A conserved histidine is essential for glycerolipid acyltransferase catalysis

Mlg1, a yeast acyltransferase located in ER membranes associated with mitochondria (MAMs), is involved in de novo synthesis and remodelling of phospholipids

In cells, phospholipids contain acyl chains of variable lengths and saturation, features that affect their functions. Their de novo synthesis in the endoplasmic reticulum takes place via the cytidine diphosphate diacylglycerol (CDP-DAG) and Kennedy pathways, which are conserved in eukaryotes. PA is a key intermediate for all phospholipids (PI, PIPs, PS, PE, PC, PG and CL). The de novo synthesis of PA occurs by acylation of glycerophosphate leading to the synthesis of 1-acyl lysoPA and subsequent acylation of 1-acyl lysoPA at the sn-2 position. Using membranes from Escherichia coli overexpressing MLG1, we showed that the yeast gene MLG1 encodes an acyltransferase, leading specifically to the synthesis of PA from 1-acyl lysoPA. Moreover, after their de novo synthesis, phospholipids can be remodelled by acyl exchange with one and/or two acyl chains exchanged at the sn-1 and/or sn-2 position. Based on shotgun lipidomics of the reference and mlg1Δ strains, as well as biochemical assays for acyltransferase activities, we identified an additional remodelling activity for Mlg1p, namely, incorporation of palmitic acid into the sn-1 position of PS and PE. By using confocal microscopy and subcellular fractionation, we also found that this acyltransferase is located in ER membranes associated with mitochondria, a finding that highlights the importance of these organelles in the global cellular metabolism of lipids.

Identification of a 1-acyl-glycerol-3-phosphate acyltransferase from Mycobacterium tuberculosis, a key enzyme involved in triacylglycerol biosynthesis

Latent tuberculosis, caused by dormant Mycobacterium tuberculosis (Mtb), poses a threat to global health through the incubation of undiagnosed infections within the community. Dormant Mtb, which is phenotypically tolerant to antibiotics, accumulates triacylglycerol (TAG) utilizing fatty acids obtained from macrophage lipid droplets. TAG is vital to mycobacteria, serving as a cell envelope component and energy reservoir during latency. TAG synthesis occurs by sequential acylation of glycerol-3-phosphate, wherein the second acylation step is catalyzed by acylglycerol-3-phosphate acyltransferase (AGPAT), resulting in the production of phosphatidic acid (PA), a precursor for the synthesis of TAG and various phospholipids. Here, we have characterized a putative acyltransferase of Mtb encoded by Rv3816c. We found that Rv3816c has all four characteristic motifs of AGPAT, exists as a membrane-bound enzyme, and functions as 1-acylglycerol-3-phosphate acyltransferase. The enzyme could transfer the acyl group to acylglycerol-3-phosphate (LPA) from monounsaturated fatty acyl-coenzyme A of chain length 16 or 18 to produce PA. Complementation of Escherichia coli PlsC mutant in vivo by Rv3816c confirmed that it functions as AGPAT. Its active site mutants, H43A and D48A, were incapable of transferring the acyl group to LPA in vitro and were not able to rescue the growth defect of E. coli PlsC mutant in vivo. Identifying Rv3816c as AGPAT and comparing its properties with other AGPAT homologs is not only a step toward understanding the TAG biosynthesis in mycobacteria but has the potential to explore it as a drug target.

Acylation of glycerolipids in mycobacteria

We report on the existence of two phosphatidic acid biosynthetic pathways in mycobacteria, a classical one wherein the acylation of the sn-1 position of glycerol-3-phosphate (G3P) precedes that of sn-2 and another wherein acylations proceed in the reverse order. Two unique acyltransferases, PlsM and PlsB2, participate in both pathways and hold the key to the unusual positional distribution of acyl chains typifying mycobacterial glycerolipids wherein unsaturated substituents principally esterify position sn-1 and palmitoyl principally occupies position sn-2. While PlsM selectively transfers a palmitoyl chain to the sn-2 position of G3P and sn-1-lysophosphatidic acid (LPA), PlsB2 preferentially transfers a stearoyl or oleoyl chain to the sn-1 position of G3P and an oleyl chain to sn-2-LPA. PlsM is the first example of an sn-2 G3P acyltransferase outside the plant kingdom and PlsB2 the first example of a 2-acyl-G3P acyltransferase. Both enzymes are unique in their ability to catalyze acyl transfer to both G3P and LPA.

Structural basis of the acyl-transfer mechanism of human GPAT1

Glycerol-3-phosphate acyltransferase (GPAT)1 is a mitochondrial outer membrane protein that catalyzes the first step of de novo glycerolipid biosynthesis. Hepatic expression of GPAT1 is linked to liver fat accumulation and the severity of nonalcoholic fatty liver diseases. Here we present the cryo-EM structures of human GPAT1 in substrate analog-bound and product-bound states. The structures reveal an N-terminal acyltransferase domain that harbors important catalytic motifs and a tightly associated C-terminal domain that is critical for proper protein folding. Unexpectedly, GPAT1 has no transmembrane regions as previously proposed but instead associates with the membrane via an amphipathic surface patch and an N-terminal loop-helix region that contains a mitochondrial-targeting signal. Combined structural, computational and functional studies uncover a hydrophobic pathway within GPAT1 for lipid trafficking. The results presented herein lay a framework for rational inhibitor development for GPAT1.

Insights into complex I assembly: Function of NDUFAF1 and a link with cardiolipin remodeling

Respiratory complex I is a ~1-MDa proton pump in mitochondria. Its structure has been revealed in great detail, but the structural basis of its assembly, in humans involving at least 15 assembly factors, is essentially unknown. We determined cryo-electron microscopy structures of assembly intermediates associated with assembly factor NDUFAF1 in a yeast model system. Subunits ND2 and NDUFC2 together with assembly factors NDUFAF1 and CIA84 form the nucleation point of the NDUFAF1-dependent assembly pathway. Unexpectedly, the cardiolipin remodeling enzyme tafazzin is an integral component of this core complex. In a later intermediate, all 12 subunits of the proximal proton pump module have assembled. NDUFAF1 locks the central ND3 subunit in an assembly-competent conformation, and major rearrangements of central subunits are required for complex I maturation.

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.

