26 and Cwh43 and suggest that undeclared suppressor mutations in certain chromosomal regions can generate false genetic interactions.

Potentiating the antitumour response of CD8(+) T cells by modulating cholesterol metabolism

CD8(+) T cells have a central role in antitumour immunity, but their activity is suppressed in the tumour microenvironment. Reactivating the cytotoxicity of CD8(+) T cells is of great clinical interest in cancer immunotherapy. Here we report a new mechanism by which the antitumour response of mouse CD8(+) T cells can be potentiated by modulating cholesterol metabolism. Inhibiting cholesterol esterification in T cells by genetic ablation or pharmacological inhibition of ACAT1, a key cholesterol esterification enzyme, led to potentiated effector function and enhanced proliferation of CD8(+) but not CD4(+) T cells. This is due to the increase in the plasma membrane cholesterol level of CD8(+) T cells, which causes enhanced T-cell receptor clustering and signalling as well as more efficient formation of the immunological synapse. ACAT1-deficient CD8(+) T cells were better than wild-type CD8(+) T cells at controlling melanoma growth and metastasis in mice. We used the ACAT inhibitor avasimibe, which was previously tested in clinical trials for treating atherosclerosis and showed a good human safety profile, to treat melanoma in mice and observed a good antitumour effect. A combined therapy of avasimibe plus an anti-PD-1 antibody showed better efficacy than monotherapies in controlling tumour progression. ACAT1, an established target for atherosclerosis, is therefore also a potential target for cancer immunotherapy.

Myeloid Acyl-CoA:Cholesterol Acyltransferase 1 Deficiency Reduces Lesion Macrophage Content and Suppresses Atherosclerosis Progression

Acyl-CoA:cholesterol acyltransferase 1 (Acat1) converts cellular cholesterol to cholesteryl esters and is considered a drug target for treating atherosclerosis. However, in mouse models for atherosclerosis, global Acat1 knockout (Acat1(-/-)) did not prevent lesion development. Acat1(-/-) increased apoptosis within lesions and led to several additional undesirable phenotypes, including hair loss, dry eye, leukocytosis, xanthomatosis, and a reduced life span. To determine the roles of Acat1 in monocytes/macrophages in atherosclerosis, we produced a myeloid-specific Acat1 knockout (Acat1(-M/-M)) mouse and showed that, in the Apoe knockout (Apoe(-/-)) mouse model for atherosclerosis, Acat1(-M/-M) decreased the plaque area and reduced lesion size without causing leukocytosis, dry eye, hair loss, or a reduced life span. Acat1(-M/-M) enhanced xanthomatosis in apoe(-/-) mice, a skin disease that is not associated with diet-induced atherosclerosis in humans. Analyses of atherosclerotic lesions showed that Acat1(-M/-M) reduced macrophage numbers and diminished the cholesterol and cholesteryl ester load without causing detectable apoptotic cell death. Leukocyte migration analysis in vivo showed that Acat1(-M/-M) caused much fewer leukocytes to appear at the activated endothelium. Studies in inflammatory (Ly6C(hi)-positive) monocytes and in cultured macrophages showed that inhibiting ACAT1 by gene knockout or by pharmacological inhibition caused a significant decrease in integrin β 1 (CD29) expression in activated monocytes/macrophages. The sparse presence of lesion macrophages without Acat1 can therefore, in part, be attributed to decreased interaction between inflammatory monocytes/macrophages lacking Acat1 and the activated endothelium. We conclude that targeting ACAT1 in a myeloid cell lineage suppresses atherosclerosis progression while avoiding many of the undesirable side effects caused by global Acat1 inhibition.

Saccharomyces cerevisiae lysophospholipid acyltransferase, Lpt1, requires Asp146 and Glu297 for catalysis

