Role of the N-terminal hydrophilic domain of acyl-coenzyme A:cholesterol acyltransferase 1 on the enzyme's quaternary structure and catalytic efficiency

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.

A basic model for cell cholesterol homeostasis

Cells manage their cholesterol by negative feedback using a battery of sterol-responsive proteins. How these activities are coordinated so as to specify the abundance and distribution of the sterol is unclear. We present a simple mathematical model that addresses this question. It assumes that almost all of the cholesterol is associated with phospholipids in stoichiometric complexes. A small fraction of the sterol is uncomplexed and thermodynamically active. It equilibrates among the organelles, setting their sterol level according to the affinity of their phospholipids. The activity of the homeostatic proteins in the cytoplasmic membranes is then set by their fractional saturation with uncomplexed cholesterol in competition with the phospholipids. The high-affinity phospholipids in the plasma membrane (PM) are filled to near stoichiometric equivalence, giving it most of the cell sterol. Notably, the affinity of the phospholipids in the endomembranes (EMs) is lower by orders of magnitude than that of the phospholipids in the PM. Thus, the small amount of sterol in the EMs rests far below stoichiometric capacity. Simulations match a variety of experimental data. The model captures the essence of cell cholesterol homeostasis, makes coherent a diverse set of experimental findings, provides a surprising prediction and suggests new experiments.

Nanodisc scaffold peptide (NSPr) replaces detergent by reconstituting acyl-CoA:cholesterol acyltransferase 1 into peptidiscs

To conduct biochemical studies in vitro, membrane proteins (MPs) must be solubilized with detergents. While detergents are great tools, they can also inhibit the biological activity and/or perturb oligomerization of individual MPs. Nanodisc scaffold peptide (NSPr), an amphipathic peptide analog of ApoA1, was recently shown to reconstitute detergent solubilized MPs into peptidiscs in vitro. Acyl-coenzyme A:cholesterol acyltransferase 1 (ACAT1), also known as sterol O-acyltransferase 1 (SOAT1), plays a key role in cellular cholesterol storage in various cell types and is a drug target to treat multiple human diseases. ACAT1 contains nine transmembrane domains (TMDs) and primarily forms a homotetramer in vitro and in intact cells; deletion of the N-terminal dimerization domain produces a homodimer with full retention in catalytic activity. ACAT1 is prone to inactivation by numerous detergents. Here we pursued the use of NSPr to overcome the detergent-induced inactivation of ACAT1 by generating near detergent-free ACAT1 peptidiscs. Based on native-PAGE analysis, we showed that NSPr reconstitutes ACAT1 into soluble peptidiscs, in which ACAT1 exists predominantly in oligomeric states greater than a homotetramer. The formation of these higher-order oligomeric states was independent of the N-terminal dimerization domain, suggesting that the oligomerization is mediated through hydrophobic interactions of multiple ACAT1 subunits. ACAT1 peptidiscs were still susceptible to heat-mediated inactivation, presumably due to the residual detergent (CHAPS) bound to ACAT1. We then conditioned ACAT1 with phosphatidylcholine (PC) to replace CHAPS prior to the formation of ACAT1 peptidiscs. The results showed, when PC was included, ACAT1 was present mainly in higher-order oligomeric states with greater enzymatic activity. With PC present, the enzymatic activity of ACAT1 peptidiscs was protected from heat-mediated inactivation. These results support the use of NSPr to create a near detergent-free solution of ACAT1 in peptidiscs for various in vitro studies. Our current results also raise the possibility that, under certain conditions, ACAT1 may form higher-order oligomeric states in vivo.

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.

A simple method to disrupt and restore subunit interaction of acyl-CoA:cholesterol acyltransferase 1

Acyl-CoA:cholestereol acyltransferase 1 (ACAT1) is a two-fold dimer (homotetramer) and has two distinct dimerization domains. One domain is in an alpha-helical rich region near the cytoplasmic N-terminus. The other is proposed to be near the C-terminus where multiple transmembrane domains promote hydrophobic interactions between two ACAT1 subunits. The truncation of the ACAT1 N-terminal dimerization domain, Δ1-65, creates a dimer which is fully enzymatically active. It is currently not known how the C-terminal dimerization domain contributes to ACAT1 enzymatic activity. Here we describe a simple method that dissociates ACAT1 dimers through the addition of the non-ionic detergents Triton X-100 or octyl glucoside which disrupt the C-terminal dimerization domain. We also document the protocols for a method to exchange Triton X-100 with CHAPS to restore C-terminal dimerization of the ACAT1 protein, and an optimized liposomal assay to assess ACAT enzymatic activity.