Transcriptome analysis of Pacific white shrimp (Litopenaeus vannamei) hepatopancreas challenged by Vibrio alginolyticus reveals lipid metabolic disturbance

Vibrio alginolyticus is a devastating bacterial pathogen of Pacific white shrimp (Litopenaeus vannamei), which often causes acute hepatopancreatic necrosis syndrome (AHPNS) and early mortality syndrome (EMS). Elucidation of molecular mechanisms of L. vannamei in responding to infection is essential for controlling the epidemic. In the present study, transcriptomic profiles of L. vannamei hepatopancreas were explored by injecting with PBS or V. alginolyticus. Hepatopancreas morphology of L. vannamei was also assessed. The result reveals that compared with the hepatopancreas of PBS group, the storage cells (R-cell), secretory cells (B-cell) and star-shaped polygonal structures of the lumen were disappeared and necrotic after challenged by V. alginolyticus at 24 h. Transcriptome data showed that a total of 314 differential expression genes were induced by V. alginolyticus, with 133 and 181 genes up- and down-regulated, respectively. These genes were mainly associated with lysosome pathway, glycerophospholipid metabolism, drug metabolism-other enzymes, cysteine and methionine metabolism, aminoacyl-tRNA biosynthesis and PPAR signal pathway. Among these pathways, the lysosome pathway, glycerophospholipid metabolism and PPAR signal pathway were both related with lipid metabolism. Therefore, we detected the lipid accumulation in hepatopancreas by Oil Red O staining, TG and CHOL detection and the relative mRNA expression of several lipid metabolism related genes in the hepatopancreas of shrimp after challenge to V. alginolyticus. The present data reveals that lipids from the L. vannamei are nutrient sources for the V. alginolyticus and define the fate of the infection by modulating lipid homeostasis. These findings may have important implication for understanding the L. vannamei and V. alginolyticus interactions, and provide a substantial dataset for further research and may deliver the basis for preventing the bacterial diseases.

Co-expressing Eranthis hyemalis lysophosphatidic acid acyltransferase 2 and elongase improves two very long chain polyunsaturated fatty acid production in Brassica carinata

Docosadienoic acid (DDA, 22:2–13,16) and docosatrienoic acid (DTA, 22:3–13,16,19) are two very long chain polyunsaturated fatty acids (VLCPUFAs) that are recently shown to possess strong anti-inflammatory and antitumor properties. An ELO type elongase (EhELO1) from wild plant Eranthis hyemalis can synthesize the two fatty acids by sequential elongation of linoleic acid and alpha-linolenic acid, respectively. Seed-specific expression of this gene in oilseed crop Brassica carinata produced a considerable amount of DDA and DTA in transgenic seeds. However, these fatty acids were excluded from the sn-2 position of triacylglycerols (TAGs). To improve the production level and nutrition value of the VLCPUFAs in the transgenic oilseed crop, a cytoplasmic lysophosphatidic acid acyltransferase (EhLPAAT2) for the incorporation of the two fatty acids into the sn-2 position of triacylglycerols was identified from E. hyemalis. RT-PCR analysis showed that it was preferentially expressed in developing seeds where EhELO1 was exclusively expressed in E. hyemalis. Seed specific expression of EhLPAAT2 along with EhELO1 in B. carinata resulted in the effective incorporation of DDA and DTA at the sn-2 position of TAGs, thereby increasing the total amount of DDA and DTA in transgenic seeds. To our knowledge, this is the first plant LPAAT that can incorporate VLCPUFAs into TAGs. Improved production of DDA and DTA in the oilseed crop using EhLPAAT2 and EhELO1 provides a real commercial opportunity for high value agriculture products for nutraceutical uses.

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The first plant LPAAT able to acylate VLCPUFAs was identified from winter aconite.

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It could complement the defective phenotype of E. coli LPAAT mutant.

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It could improve the incorporation of two VLCPUFAs into TAGs in oilseeds.

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It could enhance the total production of two VLCPUFAs in oilseeds.

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Seed-specific expression of it could also increase seed oil and seed weight.

A Novel Lysophosphatidic Acid Acyltransferase of Escherichia coli Produces Membrane Phospholipids with a cis-vaccenoyl Group and Is Related to Flagellar Formation

Lysophosphatidic acid acyltransferase (LPAAT) introduces fatty acyl groups into the sn-2 position of membrane phospholipids (PLs). Various bacteria produce multiple LPAATs, whereas it is believed that Escherichia coli produces only one essential LPAAT homolog, PlsC-the deletion of which is lethal. However, we found that E. coli possesses another LPAAT homolog named YihG. Here, we show that overexpression of YihG in E. coli carrying a temperature-sensitive mutation in plsC allowed its growth at non-permissive temperatures. Analysis of the fatty acyl composition of PLs from the yihG-deletion mutant (∆yihG) revealed that endogenous YihG introduces the cis-vaccenoyl group into the sn-2 position of PLs. Loss of YihG did not affect cell growth or morphology, but ∆yihG cells swam well in liquid medium in contrast to wild-type cells. Immunoblot analysis showed that FliC was highly expressed in ∆yihG cells, and this phenotype was suppressed by expression of recombinant YihG in ∆yihG cells. Transmission electron microscopy confirmed that the flagellar structure was observed only in ∆yihG cells. These results suggest that YihG has specific functions related to flagellar formation through modulation of the fatty acyl composition of membrane PLs.

The Function of Tafazzin, a Mitochondrial Phospholipid-Lysophospholipid Acyltransferase

Tafazzin is a mitochondrial enzyme that exchanges fatty acids between phospholipids by phospholipid-lysophospholipid transacylation. The reaction alters the molecular species composition and, as a result, the physical properties of lipids. In vivo, the most important substrate of tafazzin is the mitochondria-specific lipid cardiolipin. Tafazzin mutations cause the human disease Barth syndrome, which presents with cardiomyopathy, skeletal muscle weakness, fatigue, and other symptoms, probably all related to mitochondrial dysfunction. The reason why mitochondria require tafazzin is still not known, but recent evidence suggests that tafazzin may lower the energy cost associated with protein crowding in the inner mitochondrial membrane.