The esterification of lysophospholipids contributes to phospholipid synthesis, remodeling, and scavenging. Acyl-CoA-dependent lysophospholipid acyltransferase activity with broad substrate use is mediated by Saccharomyces cerevisiae Lpt1p. We sought to identify Lpt1p active site amino acids besides the histidine conserved among homologs and repeatedly found to be required for catalysis. In vitro Lpt1p assays with amino acid modifying agents implicated aspartate, glutamate, and lysine as active site residues. Threonine and tyrosine were not ruled out. Aligning the primary structures of functionally characterized LPT1 homologs from fungi, plants, and animals identified 11 conserved aspartate, glutamate, lysine, threonine, and tyrosine residues. Site-directed mutagenesis of the respective codons showed that changing D146 and E297 abolished activity without abolishing protein expression. The mechanism of Lpt1p was further analyzed using monounsaturated acyl-CoA species with different double bond positions. Delta 6 species showed the highest catalytic efficiency. We propose that D146 and E297 act in conjunction with H382 as nucleophiles that attack the hydroxyl group in lysophospholipids in a general acid/base mechanism. This sequential mechanism provides a precedent for other members of the membrane bound O-acyltransferase family. Also, Lpt1p optimally orients acyl-CoA substrates with 7.5 Å between a double bond and the thioester bond.

Enzyme-coupled assays for flip-flop of acyl-Coenzyme A in liposomes

Acyl-Coenzyme A is made in the cytosol. Certain enzymes using acyl-CoA seem to operate in the lumen of the ER but no corresponding flippases for acyl-CoA or an activated acyl have been described. In order to test the ability of purified candidate flippases to operate the transport of acyl-CoA through lipid bilayers in vitro we developed three enzyme-coupled assays using large unilamellar vesicles (LUVs) obtained by detergent removal. The first assay uses liposomes encapsulating a water-soluble acyl-CoA:glycerol-3-phosphate acyl transferase plus glycerol-3-phosphate (G3P). It measures formation of [(3)H]lyso-phosphatidic acid inside liposomes after [(3)H]palmitoyl-CoA has been added from outside. Two other tests use empty liposomes containing [(3)H]palmitoyl-CoA in the inner membrane leaflet, to which either soluble acyl-CoA:glycerol-3-phosphate acyl transferase plus glycerol-3-phosphate or alkaline phosphatase are added from outside. Here one can follow the appearance of [(3)H]lyso-phosphatidic acid or of dephosphorylated [(3)H]acyl-CoA, respectively, both being made outside the liposomes. Although the liposomes may retain small amounts of detergent, all these tests show that palmitoyl-CoA crosses the lipid bilayer only very slowly and that the lipid composition of liposomes barely affects the flip-flop rate. Thus, palmitoyl-CoA cannot cross the membrane spontaneously implying that in vivo some transport mechanism is required.

Mechanistic analysis of ghrelin-O-acyltransferase using substrate analogs

Ghrelin-O-Acyltransferase (GOAT) is an 11-transmembrane integral membrane protein that octanoylates the metabolism-regulating peptide hormone ghrelin at Ser3 and may represent an attractive target for the treatment of type II diabetes and the metabolic syndrome. Protein octanoylation is unique to ghrelin in humans, and little is known about the mechanism of GOAT or of related protein-O-acyltransferases HHAT or PORC. In this study, we explored an in vitro microsomal ghrelin octanoylation assay to analyze its enzymologic features. Measurement of Km for 10-mer, 27-mer, and synthetic Tat-peptide-containing ghrelin substrates provided evidence for a role of charge interactions in substrate binding. Ghrelin substrates with amino-alanine in place of Ser3 demonstrated that GOAT can catalyze the formation of an octanoyl-amide bond at a similar rate compared with the natural reaction. A pH-rate comparison of these substrates revealed minimal differences in acyltransferase activity across pH 6.0-9.0, providing evidence that these reactions may be relatively insensitive to the basicity of the substrate nucleophile. The conserved His338 residue was required both for Ser3 and amino-Ala3 ghrelin substrates, suggesting that His338 may have a key catalytic role beyond that of a general base.

Acyl-CoA:cholesterol acyltransferases (ACATs/SOATs): Enzymes with multiple sterols as substrates and as activators