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This method can be applied to dissociate ACAT1 subunits by using Triton X-100 or octyl glucoside.

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ACAT1 dimerization can be restored by exchanging Triton X-100 with CHAPS.

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The liposomal ACAT activity assay conditions have been optimized.

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.

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'.

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.

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.

Lipin proteins form homo- and hetero-oligomers

Lipin family members (lipin 1, 2 and 3) are bi-functional proteins that dephosphorylate PA (phosphatidic acid) to produce DAG (diacylglycerol) and act in the nucleus to regulate gene expression. Although other components of the triacylglycerol synthesis pathway can form oligomeric complexes, it is unknown whether lipin proteins also exist as oligomers. In the present study, using various approaches, we revealed that lipin 1 formed stable homo-oligomers with itself and hetero-oligomers with lipin 2/3. Both the N- and C-terminal regions of lipin 1 mediate its oligomerization in a head-to-head/tail-to-tail manner. We also show that lipin 1 subcellular localization can be influenced through oligomerization, and the individual lipin 1 monomers in the oligomer function independently in catalysing dephosphorylation of PA. The present study provides evidence that lipin proteins function as oligomeric complexes and that the three mammalian lipin isoforms can form combinatorial units.

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.

Acyl-coenzyme A:cholesterol acyltransferases

The enzymes acyl-coenzyme A (CoA):cholesterol acyltransferases (ACATs) are membrane-bound proteins that utilize long-chain fatty acyl-CoA and cholesterol as substrates to form cholesteryl esters. In mammals, two isoenzymes, ACAT1 and ACAT2, encoded by two different genes, exist. ACATs play important roles in cellular cholesterol homeostasis in various tissues. This chapter summarizes the current knowledge on ACAT-related research in two areas: 1) ACAT genes and proteins and 2) ACAT enzymes as drug targets for atherosclerosis and for Alzheimer's disease.

Characterization of human lysophospholipid acyltransferase 3

Esterifying lysophospholipids may serve a variety of functions, including phospholipid remodeling and limiting the abundance of bioactive lipids. Recently, a yeast enzyme, Lpt1p, that esterifies an array of lysophospholipids was identified. Described here is the characterization of a human homolog of LPT1 that we have called lysophosphatidylcholine acyltransferase 3 (LPCAT3). Expression of LPCAT3 in Sf9 insect cells conferred robust esterification of lysophosphatidylcholine in vitro. Kinetic analysis found apparent cooperativity with a saturated acyl-CoA having the lowest K0.5 (5 microM), a monounsaturated acyl-CoA having the highest apparent Vmax (759 nmol/min/mg), and two polyunsaturated acyl-CoAs showing intermediate values. Lysophosphatidylethanolamine and lysophosphatidylserine were also utilized as substrates. Electrospray ionization mass spectrometric analysis of phospholipids in Sf9 cells expressing LPCAT3 showed a relative increase in phosphatidylcholine containing saturated acyl chains and a decrease in phosphatidylcholine containing unsaturated acyl chains. Targeted reduction of LPCAT3 expression in HEK293 cells had essentially an opposite effect, resulting in decreased abundance of saturated phospholipid species and more unsaturated species. Reduced LPCAT3 expression resulted in more apoptosis and distinctly fewer lamellipodia, suggesting a necessary role for lysophospholipid esterification in maintaining cellular function and structure.

An N-terminal fragment of mouse DGAT1 binds different acyl-CoAs with varying affinity

A histidine-tagged recombinant N-terminal fragment of type-1 mouse liver diacylglycerol acyltransferase (DGAT; EC 2.3.1.20), MmDGAT1(1-95)His6, was expressed in Escherichia coli, and used to investigate possible acyl-CoA-binding properties. Analysis of the purified fragment by MALDI-TOF mass spectrometry revealed a polypeptide with molecular mass of about 11 kDa which was consistent with the calculated molecular mass based on the deduced amino acid sequence. Lipidex-1000 binding assays indicated that MmDGAT1(1-95)His(6) interacted with long chain fatty acyl-CoAs similar to observations on DGAT1 from oilseed rape (Brassica napus). Binding, as a function of acyl-CoA concentration, differed for palmitoyl (16:0), stearoyl (18:0), and erucoyl (cisDelta(13)22:1)-CoA. Binding of stearoyl- or erucoyl-CoA to MmDGAT1(1-95)His(6) as a function of acyl-CoA concentration, however, was sigmoid and displayed positive cooperativity suggesting that MmDGAT1 may be subject to allosteric modulation by acyl-CoAs. An intra-polypeptide segment within the N-terminal region of MmDGAT1 contained remnants of an acyl-CoA-binding signature initially identified in plant DGAT1. The acyl-CoA-binding site in mammalian DGAT1 could represent a potential target for therapeutic interventions for disorders such as type-2 diabetes and obesity.