Structure and Functional Characterization of Membrane Integral Proteins in the Lipid Cubic Phase

The lipid cubic phase (LCP) has been used extensively as a medium for crystallizing membrane proteins. It is an attractive environment in which to perform such studies because it incorporates a lipid bilayer. It is therefore considered a useful and a faithful biomembrane mimetic. Here, we bring together evidence that supports this view. Biophysical characterizations are described demonstrating that the cubic phase is a porous medium into and out of which water-soluble molecules can diffuse for binding to and reaction with reconstituted proteins. The proteins themselves are shown to be functionally reconstituted into and to have full mobility in the bilayered membrane, a prerequisite for LCP crystallogenesis. Spectroscopic methods have been used to characterize the conformation and disposition of proteins in the mesophase. Procedures for performing activity assays on enzymes directly in the cubic phase have been reported. Specific examples described here include a kinase and two transferases, where quantitative kinetics and mechanism-defining measurements were performed directly or via a coupled assay system. Finally, ligand-binding assays are described, where binding to proteins in the mesophase membrane was monitored directly by eye and indirectly by fluorescence quenching, enabling binding constant determinations for targets with affinity values in the micromolar and nanomolar range. These results make a convincing case that the lipid bilayer of the cubic mesophase is an excellent membrane mimetic and a suitable medium in which to perform not only crystallogenesis but also biochemical and biophysical characterizations of membrane proteins.

Biosynthetic Enzymes of Membrane Glycerophospholipid Diversity as Therapeutic Targets for Drug Development

Biophysical properties of membranes are dependent on their glycerophospholipid compositions. Lysophospholipid acyltransferases (LPLATs) selectively incorporate fatty chains into lysophospholipids to affect the fatty acid composition of membrane glycerophospholipids. Lysophosphatidic acid acyltransferases (LPAATs) of the 1-acylglycerol-3-phosphate O-acyltransferase (AGPAT) family incorporate fatty chains into phosphatidic acid during the de novo glycerophospholipid synthesis in the Kennedy pathway. Other LPLATs of both the AGPAT and the membrane bound O-acyltransferase (MBOAT) families further modify the fatty chain compositions of membrane glycerophospholipids in the remodeling pathway known as the Lands' cycle. The LPLATs functioning in these pathways possess unique characteristics in terms of their biochemical activities, regulation of expressions, and functions in various biological contexts. Essential physiological functions for LPLATs have been revealed in studies using gene-deficient mice, and important roles for several enzymes are also indicated in human diseases where their mutation or dysregulation causes or contributes to the pathological condition. Now several LPLATs are emerging as attractive therapeutic targets, and further understanding of the mechanisms underlying their physiological and pathological roles will aid in the development of novel therapies to treat several diseases that involve altered glycerophospholipid metabolism.

Novel Dual-Functional Enzyme Lip10 Catalyzes Lipase and Acyltransferase Activities in the Oleaginous Fungus Mucor circinelloides

Lipases or triacylglycerol (TAG) lipases belong to the α/β-hydrolases superfamily, which are enzymes capable of catalyzing the hydrolysis of the ester bond between fatty acids and glycerol. Interestingly, some lipases have been found to not only possess hydrolysis activity but also acyltransferase activity in yeasts and microalgae. Our present study reported a novel dual-functional Mucor circinelloides lipase Lip10 with a slight lipolysis activity but a noteworthy phospholipid/diacylglycerol acyltransferase (PDAT) activity. The purified Lip10 mutants prefer to utilize phosphatidyl serine to form TAG over phosphatidyl ethanolamine and phosphatidylcholine. Site-directed mutagenesis indicated that the histidine residue in the acyltransferase motif H-(X)₄-D is indispensable for the PDAT activity of Lip10. Overexpression of the acyltransferase motif of Lip10 promoted cell growth by 12% and increased lipid production by 14% compared to the control, whilst overexpression of the lipase motif induced lipid degradation in M. circinelloides.

TAZ encodes tafazzin, a transacylase essential for cardiolipin formation and central to the etiology of Barth syndrome

Tafazzin, which is encoded by the TAZ gene, catalyzes transacylation to form mature cardiolipin and shows preference for the transfer of a linoleic acid (LA) group from phosphatidylcholine (PC) to monolysocardiolipin (MLCL) with influence from mitochondrial membrane curvature. The protein contains domains and motifs involved in targeting, anchoring, and an active site for transacylase activity. Tafazzin activity affects many aspects of mitochondrial structure and function, including that of the electron transport chain, fission-fusion, as well as apoptotic signaling. TAZ mutations are implicated in Barth syndrome, an underdiagnosed and devastating disease that primarily affects male pediatric patients with a broad spectrum of disease pathologies that impact the cardiovascular, neuromuscular, metabolic, and hematologic systems.

Developing a High-Throughput Assay for the Integral Membrane Glycerol 3-Phosphate Acyltransferase

Phospholipid biosynthesis begins with the acylation of glycerol 3-phosphate (G3P). In most Gram-positive bacteria including many pathogens, a membrane protein called PlsY is the only acyltransferase that catalyzes this essential step, making it a potential target for the development of antibiotics. A convenient enzymatic assay should facilitate such drug discovery activities. Previously, we developed a continuous assay by monitoring phosphate, one of the enzymatic product, using a fluorescently labeled phosphate binding protein in a bilayer environment called lipid cubic phase (LCP). However, some intrinsic characteristics of LCP, such as high viscosity, make the assay incompatible with common high-throughput liquid-handling platforms. Here, we adapted the assay by hosting PlsY in detergent micelles, enabling us to conduct the assay using standard multi-channel pipets in a high-throughput manner. With optimal enzyme loading, the reaction velocity was linear up to 30 min. PlsY showed Michaelis-Menten kinetics behavior in micelles with a V_max of 57.5 μmol min^-1 mg^-1, and K_m of 1.14 mM G3P and 6.2 μM acyl phosphate. The inhibitory product lysophosphatidic acid inhibited PlsY with the IC₅₀ of 19 μM. The results principally demonstrated the feasibility of using the assay for high-throughput screening, and the protocol provided an encouraging starting point for further optimization and validation of the assay for automated platforms.