Cholesterol is essential to the growth and viability of cells. The metabolites of cholesterol include: steroids, oxysterols, and bile acids, all of which play important physiological functions. Cholesterol and its metabolites have been implicated in the pathogenesis of multiple human diseases, including: atherosclerosis, cancer, neurodegenerative diseases, and diabetes. Thus, understanding how cells maintain the homeostasis of cholesterol and its metabolites is an important area of study. Acyl-coenzyme A:cholesterol acyltransferases (ACATs, also abbreviated as SOATs) converts cholesterol to cholesteryl esters and play key roles in the regulation of cellular cholesterol homeostasis. ACATs are most unusual enzymes because (i) they metabolize diverse substrates including both sterols and certain steroids; (ii) they contain two different binding sites for steroidal molecules. In mammals, there are two ACAT genes that encode two different enzymes, ACAT1 and ACAT2. Both are allosteric enzymes that can be activated by a variety of sterols. In addition to cholesterol, other sterols that possess the 3-beta OH at C-3, including PREG, oxysterols (such as 24(S)-hydroxycholesterol and 27-hydroxycholesterol, etc.), and various plant sterols, could all be ACAT substrates. All sterols that possess the iso-octyl side chain including cholesterol, oxysterols, various plant sterols could all be activators of ACAT. PREG can only be an ACAT substrate because it lacks the iso-octyl side chain required to be an ACAT activator. The unnatural cholesterol analogs epi-cholesterol (with 3-alpha OH in steroid ring B) and ent-cholesterol (the mirror image of cholesterol) contain the iso-octyl side chain but do not have the 3-beta OH at C-3. Thus, they can only serve as activators and cannot serve as substrates. Thus, within the ACAT holoenzyme, there are site(s) that bind sterol as substrate and site(s) that bind sterol as activator; these sites are distinct from each other. These features form the basis to further pursue ACAT structure-function analysis, and can be explored to develop novel allosteric ACAT inhibitors for therapeutic purposes. This article is part of a Special Issue entitled 'Steroid/Sterol signaling'.

Membrane bound O-acyltransferases and their inhibitors

Since the identification of the membrane-bound O-acyltransferase (MBOATs) protein family in the early 2000s, three distinct members [porcupine (PORCN), hedgehog (Hh) acyltransferase (HHAT) and ghrelin O-acyltransferase (GOAT)] have been shown to acylate specific proteins or peptides. In this review, topology determination, development of assays to measure enzymatic activities and discovery of small molecule inhibitors are compared and discussed for each of these enzymes.

The active site of yeast phosphatidylinositol synthase Pis1 is facing the cytosol

Five yeast enzymes synthesizing various glycerophospholipids belong to the CDP-alcohol phosphatidyltransferase (CAPT) superfamily. They only share the so-called CAPT motif, which forms the active site of all these enzymes. Bioinformatic tools predict the CAPT motif of phosphatidylinositol synthase Pis1 as either ER luminal or cytosolic. To investigate the membrane topology of Pis1, unique cysteine residues were introduced into either native or a Cys-free form of Pis1 and their accessibility to the small, membrane permeating alkylating reagent N-ethylmaleimide (NEM) and mass tagged, non-permeating maleimides, in the presence and absence of non-denaturing detergents, was monitored. The results clearly point to a cytosolic location of the CAPT motif. Pis1 is highly sensitive to non-denaturing detergent, and low concentrations (0.05%) of dodecylmaltoside change the accessibility of single substituted Cys in the active site of an otherwise cysteine free version of Pis1. Slightly higher detergent concentrations inactivate the enzyme. Removal of the ER retrieval sequence from (wt) Pis1 enhances its activity, again suggesting an influence of the lipid environment. The central 84% of the Pis1 sequence can be aligned and fitted onto the 6 transmembrane helices of two recently crystallized archaeal members of the CAPT family. Results delineate the accessibility of different parts of Pis1 in their natural context and allow to critically evaluate the performance of different cysteine accessibility methods. Overall the results show that cytosolically made inositol and CDP-diacylglycerol can access the active site of the yeast PI synthase Pis1 from the cytosolic side and that Pis1 structure is strongly affected by mild detergents.

Identification of key residues and regions important for porcupine-mediated Wnt acylation

Wnts comprise a family of lipid-modified, secreted signaling proteins that control embryogenesis, as well as tissue homeostasis in adults. Post-translational attachment of palmitoleate (C16:1) to a conserved Ser in Wnt proteins is catalyzed by Porcupine (Porcn), a member of the membrane bound O-acyltransferase (MBOAT) family, and is required for Wnt secretion and signaling. Moreover, genetic alterations in the PORCN gene lead to focal dermal hypoplasia, an X-linked developmental disorder. Despite its physiological importance, the biochemical mechanism governing Wnt acylation by Porcn is poorly understood. Here, we use a cell-based fatty acylation assay that is a direct readout of Porcn acyltransferase activity to perform structure-function analysis of highly conserved residues in Porcn and Wnt3a. In total, 16-point mutations in Porcn and 13 mutations in Wnt3a were generated and analyzed. We identified key residues within Porcn required for enzymatic activity, stability, and Wnt3a binding and mapped these active site residues to predicted transmembrane domain 9. Analysis of focal dermal hypoplasia-associated mutations in Porcn revealed that loss of enzymatic activity arises from altered stability. A consensus sequence within Wnt3a was identified (CXCHGXSXXCXXKXC) that contains residues that mediate Porcn binding, fatty acid transfer, and Wnt signaling. We also showed that Ser or Thr, but not Cys, can serve as a fatty acylation site in Wnt, establishing Porcn as an O-acyltransferase. This analysis sheds light into the mechanism by which Porcn transfers fatty acids to Wnt proteins and provides insight into the mechanisms of fatty acid transfer by MBOAT family members.