Acyl-CoA-binding and self-associating properties of a recombinant 13.3 kDa N-terminal fragment of diacylglycerol acyltransferase-1 from oilseed rape

Background

Diacylglycerol acyltransferase (DGAT, EC 2.3.1.20) catalyzes the acyl-CoA-dependent acylation of sn-1, 2-diacylglycerol to generate triacylglycerol and CoA. The deduced amino acid sequence of cDNAs encoding DGAT1 from plants and mammals exhibit a hydrophilic N-terminal region followed by a number of potential membrane-spanning segments, which is consistent with the membrane-bound nature of this enzyme family. In order to gain insight into the structure/function properties of DGAT1 from Brassica napus (BnDGAT1), we produced and partially characterized a recombinant polyHis-tagged N-terminal fragment of the enzyme, BnDGAT1_(1–116)His₆, with calculated molecular mass of 13,278 Da.

Results

BnDGAT1_(1–116)His₆ was highly purified from bacterial lysate and plate-like monoclinic crystals were grown using this preparation. Lipidex-1000 binding assays and gel electrophoresis indicated that BnDGAT1_(1–116)His₆ interacts with long chain acyl-CoA. The enzyme fragment displayed enhanced affinity for erucoyl (22:1cis^Δ13)-CoA over oleoyl (18:1cis^Δ9)-CoA, and the binding process displayed positive cooperativity. Gel filtration chromatography and cross-linking studies indicated that BnDGAT1_(1–116)His₆ self-associated to form a tetramer. Polyclonal antibodies raised against a peptide of 15 amino acid residues representing a segment of BnDGAT1_(1–116)His₆ failed to react with protein in microsomal vesicles following treatment with proteinase K, suggesting that the N-terminal fragment of BnDGAT1 was localized to the cytosolic side of the ER.

Conclusion

Collectively, these results suggest that BnDGAT1 may be allosterically modulated by acyl-CoA through the N-terminal region and that the enzyme self-associates via interactions on the cytosolic side of the ER.

Cholesterol sensing, trafficking, and esterification

Mammalian cells acquire cholesterol from low-density lipoprotein (LDL) and from endogenous biosynthesis. The roles of the Niemann-Pick type C1 protein in mediating the endosomal transport of LDL-derived cholesterol and endogenously synthesized cholesterol are discussed. Excess cellular cholesterol is converted to cholesteryl esters by the enzyme acyl-coenzyme A:cholesterol acyltransferase (ACAT) 1 or is removed from a cell by cellular cholesterol efflux at the plasma membrane. A close relationship between the ACAT substrate pool and the cholesterol efflux pool is proposed. Sterol-sensing domains (SSDs) are present in several membrane proteins, including NPC1, HMG-CoA reductase, and the SREBP cleavage-activating protein. The functions of SSDs are described. ACAT1 is an endoplasmic reticulum cholesterol sensor and contains a signature motif characteristic of the membrane-bound acyltransferase family. The nonvesicular cholesterol translocation processes involve the START domain proteins and the oxysterol binding protein-related proteins (ORPs). The properties of these proteins are summarized.

Diacylglycerol acyltransferase: A key mediator of plant triacylglycerol synthesis

Many plants deposit TAG in seeds and fruits as the major form of storage lipid. TAG production is of tremendous socioeconomic value in food, nutraceutical, and industrial applications, and thus numerous conventional and molecular genetic strategies have been explored in attempts to increase TAG content and modify the FA composition of plant seed oils. Much research has focused on the acyl-CoA-dependent reaction catalyzed by diacylglycerol acyltransferase (DGAT), which is an integral endoplasmic reticulum protein and has also been shown to be present in oil bodies and plastids. DGAT enzymes exhibit diverse biochemical properties among different plant species, many of which are summarized here. In addition to catalyzing a critical step in TAG biosynthesis, there is evidence that DGAT has roles in lipid metabolism associated with germination and leaf senescence. TAG can also be formed in plants via two different acyl-CoA-independent pathways, catalyzed by phospholipid: diacylglycerol acyltransferase and diacylglycerol transacylase. The current understanding of the terminal step in TAG formation in plants and the development of molecular genetic approaches aimed at altering TAG yield and FA composition of TAG are discussed.