Substrate channeling in the glycerol-3-phosphate pathway regulates the synthesis, storage and secretion of glycerolipids

The successive acylation of glycerol-3-phosphate (G3P) by glycerol-3-phosphate acyltransferases and acylglycerol-3-phosphate acyltransferases produces phosphatidic acid (PA), a precursor for CDP-diacylglycerol-dependent phospholipid synthesis. PA is further dephosphorylated by LIPINs to produce diacylglycerol (DG), a substrate for the synthesis of triglyceride (TG) by DG acyltransferases and a precursor for phospholipid synthesis via the CDP-choline and CDP-ethanolamine (Kennedy) pathways. The channeling of fatty acids into TG for storage in lipid droplets and secretion in lipoproteins or phospholipids for membrane biogenesis is dependent on isoform expression, activity and localization of G3P pathway enzymes, as well as dietary and hormonal and tissue-specific factors. Here, we review the mechanisms that control partitioning of substrates into lipid products of the G3P pathway.

Rickettsia Lipid A Biosynthesis Utilizes the Late Acyltransferase LpxJ for Secondary Fatty Acid Addition

Members of the Rickettsia genus are obligate intracellular, Gram-negative coccobacilli that infect mammalian and arthropod hosts. Several rickettsial species are human pathogens and are transmitted by blood-feeding arthropods. In Gram-negative parasites, the outer membrane (OM) sits at the nexus of the host-pathogen interaction and is rich in lipopolysaccharide (LPS). The lipid A component of LPS anchors the molecule to the bacterial surface and is an endotoxic agonist of Toll-like receptor 4 (TLR4). Despite the apparent importance of lipid A in maintaining OM integrity, as well as its inflammatory potential during infection, this molecule is poorly characterized in Rickettsia pathogens. In this work, we have identified and characterized new members of the recently discovered LpxJ family of lipid A acyltransferases in both Rickettsia typhi and Rickettsia rickettsii, the etiological agents of murine typhus and Rocky Mountain spotted fever, respectively. Our results demonstrate that these enzymes catalyze the addition of a secondary acyl chain (C₁₄/C₁₆) to the 3'-linked primary acyl chain of the lipid A moiety in the final steps of the Raetz pathway of lipid A biosynthesis. Since lipid A architecture is fundamental to bacterial OM integrity, we believe that rickettsial LpxJ may be important in maintaining membrane dynamics to facilitate molecular interactions at the host-pathogen interface that are required for adhesion and invasion of mammalian cells. This work contributes to our understanding of rickettsial outer membrane physiology and sets a foundation for further exploration of the envelope and its role in pathogenesis.IMPORTANCE Lipopolysaccharide (LPS) triggers an inflammatory response through the TLR4-MD2 receptor complex and inflammatory caspases, a process mediated by the lipid A moiety of LPS. Species of Rickettsia directly engage both extracellular and intracellular immunosurveillance, yet little is known about rickettsial lipid A. Here, we demonstrate that the alternative lipid A acyltransferase, LpxJ, from Rickettsia typhi and R. rickettsii catalyzes the addition of C₁₆ fatty acid chains into the lipid A 3'-linked primary acyl chain, accounting for major structural differences relative to the highly inflammatory lipid A of Escherichia coli.

Dual Functions of Lip6 and Its Regulation of Lipid Metabolism in the Oleaginous Fungus Mucor circinelloides

Although multiple roles of lipases have been reported in yeasts and microalgae, the functions of lipases have not been studied in oleaginous filamentous fungi. Lipase Lip6 has been reported in the oleaginous filamentous fungus Mucor circinelloides with the consensus lipase motif GXSXG and the typical acyltransferase motif of H-(X)₄-D. To demonstrate that Lip6 might play dual roles as a lipase and an acyltransferase, we performed site-directed mutagenesis in the lipase motif and the acyltransferase motif of Lip6. Mutation in the lipase motif increased cell biomass by 12%-18% and promoted lipid accumulation by 9%-24%, while mutation in the acyltransferase motif induced lipid degradation. In vitro, purified Lip6 had a slight lipase activity but had a stronger phospholipid:DAG acyltransferase activity. Enzyme activity assays in vivo and phospholipid synthesis pathway analysis suggested that phosphatidyl serine and phosphatidyl ethanolamine can be the supplier of a fatty acyl moiety to form TAG in M. circinelloides.

The phospholipid-repair system LplT/Aas in Gram-negative bacteria protects the bacterial membrane envelope from host phospholipase A2 attack

Secretory phospholipases A2 (sPLA2s) are potent components of mammalian innate-immunity antibacterial mechanisms. sPLA2 enzymes attack bacteria by hydrolyzing bacterial membrane phospholipids, causing membrane disorganization and cell lysis. However, most Gram-negative bacteria are naturally resistant to sPLA2 Here we report a novel resistance mechanism to mammalian sPLA2 in Escherichia coli, mediated by a phospholipid repair system consisting of the lysophospholipid transporter LplT and the acyltransferase Aas in the cytoplasmic membrane. Mutation of the lplT or aas gene abolished bacterial lysophospholipid acylation activity and drastically increased bacterial susceptibility to the combined actions of inflammatory fluid components and sPLA2, resulting in bulk phospholipid degradation and loss of colony-forming ability. sPLA2-mediated hydrolysis of the three major bacterial phospholipids exhibited distinctive kinetics and deacylation of cardiolipin to its monoacyl-derivative closely paralleled bacterial death. Characterization of the membrane envelope in lplT- or aas-knockout mutant bacteria revealed reduced membrane packing and disruption of lipid asymmetry with more phosphatidylethanolamine present in the outer leaflet of the outer membrane. Moreover, modest accumulation of lysophospholipids in these mutant bacteria destabilized the inner membrane and rendered outer membrane-depleted spheroplasts much more sensitive to sPLA2 These findings indicated that LplT/Aas inactivation perturbs both the outer and inner membranes by bypassing bacterial membrane maintenance mechanisms to trigger specific interfacial activation of sPLA2 We conclude that the LplT/Aas system is important for maintaining the integrity of the membrane envelope in Gram-negative bacteria. Our insights may help inform new therapeutic strategies to enhance host sPLA2 antimicrobial activity.