Acyl-coenzyme A:cholesterol acyltransferase 1 - significance of single-nucleotide polymorphism at residue 526 and the role of Pro347 near the fifth transmembrane domain

Acyl-coenzyme A:cholesterol acyltransferases (ACATs), which are members of the membrane-bound O-acyltransferase family, catalyze the conversion of cholesterol to cholesteryl esters. Mammals have two isoenzymes: ACAT1 and ACAT2. Both enzymes are drug targets for treating human diseases. ACAT1 is present in various cell types. It contains nine transmembrane domains (TMDs), with the active site His460 located within TMD7, and the active site Asn421 located within the fourth large cytoplasmic loop. In human ACAT1, a single-nucleotide polymorphism exists for residue 526: the codon is either CAG for Gln, or CGG for Arg. Gln526/Arg526 is present within the C-terminal loop. Its biochemical significance is unknown. In addition, within the C-terminal half of ACAT1, numerous residues conserved with those of ACAT2 are present; the functions of these conserved residues are largely unknown. Here, we performed single-substitution mutagenesis experiments to investigate the roles of individual residues present in the C-terminal loop, including Gln526/Arg526, and the eight conserved Pro residues located near/in various TMDs. The results show that the enzyme activity of ACAT1 with Gln526 is less active than that of ACAT1 with Arg526 by 40%. In addition, several residues in the C-terminal loop are important for maintaining proper ACAT1 protein stability. Other results show that Pro347 plays an important role in modulating enzyme catalysis. Overall, our results imply that the CAG/CGG polymorphism can be utilized to perform ACAT1 activity/human disease susceptibility studies, and that Pro347 located near TMD5 plays an important role in modulating enzyme catalysis.

Architectural organization of the metabolic regulatory enzyme ghrelin O-acyltransferase

Ghrelin O-acyltransferase (GOAT) is a polytopic integral membrane protein required for activation of ghrelin, a secreted metabolism-regulating peptide hormone. Although GOAT is a potential therapeutic target for the treatment of obesity and diabetes and plays a key role in other physiologic processes, little is known about its structure or mechanism. GOAT is a member of the membrane-bound O-acyltransferase (MBOAT) family, a group of polytopic integral membrane proteins involved in lipid-biosynthetic and lipid-signaling reactions from prokaryotes to humans. Here we use phylogeny and a variety of bioinformatic tools to predict the topology of GOAT. Using selective permeabilization indirect immunofluorescence microscopy in combination with glycosylation shift immunoblotting, we demonstrate that GOAT contains 11 transmembrane helices and one reentrant loop. Development of the V5Glyc tag, a novel, small, and sensitive dual topology reporter, facilitated these experiments. The MBOAT family invariant residue His-338 is in the ER lumen, consistent with other family members, but conserved Asn-307 is cytosolic, making it unlikely that both are involved in catalysis. Photocross-linking of synthetic ghrelin analogs and inhibitors demonstrates binding to the C-terminal region of GOAT, consistent with a role of His-338 in the active site. This knowledge of GOAT architecture is important for a deeper understanding of the mechanism of GOAT and other MBOATs and could ultimately advance the discovery of selective inhibitors for these enzymes.