Human ACAT1 gene expression and its involvement in the development of atherosclerosis

Atherosclerosis is caused by a series of pathologic changes at the cellular level, with formation of macrophage-derived foam cells occurring at an early stage. Most of the cholesteryl esters in macrophage foam cells are produced by the enzyme acyl-coenzyme A:cholesterol acyltransferase (ACAT). Two ACAT genes, Acat1 and Acat2, exist in mammals. In the monocyte–macrophages, ACAT1 is the major isoenzyme and is a drug target for atherosclerosis treatment. Various proatherogenic stimuli, including interferon-γ and dexamethasone, cause upregulation of human Acat1 expression in macrophages. Thus, it should be possible to find antagonist(s) to downregulate human Acat1 expression. A greater understanding of human Acat1 expression may provide scientists with opportunities for novel therapeutic approaches to combat atherosclerosis.

Potential role of acyl-coenzyme A:cholesterol transferase (ACAT) Inhibitors as hypolipidemic and antiatherosclerosis drugs

Acyl-coenzyme A:cholesterol transferase (ACAT) is an integral membrane protein localized in the endoplasmic reticulum. ACAT catalyzes the formation of cholesteryl esters from cholesterol and fatty acyl coenzyme A. The cholesteryl esters are stored as cytoplasmic lipid droplets inside the cell. This process is very important to the organism as high cholesterol levels have been associated with cardiovascular disease. In mammals, two ACAT genes have been identified, ACAT1 and ACAT2. ACAT1 is ubiquitous and is responsible for cholesteryl ester formation in brain, adrenal glands, macrophages, and kidneys. ACAT2 is expressed in the liver and intestine. The inhibition of ACAT activity has been associated with decreased plasma cholesterol levels by suppressing cholesterol absorption and by diminishing the assembly and secretion of apolipoprotein B-containing lipoproteins such as very low density lipoprotein (VLDL). ACAT inhibition also prevents the conversion of macrophages into foam cells in the arterial walls, a critical event in the development of atherosclerosis. This review paper will focus on the role of ACAT in cholesterol metabolism, in particular as a target to develop novel therapeutic agents to control hypercholesterolemia, atherosclerosis, and Alzheimer's disease.

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

Human acyl-coenzyme A:cholesterol acyltransferase 1 (hACAT1) esterifies cholesterol at the endoplasmic reticulum (ER). We had previously reported that hACAT1 contains seven transmembrane domains (TMD) (Lin, S., Cheng, D., Liu, M. S., Chen, J., and Chang, T. Y. (1999) J. Biol. Chem. 274, 23276-23285) and nine cysteines. The Cys near the N-terminal is located at the cytoplasm; the two cysteines near the C-terminal form a disulfide bond and are located in the ER lumen. The other six free cysteines are located in buried region(s) of the enzyme (Guo, Z.-Y., Chang, C. C. Y., Lu, X., Chen, J., Li, B.-L., and Chang, T.-Y. (2005) Biochemistry 44, 6537-6548). In the current study, we show that the conserved His-460 is a key active site residue for hACAT1. We next performed Cys-scanning mutagenesis within the region of amino acids 354-493, expressed these mutants in Chinese hamster ovary cells lacking ACAT1, and prepared microsomes from transfected cells. The microsomes are either left intact or permeabilized with detergent. The accessibility of the engineered cysteines of microsomal hACAT1 to various maleimide derivatives, including mPEG(5000)-maleimide (large, hydrophilic, and membrane-impermeant), N-ethylmaleimide, 4-acetamido-4'-maleimidylstilbene-2,2'-disulfonic acid (small, hydrophilic, and ER membrane-permeant), and N-phenylmaleimide (small, hydrophobic, and ER membrane-permeant), were monitored by Western blot analysis. The results led us to construct a revised, nine-TMD model, with the active site His-460 located within a hitherto undisclosed transmembrane domain, between Arg-443 and Tyr-462.