Structural insights into the committed step of bacterial phospholipid biosynthesis

The membrane-integral glycerol 3-phosphate (G3P) acyltransferase PlsY catalyses the committed and essential step in bacterial phospholipid biosynthesis by acylation of G3P, forming lysophosphatidic acid. It contains no known acyltransferase motifs, lacks eukaryotic homologs, and uses the unusual acyl-phosphate as acyl donor, as opposed to acyl-CoA or acyl-carrier protein for other acyltransferases. Previous studies have identified several PlsY inhibitors as potential antimicrobials. Here we determine the crystal structure of PlsY at 1.48 Å resolution, revealing a seven-transmembrane helix fold. Four additional substrate- and product-bound structures uncover the atomic details of its relatively inflexible active site. Structure and mutagenesis suggest a different acylation mechanism of ‘substrate-assisted catalysis’ that, unlike other acyltransferases, does not require a proteinaceous catalytic base to complete. The structure data and a high-throughput enzymatic assay developed in this work should prove useful for virtual and experimental screening of inhibitors against this vital bacterial enzyme.

The first step in bacterial phospholipid biosynthesis is the acylation of glycerol 3-phosphate to form lysophosphatidic acid. Here, the authors present the high resolution crystal structure of the glycerol 3-phosphate acyltransferase PlsY, a membrane protein and give insights into its catalytical mechanism.

AlmG, responsible for polymyxin resistance in pandemic Vibrio cholerae, is a glycyltransferase distantly related to lipid A late acyltransferases

Cationic antimicrobial peptides (CAMPs), such as polymyxins, are used as a last-line defense in treatment of many bacterial infections. However, some bacteria have developed resistance mechanisms to survive these compounds. Current pandemic O1 Vibrio cholerae biotype El Tor is resistant to polymyxins, whereas a previous pandemic strain of the biotype Classical is polymyxin-sensitive. The almEFG operon found in El Tor V. cholerae confers >100-fold resistance to antimicrobial peptides through aminoacylation of lipopolysaccharide (LPS), expected to decrease the negatively charged surface of the V. cholerae outer membrane. This Gram-negative system bears striking resemblance to a related Gram-positive cell-wall remodeling strategy that also promotes CAMP resistance. Mutants defective in AlmEF-dependent LPS modification exhibit reduced fitness in vivo Here, we present investigation of AlmG, the hitherto uncharacterized member of the AlmEFG pathway. Evidence for AlmG glycyl to lipid substrate transferase activity is demonstrated in vivo by heterologous expression of V. cholerae pathway enzymes in a specially engineered Escherichia coli strain. Development of a minimal keto-deoxyoctulosonate (Kdo)-lipid A domain in E. coli was necessary to facilitate chemical structure analysis and to produce a mimetic Kdo-lipid A domain AlmG substrate to that synthesized by V. cholerae. Our biochemical studies support a uniquely nuanced pathway of Gram-negative CAMPs resistance and provide a more detailed description of an enzyme of the pharmacologically relevant lysophosphospholipid acyltransferase (LPLAT) superfamily.

How lipid droplets "TAG" along: Glycerolipid synthetic enzymes and lipid storage

Triacylglycerols (TAG) serve as the predominant form of energy storage in mammalian cells, and TAG synthesis influences conditions such as obesity, fatty liver, and insulin resistance. In most tissues, the glycerol 3-phosphate pathway enzymes are responsible for TAG synthesis, and the regulation and function of these enzymes is therefore important for metabolic homeostasis. Here we review the sites and regulation of glycerol-3-phosphate acyltransferase (GPAT), acylglycerol-3-phosphate acyltransferase (AGPAT), lipin phosphatidic acid phosphatase (PAP), and diacylglycerol acyltransferase (DGAT) enzyme action. We highlight the critical roles that these enzymes play in human health by reviewing Mendelian disorders that result from mutation in the corresponding genes. We also summarize the valuable insights that genetically engineered mouse models have provided into the cellular and physiological roles of GPATs, AGPATs, lipins and DGATs. Finally, we comment on the status and feasibility of therapeutic approaches to metabolic disease that target enzymes of the glycerol 3-phosphate pathway. This article is part of a Special Issue entitled: Recent Advances in Lipid Droplet Biology edited by Rosalind Coleman and Matthijs Hesselink.

Structural basis of phosphatidyl-myo-inositol mannosides biosynthesis in mycobacteria

Phosphatidyl-myo-inositol mannosides (PIMs) are glycolipids of unique chemical structure found in the inner and outer membranes of the cell envelope of all Mycobacterium species. The PIM family of glycolipids comprises phosphatidyl-myo-inositol mono-, di-, tri-, tetra-, penta-, and hexamannosides with different degrees of acylation. PIMs are considered not only essential structural components of the cell envelope but also the precursors of lipomannan and lipoarabinomannan, two major lipoglycans implicated in host-pathogen interactions. Since the description of the complete chemical structure of PIMs, major efforts have been committed to defining the molecular bases of its biosynthetic pathway. The structural characterization of the integral membrane phosphatidyl-myo-inositol phosphate synthase (PIPS), and that of three enzymes working at the protein-membrane interface, the phosphatidyl-myo-inositol mannosyltransferases A and B, and the acyltransferase PatA, established the basis of the early steps of the PIM pathway at the molecular level. This article is part of a Special Issue entitled: Bacterial Lipids edited by Russell E. Bishop.

Structure-guided enzymology of the lipid A acyltransferase LpxM reveals a dual activity mechanism

Gram-negative bacteria possess a characteristic outer membrane, of which the lipid A constituent elicits a strong host immune response through the Toll-like receptor 4 complex, and acts as a component of the permeability barrier to prevent uptake of bactericidal compounds. Lipid A species comprise the bulk of the outer leaflet of the outer membrane and are produced through a multistep biosynthetic pathway conserved in most Gram-negative bacteria. The final steps in this pathway involve the secondary acylation of lipid A precursors. These are catalyzed by members of a superfamily of enzymes known as lysophospholipid acyltransferases (LPLATs), which are present in all domains of life and play important roles in diverse biological processes. To date, characterization of this clinically important class of enzymes has been limited by a lack of structural information and the availability of only low-throughput biochemical assays. In this work, we present the structure of the bacterial LPLAT protein LpxM, and we describe a high-throughput, label-free mass spectrometric assay to characterize acyltransferase enzymatic activity. Using our structure and assay, we identify an LPLAT thioesterase activity, and we provide experimental evidence to support an ordered-binding and "reset" mechanistic model for LpxM function. This work enables the interrogation of other bacterial acyltransferases' structure-mechanism relationships, and the assay described herein provides a foundation for quantitatively characterizing the enzymology of any number of clinically relevant LPLAT proteins.