Production of ACAT1 56-kDa isoform in human cells via trans-splicing involving the ampicillin resistance gene

Trans-splicing, a process involving the cleavage and joining of two separate transcripts, can expand the transcriptome and proteome in eukaryotes. Chimeric RNAs generated by trans-splicing are increasingly described in literatures. The widespread presence of antibiotic resistance genes in natural environments and human intestines is becoming an important challenge for public health. Certain antibiotic resistance genes, such as ampicillin resistance gene (Amp^r), are frequently used in recombinant plasmids. Until now, trans-splicing involving recombinant plasmid-derived exogenous transcripts and endogenous cellular RNAs has not been reported. Acyl-CoA:cholesterol acyltransferase 1 (ACAT1) is a key enzyme involved in cellular cholesterol homeostasis. The 4.3-kb human ACAT1 chimeric mRNA can produce 50-kDa and 56-kDa isoforms with different enzymatic activities. Here, we show that human ACAT1 56-kDa isoform is produced from an mRNA species generated through the trans-splicing of an exogenous transcript encoded by the antisense strand of Amp^r (asAmp) present in common Amp^r-plasmids and the 4.3-kb endogenous ACAT1 chimeric mRNA, which is presumably processed through a prior event of interchromosomal trans-splicing. Strikingly, DNA fragments containing the asAmp with an upstream recombined cryptic promoter and the corresponding exogenous asAmp transcripts have been detected in human cells. Our findings shed lights on the mechanism of human ACAT1 56-kDa isoform production, reveal an exogenous-endogenous trans-splicing system, in which recombinant plasmid-derived exogenous transcripts are linked with endogenous cellular RNAs in human cells, and suggest that exogenous DNA might affect human gene expression at both DNA and RNA levels.

Adaptation of low-resolution methods for the study of yeast microsomal polytopic membrane proteins: a methodological review

Most integral membrane proteins of yeast with two or more membrane-spanning sequences have not yet been crystallized and for many of them the side on which the active sites or ligand-binding domains reside is unknown. Also, bioinformatic topology predictions are not yet fully reliable. However, so-called low-resolution biochemical methods can be used to locate hydrophilic loops or individual residues of polytopic membrane proteins at one or the other side of the membrane. The advantages and limitations of several such methods for topological studies with yeast ER integral membrane proteins are discussed. We also describe new tools that allow us to better control and validate results obtained with SCAM (substituted cysteine accessibility method), an approach that determines the position of individual residues with respect to the membrane plane, whereby only minimal changes in the primary sequence have to be introduced into the protein of interest.

A look at diacylglycerol acyltransferases (DGATs) in algae

Triacylglycerols (TAGs) from algae are considered to be a potentially viable source of biodiesel and thereby renewable energy, but at the moment very little is known about the biosynthetic pathway in these organisms. Here we compare what is currently known in eukaryotic algal species, in particular the characteristics of algal diacylglycerol acyltransferase (DGAT), the last enzyme of de novo TAG biosynthesis. Several studies in plants and mammals have shown that there are two DGAT isoforms, DGAT1 and DGAT2, which catalyse the same reaction but have no clear sequence similarities. Instead, they have differences in functionality and spatial and temporal expression patterns. Bioinformatic searches of sequenced algal genomes reveal that most algae have multiple copies of putative DGAT2s, whereas other eukaryotes have single genes. Investigating whether these putative isoforms are indeed functional and whether they confer significantly different phenotypes to algal cells will be vital for future efforts to genetically modify algae for biofuel production.

Palmitoylation of Hedgehog proteins

Hedgehog (Hh) proteins are secreted signaling proteins that contain amide-linked palmitate at the N-terminus and cholesterol at the C-terminus. Palmitoylation of Hh proteins is critical for effective long- and short-range signaling. The palmitoylation reaction occurs during transit of Hh through the secretory pathway, most likely in the lumen of the ER. Attachment of palmitate to Hh proteins is independent of cholesterol modification and autoprocessing and is catalyzed by Hhat (Hedgehog acyltransferase). Hhat is a member of the membrane bound O-acyltransferase (MBOAT) family, a subgroup of multipass membrane proteins that catalyze transfer of fatty acyl groups to lipids and proteins. Several classes of secreted proteins have recently been shown to be substrates for MBOAT acyltransferases, including Hh proteins and Spitz (palmitoylated by Hhat), Wg/Wnt proteins (modified with palmitate and/or palmitoleate by Porcupine) and ghrelin (octanoylated by ghrelin O-acyltransferase). These findings highlight protein fatty acylation as a mechanism that not only influences membrane binding of intracellular proteins but also regulates the signaling range and efficacy of secreted proteins.