Human Acyl-Coenzyme A:Cholesterol Acyltransferase 1 (acat1) Sequences Located in Two Different Chromosomes (7 and 1) Are Required to Produce a Novel ACAT1 Isoenzyme with Additional Sequence at the N Terminus

A rare form of human ACAT1 mRNA, containing the optional long 5'-untranslated region, is produced as a 4.3-kelonucleotide chimeric mRNA through a novel interchromosomal trans-splicing of two discontinuous RNAs transcribed from chromosomes 1 and 7. To investigate its function, we express the chimeric ACAT1 mRNA in Chinese hamster ovary cells and show that it can produce a larger ACAT1 protein, with an apparent molecular mass of 56 kDa on SDS-PAGE, in addition to the normal, 50-kDa ACAT1 protein, which is produced from the ACAT1 mRNAs without the optional long 5'-untranslated repeat. To produce the 56-kDa ACAT1, acat1 sequences located at both chromosomes 7 and 1 are required. The 56-kDa ACAT1 can be recognized by specific antibodies prepared against the predicted additional amino acid sequence located upstream of the N-terminal of the ACAT1(ORF). The translation initiation codon for the 56-kDa protein is GGC, which encodes for glycine, as deduced by mutation analysis and mass spectrometry. Similar to the 50-kDa protein, when expressed alone, the 56-kDa ACAT1 is located in the endoplasmic reticulum and is enzymatically active. The 56-kDa ACAT1 is present in native human cells, including human monocyte-derived macrophages. Our current results show that the function of the chimeric ACAT1 mRNA is to increase the ACAT enzyme diversity by producing a novel isoenzyme. To our knowledge, our result provides the first mammalian example that a trans-spliced mRNA produces a functional protein.

Human acyl-coenzyme A:cholesterol acyltransferase expressed in chinese hamster ovary cells: membrane topology and active site location

Acyl-CoA:cholesterol acyltransferase (ACAT) is a membrane-bound enzyme that produces cholesteryl esters intracellularly. Two ACAT genes (ACAT1 and ACAT2) have been identified. The expression of ACAT1 is ubiquitous, whereas that of ACAT2 is tissue restricted. Previous research indicates that ACAT1 may contain seven transmembrane domains (TMDs). To study ACAT2 topology, we inserted two different antigenic tags (hemagglutinin, monoclonal antibody Mab1) at various hydrophilic regions flanking each of its predicted TMDs, and expressed the recombinant proteins in mutant Chinese hamster ovary cells lacking endogenous ACAT. Each tagged ACAT2 was expressed in the endoplasmic reticulum as a single undegraded protein band and was at least partially active enzymatically. We then used cytoimmunofluorescence and protease protection assays to monitor the sidedness of the hemagglutinin and Mab1 tags along the ER membranes. The results indicated that ACAT2 contains only two detectable TMDs, located near the N terminal region. We also show that a conserved serine (S245), a candidate active site residue, is not essential for ACAT catalysis. Instead, a conserved histidine (H434) present within a hydrophobic peptide segment, may be essential for ACAT catalysis. H434 may be located at the cytoplasmic side of the membrane.

Cholesterol is superior to 7-ketocholesterol or 7 alpha-hydroxycholesterol as an allosteric activator for acyl-coenzyme A:cholesterol acyltransferase 1

We compared the abilities of cholesterol versus various oxysterols as substrate and/or as activator for the enzyme acyl-coenzyme A:cholesterol acyltransferase (ACAT), by monitoring the activity of purified human ACAT1 in response to sterols solubilized in mixed micelles or in reconstituted vesicles. The results showed that 5 alpha,6 alpha-epoxycholesterol and 7 alpha-hydroxycholesterol are comparable with cholesterol as the favored substrates, whereas 7-ketocholesterol, 7 beta-hydroxycholesterol, 5 beta,6 beta-epoxycholesterol, and 24(S),25-epoxycholesterol are very poor substrates for the enzyme. We then tested the ability of 7-ketocholesterol as an activator when cholesterol was measured as the substrate, and vice versa. When cholesterol was measured as the substrate, the addition of 7-ketocholesterol could not activate the enzyme. In contrast, when 7-ketocholesterol was measured as the substrate, the addition of cholesterol significantly activated the enzyme and changed the shape of the substrate saturation curve from sigmoidal to essentially hyperbolic. Additional results show that, as an activator, cholesterol is much better than all the oxysterols tested. These results suggest that ACAT1 contains two types of sterol binding sites; the structural requirement for the ACAT activator site is more stringent than it is for the ACAT substrate site. Upon activation by cholesterol, ACAT1 becomes promiscuous toward various sterols as its substrate.