Characterization of the Plasmodium falciparum and P. berghei glycerol 3-phosphate acyltransferase involved in FASII fatty acid utilization in the malaria parasite apicoplast

Malaria parasites can synthesize fatty acids via a type II fatty acid synthesis (FASII) pathway located in their apicoplast. The FASII pathway has been pursued as an anti‐malarial drug target, but surprisingly little is known about its role in lipid metabolism. Here we characterize the apicoplast glycerol 3‐phosphate acyltransferase that acts immediately downstream of FASII in human (Plasmodium falciparum) and rodent (Plasmodium berghei) malaria parasites and investigate how this enzyme contributes to incorporating FASII fatty acids into precursors for membrane lipid synthesis. Apicoplast targeting of the P. falciparum and P. berghei enzymes are confirmed by fusion of the N‐terminal targeting sequence to GFP and 3′ tagging of the full length protein. Activity of the P. falciparum enzyme is demonstrated by complementation in mutant bacteria, and critical residues in the putative active site identified by site‐directed mutagenesis. Genetic disruption of the P. falciparum enzyme demonstrates it is dispensable in blood stage parasites, even in conditions known to induce FASII activity. Disruption of the P. berghei enzyme demonstrates it is dispensable in blood and mosquito stage parasites, and only essential for development in the late liver stage, consistent with the requirement for FASII in rodent malaria models. However, the P. berghei mutant liver stage phenotype is found to only partially phenocopy loss of FASII, suggesting newly made fatty acids can take multiple pathways out of the apicoplast and so giving new insight into the role of FASII and apicoplast glycerol 3‐phosphate acyltransferase in malaria parasites.

Apicoplast-Localized Lysophosphatidic Acid Precursor Assembly Is Required for Bulk Phospholipid Synthesis in Toxoplasma gondii and Relies on an Algal/Plant-Like Glycerol 3-Phosphate Acyltransferase

Most apicomplexan parasites possess a non-photosynthetic plastid (the apicoplast), which harbors enzymes for a number of metabolic pathways, including a prokaryotic type II fatty acid synthesis (FASII) pathway. In Toxoplasma gondii, the causative agent of toxoplasmosis, the FASII pathway is essential for parasite growth and infectivity. However, little is known about the fate of fatty acids synthesized by FASII. In this study, we have investigated the function of a plant-like glycerol 3-phosphate acyltransferase (TgATS1) that localizes to the T. gondii apicoplast. Knock-down of TgATS1 resulted in significantly reduced incorporation of FASII-synthesized fatty acids into phosphatidic acid and downstream phospholipids and a severe defect in intracellular parasite replication and survival. Lipidomic analysis demonstrated that lipid precursors are made in, and exported from, the apicoplast for de novo biosynthesis of bulk phospholipids. This study reveals that the apicoplast-located FASII and ATS1, which are primarily used to generate plastid galactolipids in plants and algae, instead generate bulk phospholipids for membrane biogenesis in T. gondii.

Structural basis for selective recognition of acyl chains by the membrane-associated acyltransferase PatA

The biosynthesis of phospholipids and glycolipids are critical pathways for virtually all cell membranes. PatA is an essential membrane associated acyltransferase involved in the biosynthesis of mycobacterial phosphatidyl-myo-inositol mannosides (PIMs). The enzyme transfers a palmitoyl moiety from palmitoyl–CoA to the 6-position of the mannose ring linked to 2-position of inositol in PIM₁/PIM₂. We report here the crystal structures of PatA from Mycobacterium smegmatis in the presence of its naturally occurring acyl donor palmitate and a nonhydrolyzable palmitoyl–CoA analog. The structures reveal an α/β architecture, with the acyl chain deeply buried into a hydrophobic pocket that runs perpendicular to a long groove where the active site is located. Enzyme catalysis is mediated by an unprecedented charge relay system, which markedly diverges from the canonical HX₄D motif. Our studies establish the mechanistic basis of substrate/membrane recognition and catalysis for an important family of acyltransferases, providing exciting possibilities for inhibitor design.

PatA is a membrane-associated acyltransferase that is essential for the biosynthesis of mycobacterial glycolipids. Here, Albesa-Jové et al. describe structures of PatA from Mycobacterium smegmatis in complex with acyl donors and show that catalysis occurs by an unusual charge-relay mechanism.

Site-Directed Mutagenesis from Arg195 to His of a Microalgal Putatively Chloroplastidial Glycerol-3-Phosphate Acyltransferase Causes an Increase in Phospholipid Levels in Yeast

To analyze the contribution of glycerol-3-phosphate acyltransferase (GPAT) to the first acylation of glycerol-3-phosphate (G-3-P), the present study focused on a functional analysis of the GPAT gene from Lobosphaera incisa (designated as LiGPAT). A full-length cDNA of LiGPAT consisting of a 1,305-bp ORF, a 1,652-bp 5'-UTR, and a 354-bp 3'-UTR, was cloned. The ORF encoded a 434-amino acid peptide, of which 63 residues at the N-terminus defined a chloroplast transit peptide. Multiple sequence alignment and phylogeny analysis of GPAT homologs provided the convincible bioinformatics evidence that LiGPAT was localized to chloroplasts. Considering the conservation of His among the G-3-P binding sites from chloroplastidial GPATs and the substitution of His by Arg at position 195 in the LiGPAT mature protein (designated mLiGPAT), we established the heterologous expression of either mLiGPAT or its mutant (Arg195His) (sdmLiGPAT) in the GPAT-deficient yeast mutant gat1Δ. Lipid profile analyses of these transgenic yeasts not only validated the acylation function of LiGPAT but also indicated that the site-directed mutagenesis from Arg(195) to His led to an increase in the phospholipid level in yeast. Semi-quantitative analysis of mLiGPAT and sdmLiGPAT, together with the structural superimposition of their G-3-P binding sites, indicated that the increased enzymatic activity was caused by the enlarged accessible surface of the phosphate group binding pocket when Arg(195) was mutated to His. Thus, the potential of genetic manipulation of GPAT to increase the glycerolipid level in L. incisa and other microalgae would be of great interest.