Acyl-CoA:diacylglycerol acyltransferase: molecular biology, biochemistry and biotechnology

Triacylglycerol (TG) is a storage lipid which serves as an energy reservoir and a source of signalling molecules and substrates for membrane biogenesis. TG is essential for many physiological processes and its metabolism is widely conserved in nature. Acyl-CoA:diacylglycerol acyltransferase (DGAT, EC 2.3.1.20) catalyzes the final step in the sn-glycerol-3-phosphate pathway leading to TG. DGAT activity resides mainly in two distinct membrane bound polypeptides, known as DGAT1 and DGAT2 which have been identified in numerous organisms. In addition, a few other enzymes also hold DGAT activity, including the DGAT-related acyl-CoA:monoacylglycerol acyltransferases (MGAT). Progress on understanding structure/function in DGATs has been limited by the lack of detailed three-dimensional structural information due to the hydrophobic properties of theses enzymes and difficulties associated with purification. This review examines several aspects of DGAT and MGAT genes and enzymes, including current knowledge on their gene structure, expression pattern, biochemical properties, membrane topology, functional motifs and subcellular localization. Recent progress in probing structural and functional aspects of DGAT1 and DGAT2, using a combination of molecular and biochemical techniques, is emphasized. Biotechnological applications involving DGAT enzymes ranging from obesity therapeutics to oilseed engineering are also discussed.

Cellular pregnenolone esterification by acyl-CoA:cholesterol acyltransferase

Pregnenolone (PREG) can be converted to PREG esters (PE) by the plasma enzyme lecithin: cholesterol acyltransferase (LCAT), and by other enzyme(s) with unknown identity. Acyl-CoA:cholesterol acyltransferase 1 and 2 (ACAT1 and ACAT2) convert various sterols to steryl esters; their activities are activated by cholesterol. PREG is a sterol-like molecule, with 3-β-hydroxy moiety at steroid ring A, but with much shorter side chain at steroid ring D. Here we show that without cholesterol, PREG is a poor ACAT substrate; with cholesterol, the V(max) for PREG esterification increases by 100-fold. The binding affinity of ACAT1 for PREG is 30-50-fold stronger than that for cholesterol; however, PREG is only a substrate but not an activator, while cholesterol is both a substrate and an activator. These results indicate that the sterol substrate site in ACAT1 does not involve significant sterol-phospholipid interaction, while the sterol activator site does. Studies utilizing small molecule ACAT inhibitors show that ACAT plays a key role in PREG esterification in various cell types examined. Mice lacking ACAT1 or ACAT2 do not have decreased PREG ester contents in adrenals, nor do they have altered levels of the three major secreted adrenal steroids in serum. Mice lacking LCAT have decreased levels of PREG esters in the adrenals. These results suggest LCAT along with ACAT1/ACAT2 contribute to control pregnenolone ester content in different cell types and tissues.

Ghrelin O-acyltransferase assays and inhibition

Ghrelin O-acyltransferase (GOAT) is responsible for catalyzing the attachment of the eight-carbon fatty acid octanoyl to the Ser3 side chain of the peptide ghrelin to generate the active form of this metabolic hormone. As such, GOAT is viewed as a potential therapeutic target for the treatment of obesity and diabetes mellitus. Here, we review recent progress in the development of cell and in vitro assays to measure GOAT action and the identification of several synthetic GOAT inhibitors. In particular, we discuss the design, synthesis, and characterization of the bisubstrate analog GO-CoA-Tat and its ability to modulate weight and blood glucose in mice. We also highlight current challenges and future research directions in our biomedical understanding of this fascinating ghrelin processing enzyme.

Isoform-specific inhibitors of ACATs: recent advances and promising developments

Acyl-CoA:cholesterol acyltransferase (ACAT) is a promising therapeutic target for cardiovascular diseases. Although a number of synthetic ACAT inhibitors have been developed, they have failed to show efficacy in clinical trials. Now, the presence of two ACAT isoforms with distinct functions, ACAT1 and ACAT2, has been discovered. Thus, the selectivity of ACAT inhibitors toward the two isoforms is important for their development as novel anti-atherosclerotic agents. The selectivity study indicated that fungal pyripyropene A (PPPA) is only an ACAT2-specific inhibitor. Furthermore, PPPA proved orally active in atherogenic mouse models, indicating it possessed cholesterol-lowering and atheroprotective activities. Certain PPPA derivatives, semi-synthetically prepared, possessed more potent and selective in vitro activity than PPPA against ACAT2. This review covers these studies and describes the future prospects of ACAT2-specific inhibitors.