Reassessing the Potential Activities of Plant CGI-58 Protein

Comparative Gene Identification-58 (CGI-58) is a widespread protein found in animals and plants. This protein has been shown to participate in lipolysis in mice and humans by activating Adipose triglyceride lipase (ATGL), the initial enzyme responsible for the triacylglycerol (TAG) catabolism cascade. Human mutation of CGI-58 is the cause of Chanarin-Dorfman syndrome, an orphan disease characterized by a systemic accumulation of TAG which engenders tissue disorders. The CGI-58 protein has also been shown to participate in neutral lipid metabolism in plants and, in this case, a mutation again provokes TAG accumulation. Although its roles as an ATGL coactivator and in lipid metabolism are quite clear, the catalytic activity of CGI-58 is still in question. The acyltransferase activities of CGI-58 have been speculated about, reported or even dismissed and experimental evidence that CGI-58 expressed in E. coli possesses an unambiguous catalytic activity is still lacking. To address this problem, we developed a new set of plasmids and site-directed mutants to elucidate the in vivo effects of CGI-58 expression on lipid metabolism in E. coli. By analyzing the lipid composition in selected E. coli strains expressing CGI-58 proteins, and by reinvestigating enzymatic tests with adequate controls, we show here that recombinant plant CGI-58 has none of the proposed activities previously described. Recombinant plant and mouse CGI-58 both lack acyltransferase activity towards either lysophosphatidylglycerol or lysophosphatidic acid to form phosphatidylglycerol or phosphatidic acid and recombinant plant CGI-58 does not catalyze TAG or phospholipid hydrolysis. However, expression of recombinant plant CGI-58, but not mouse CGI-58, led to a decrease in phosphatidylglycerol in all strains of E. coli tested, and a mutation of the putative catalytic residues restored a wild-type phenotype. The potential activities of plant CGI-58 are subsequently discussed.

Endogenous and Synthetic ABHD5 Ligands Regulate ABHD5-Perilipin Interactions and Lipolysis in Fat and Muscle

Fat and muscle lipolysis involves functional interactions of adipose triglyceride lipase (ATGL), α-β hydrolase domain-containing protein 5 (ABHD5), and tissue-specific perilipins 1 and 5 (PLIN1 and PLIN5). ABHD5 potently activates ATGL, but this lipase-promoting activity is suppressed when ABHD5 is bound to PLIN proteins on lipid droplets. In adipocytes, protein kinase A (PKA) phosphorylation of PLIN1 rapidly releases ABHD5 to activate ATGL, but mechanisms for rapid regulation of PLIN5-ABHD5 interaction in muscle are unknown. Here, we identify synthetic ligands that release ABHD5 from PLIN1 or PLIN5 without PKA activation and rapidly activate adipocyte and muscle lipolysis. Molecular imaging and affinity probe labeling demonstrated that ABHD5 is directly targeted by these synthetic ligands and additionally revealed that ABHD5-PLIN interactions are regulated by endogenous ligands, including long-chain acyl-CoA. Our results reveal a new locus of lipolysis control and suggest ABHD5 ligands might be developed into novel therapeutics that directly promote fat catabolism.

Biosynthesis of Membrane Lipids

The pathways in Escherichia coli and (largely by analogy) S. enterica remain the paradigm of bacterial lipid synthetic pathways, although recently considerable diversity among bacteria in the specific areas of lipid synthesis has been demonstrated. The structural biology of the fatty acid synthetic proteins is essentially complete. However, the membrane-bound enzymes of phospholipid synthesis remain recalcitrant to structural analyses. Recent advances in genetic technology have allowed the essentialgenes of lipid synthesis to be tested with rigor, and as expected most genes are essential under standard growth conditions. Conditionally lethal mutants are available in numerous genes, which facilitates physiological analyses. The array of genetic constructs facilitates analysis of the functions of genes from other organisms. Advances in mass spectroscopy have allowed very accurate and detailed analyses of lipid compositions as well as detection of the interactions of lipid biosynthetic proteins with one another and with proteins outside the lipid pathway. The combination of these advances has resulted in use of E. coli and S. enterica for discovery of new antimicrobials targeted to lipid synthesis and in deciphering the molecular actions of known antimicrobials. Finally,roles for bacterial fatty acids other than as membrane lipid structural components have been uncovered. For example, fatty acid synthesis plays major roles in the synthesis of the essential enzyme cofactors, biotin and lipoic acid. Although other roles for bacterial fatty acids, such as synthesis of acyl-homoserine quorum-sensing molecules, are not native to E. coli introduction of the relevant gene(s) synthesis of these foreign molecules readily proceeds and the sophisticated tools available can used to decipher the mechanisms of synthesis of these molecules.

Structural and functional analyses of Barth syndrome-causing mutations and alternative splicing in the tafazzin acyltransferase domain

Tafazzin is a mitochondrial phospholipid transacylase, and its mutations cause Barth syndrome (BTHS). Human tafazzin gene produces four distinct alternatively spliced transcripts. To understand the molecular mechanisms of tafazzin deficiency, we performed an atomic resolution analysis of the influence of the BTHS mutations and of alternative splicing on the structure and function of tafazzin. From the three-dimensional (3D) homology modeling of tafazzin, we identified candidate amino acid residues that contribute to cardiolipin binding and to mitochondrial membrane associations that facilitate acyl-transfer reactions. Primate specific exon 5, which is alternatively spliced, is predicted to correspond to an intrinsically unstructured region in the protein. We proposed that this region should change the substrate-binding affinity and/or contribute to primate-specific molecular interactions. Exon 7, another alternatively spliced exon, encodes a region forming a part of the putative substrate-binding cleft, suggesting that the gene products lacking exon 7 will lose their substrate-binding ability. We demonstrate a clear localization of the BTHS mutations at residues responsible for membrane association, substrate binding, and the conformational stability of tafazzin. These findings provide new insights into the function of defective tafazzin and the pathogenesis of BTHS at the level of protein 3D structure and the evolution of alternatively spliced exons in primates.