Topology of 1-acyl-sn-glycerol-3-phosphate acyltransferases SLC1 and ALE1 and related membrane-bound O-acyltransferases (MBOATs) of Saccharomyces cerevisiae

In yeast, phosphatidic acid, the biosynthetic precursor for all glycerophospholipids and triacylglycerols, is made de novo by the 1-acyl-sn-glycerol-3-phosphate acyltransferases Ale1p and Slc1p. Ale1p belongs to the membrane-bound O-acyltransferase (MBOAT) family, which contains many enzymes acylating lipids but also others that acylate secretory proteins residing in the lumen of the ER. A histidine present in a very short loop between two predicted transmembrane domains is the only residue that is conserved throughout the MBOAT gene family. The yeast MBOAT proteins of known function comprise Ale1p, the ergosterol acyltransferases Are1p and Are2p, and Gup1p, the last of which acylates lysophosphatidylinositol moieties of GPI anchors on ER lumenal GPI proteins. C-terminal topology reporters added to truncated versions of Gup1p yield a topology predicting a lumenal location of its uniquely conserved histidine 447 residue. The same approach shows that Ale1p and Are2p also have the uniquely conserved histidine residing in the ER lumen. Because these data raised the possibility that phosphatidic acid could be made in the lumen of the ER, we further investigated the topology of the second yeast 1-acyl-sn-glycerol-3-phosphate acyltransferase, Slc1p. The location of C-terminal topology reporters, microsomal assays probing the protease sensitivity of inserted tags, and the accessibility of natural or artificially inserted cysteines to membrane-impermeant alkylating agents all indicate that the most conserved motif containing the presumed active site histidine of Slc1p is oriented toward the ER lumen, whereas other conserved motifs are cytosolic. The implications of these findings are discussed.

Functional and topological analysis of yeast acyl-CoA:diacylglycerol acyltransferase 2, an endoplasmic reticulum enzyme essential for triacylglycerol biosynthesis

Acyl-CoA:diacylglycerol acyltransferase (EC 2.3.1.20) is a membrane protein present mainly in the endoplasmic reticulum. It catalyzes the final and committed step in the biosynthesis of triacylglycerol, which is the principal repository of fatty acids for energy utilization and membrane formation. Two distinct family members of acyl-CoA:diacylglycerol acyltransferase, known as DGAT1 and DGAT2, have been characterized in different organisms, including mammals, fungi, and plants. In this study, we characterized the functional role and topological orientation of signature motifs in yeast (Saccharomyces cerevisiae) DGAT2 using mutagenesis in conjunction with chemical modification. Our data provide evidence that both the N and C termini are oriented toward the cytosol and have different catalytic roles. A highly conserved motif, (129)YFP(131), and a hydrophilic segment exclusive to yeast DGAT2 reside in a long endoplasmic reticulum luminal loop following the first transmembrane domain and play an essential role in enzyme catalysis. In addition, the strongly conserved His(195) within the motif HPHG, which may play a role in the active site of DGAT2, is likely embedded in the membrane. These results indicate some similarities to the topology model of murine DGAT2 but also reveal striking differences suggesting that the topological organization of DGAT2 is not ubiquitously conserved.

Purification of recombinant acyl-coenzyme A:cholesterol acyltransferase 1 (ACAT1) from H293 cells and binding studies between the enzyme and substrates using difference intrinsic fluorescence spectroscopy

Acyl-coenzyme A:cholesterol acyltransferase 1 (ACAT1) is a membrane-bound enzyme utilizing long-chain fatty acyl-coenzyme A and cholesterol to form cholesteryl esters and coenzyme A. Previously, we had expressed tagged human ACAT1 (hACAT1) in CHO cells and purified it to homogeneity; however, only a sparse amount of purified protein could be obtained. Here we report that the hACAT1 expression level in H293 cells is 18-fold higher than that in CHO cells. We have developed a milder purification procedure to purify the enzyme to homogeneity. The abundance of the purified protein enabled us to conduct difference intrinsic fluorescence spectroscopy to study the binding between the enzyme and its substrates in CHAPS/phospholipid mixed micelles. The results show that oleoyl-CoA binds to ACAT1 with K(d) = 1.9 μM and elicits significant structural changes of the protein as manifested by the significantly positive changes in its fluorescence spectrum; stearoyl-CoA elicits a similar spectrum change but much lower in magnitude. Previously, kinetic studies had shown that cholesterol is an efficient substrate and an allosteric activator of ACAT1, while its diastereomer epicholesterol is neither a substrate nor an activator. Here we show that both cholesterol and epicholesterol induce positive changes in the ACAT1 fluorescence spectrum; however, the magnitude of spectrum changes induced by cholesterol is much larger than epicholesterol. These results show that stereospecificity, governed by the 3β-OH moiety in steroid ring A, plays an important role in the binding of cholesterol to ACAT1.

Topological orientation of acyl-CoA:diacylglycerol acyltransferase-1 (DGAT1) and identification of a putative active site histidine and the role of the n terminus in dimer/tetramer formation

Acyl CoA:diacylglycerol acyltransferase (DGAT) is an integral membrane protein of the endoplasmic reticulum that catalyzes the synthesis of triacylglycerols. Two DGAT enzymes have been identified (DGAT1 and DGAT2) with unique roles in lipid metabolism. DGAT1 is a multifunctional acyltransferase capable of synthesizing diacylglycerol, retinyl, and wax esters in addition to triacylglycerol. Here, we report the membrane topology for murine DGAT1 using protease protections assays and indirect immunofluorescence in conjunction with selective permeabilization of cellular membranes. Topology models based on prediction algorithms suggested that DGAT1 had eight transmembrane domains. In contrast, our data indicate that DGAT1 has three transmembrane domains with the N terminus oriented toward the cytosol. The C-terminal region of DGAT1, which accounts for ∼50% of the protein, is present in the endoplasmic reticulum lumen and contains a highly conserved histidine residue (His-426) that may be part of the active site. Mutagenesis of His-426 to alanine impaired the ability of DGAT1 to synthesize triacylglycerols as well as retinyl and wax esters in an in vitro acyltransferase assay. Finally, we show that the N-terminal domain of DGAT1 is not required for the catalytic activity of DGAT1 but, instead, may be involved in regulating enzyme activity and dimer/tetramer formation.

Identification of conserved regions and residues within Hedgehog acyltransferase critical for palmitoylation of Sonic Hedgehog

Background

Sonic hedgehog (Shh) is a palmitoylated protein that plays key roles in mammalian development and human cancers. Palmitoylation of Shh is required for effective long and short range Shh-mediated signaling. Attachment of palmitate to Shh is catalyzed by Hedgehog acyltransferase (Hhat), a member of the membrane bound O-acyl transferase (MBOAT) family of multipass membrane proteins. The extremely hydrophobic composition of MBOAT proteins has limited their biochemical characterization. Except for mutagenesis of two conserved residues, there has been no structure-function analysis of Hhat, and the regions of the protein required for Shh palmitoylation are unknown.

Methodology/Principal Findings

Here we undertake a systematic approach to identify residues within Hhat that are required for protein stability and/or enzymatic activity. We also identify a second, novel MBOAT homology region (residues 196–234) that is required for Hhat activity. In total, ten deletion mutants and eleven point mutants were generated and analyzed. Truncations at the N- and C-termini of Hhat yielded inactive proteins with reduced stability. Four Hhat mutants with deletions within predicted loop regions and five point mutants retained stability but lost palmitoylation activity. We purified two point mutants, W378A and H379A, with defective Hhat activity. Kinetic analyses revealed alterations in apparent K_m and V_max for Shh and/or palmitoyl CoA, changes that likely explain the catalytic defects observed for these mutants.

Conclusions/Significance

This study has pinpointed specific regions and multiple residues that regulate Hhat stability and catalysis. Our findings should be applicable to other MBOAT proteins that mediate lipid modification of Wnt proteins and ghrelin, and should serve as a model for understanding how secreted morphogens are modified by palmitoyl acyltransferases.

Neuronal cholesterol esterification by ACAT1 in Alzheimer's disease

Cholesterol has been implicated in various neurodegenerative diseases. Here we review the connection between cholesterol and Alzheimer's disease (AD), focusing on a recent study that links neuronal cholesterol esterification with biosynthesis of 24(S)-hydroxycholesterol and the fate of human amyloid precursor protein in a mouse model of AD. We also briefly evaluate the potential of ACAT1 as a drug target for AD.