Using modern tools to probe the structure-function relationship of fatty acid synthases

Fatty acid biosynthesis is essential to life and represents one of the most conserved pathways in nature, preserving the same handful of chemical reactions across all species. Recent interest in the molecular details of the de novo fatty acid synthase (FAS) has been heightened by demand for renewable fuels and the emergence of multidrug-resistant bacterial strains. Central to FAS is the acyl carrier protein (ACP), a protein chaperone that shuttles the growing acyl chain between catalytic enzymes within the FAS. Human efforts to alter fatty acid biosynthesis for oil production, chemical feedstock, or antimicrobial purposes has been met with limited success, due in part to a lack of detailed molecular information behind the ACP-partner protein interactions inherent to the pathway. This review will focus on recently developed tools for the modification of ACP and analysis of protein-protein interactions, such as mechanism-based crosslinking, and the studies exploiting them. Discussion specific to each enzymatic domain will focus first on mechanism and known inhibitors, followed by available structures and known interactions with ACP. Although significant unknowns remain, new understandings of the intricacies of FAS point to future advances in manipulating this complex molecular factory.

Glycerophosphate/Acylglycerophosphate acyltransferases

Acyl-CoA:glycerol-3-phosphate acyltransferase (GPAT) and acyl-CoA: 1-acyl-glycerol-3-phosphate acyltransferase (AGPAT) are involved in the de novo synthesis of triacylglycerol (TAG) and glycerophospholipids. Many enzymes belonging to the GPAT/AGPAT family have recently been identified and their physiological or pathophysiological roles have been proposed. The roles of GPAT/AGPAT in the synthesis of TAG and obesity-related diseases were revealed through the identification of causative genes of these diseases or analyses of genetically manipulated animals. Recent studies have suggested that some isoforms of GPAT/AGPAT family enzymes are involved in the fatty acid remodeling of phospholipids. The enzymology of GPAT/AGPAT and their physiological/pathological roles in the metabolism of glycerolipids have been described and discussed in this review.

Genome-wide identification and evolutionary analysis of algal LPAT genes involved in TAG biosynthesis using bioinformatic approaches

Lysophosphatidyl acyltransferase (LPAT) is one of the major triacylglycerol synthesis enzymes, controlling the metabolic flow of lysophosphatidic acid to phosphatidic acid. Experimental studies in Arabidopsis have shown that LPAT activity is exhibited primarily by three distinct isoforms, namely the plastid-located LPAT1, the endoplasmic reticulum-located LPAT2, and the soluble isoform of LPAT (solLPAT). In this study, 24 putative genes representing all LPAT isoforms were identified from the analysis of 11 complete genomes including green algae, red algae, diatoms and higher plants. We observed LPAT1 and solLPAT genes to be ubiquitously present in nearly all genomes examined, whereas LPAT2 genes to have evolved more recently in the plant lineage. Phylogenetic analysis indicated that LPAT1, LPAT2 and solLPAT have convergently evolved through separate evolutionary paths and belong to three different gene families, which was further evidenced by their wide divergence at gene structure and sequence level. The genome distribution supports the hypothesis that each gene encoding a LPAT is not duplicated. Mapping of exon-intron structure of LPAT genes to the domain structure of proteins across different algal and plant species indicates that exon shuffling plays no role in the evolution of LPAT genes. Besides the previously defined motifs, several conserved consensus sequences were discovered which could be useful to distinguish different LPAT isoforms. Taken together, this study will enable the generation of experimental approximations to better understand the functional role of algal LPAT in lipid accumulation.

Discovery of a bifunctional acyltransferase responsible for ornithine lipid synthesis in Serratia proteamaculans

Ornithine lipids (OLs) are phosphorus-free membrane lipids that can be formed by many bacteria but that are absent from archaea and eukaryotes. A function for OLs in stress conditions and in host-bacteria interactions has been shown in some bacteria. Some bacterial species have been described that can form OLs, but lack the known genes (olsBA) involved in its biosynthesis, which implied the existence of a second pathway. Here we describe the bifunctional protein OlsF from Serratia proteamaculans involved in OL formation. Expression of OlsF and its homologue from Flavobacterium johnsoniae in Escherichia coli causes OL formation. Deletion of OlsF in S. proteamaculans caused the absence of OL formation. Homologues of OlsF are widely distributed among γ-, δ- and ε-Proteobacteria and in the Cytophaga-Flavobacterium-Bacteroidetes group of bacteria, including several well-studied pathogens for which the presence of OLs has not been suspected, such as for example Vibrio cholerae and Klebsiella pneumonia. Using genomic data, we predict that about 50% of bacterial species can form OLs.

Divergent metabolic phenotype between two sisters with congenital generalized lipodystrophy due to double AGPAT2 homozygous mutations. a clinical, genetic and in silico study

Congenital generalized lipodystrophy (CGL) is a rare autosomal recessive disorder characterized by extreme reduction of white adipose tissue (WAT) mass. CGL type 1 is the most frequent form and is caused by mutations in AGPAT2. Genetic and clinical studies were performed in two affected sisters of a Chilean family. These patients have notoriously dissimilar metabolic abnormalities that correlate with differential levels of circulating leptin and soluble leptin receptor fraction. Sequencing of AGPAT2 exons and exon-intron boundaries revealed two homozygous mutations in both sisters. Missense mutation c.299G>A changes a conserved serine in the acyltransferase NHX4D motif of AGPAT2 (p.Ser100Asn). Intronic c.493-1G>C mutation destroy a conserved splicing site that likely leads to exon 4 skipping and deletion of whole AGPAT2 substrate binding domain. In silico protein modeling provided insights of the mechanisms of lack of catalytic activity owing to both mutations.

Identification of a broad family of lipid A late acyltransferases with non-canonical substrate specificity

Most Gram-negative organisms produce lipopolysaccharide (LPS), a complex macromolecule anchored to the bacterial membrane by the lipid A moiety. Lipid A is synthesized via the Raetz pathway, a conserved nine-step enzymatic process first characterized in Escherichia coli. The Epsilonproteobacterium Helicobacter pylori uses the Raetz pathway to synthesize lipid A; however, only eight of nine enzymes in the pathway have been identified in this organism. Here, we identify the missing acyltransferase, Jhp0255, which transfers a secondary acyl chain to the 3'-linked primary acyl chain of lipid A, an activity similar to that of E. coli LpxM. This enzyme, reannotated as LpxJ due to limited sequence similarity with LpxM, catalyses addition of a C12:0 or C14:0 acyl chain to the 3'-linked primary acyl chain of lipid A, complementing an E. coli LpxM mutant. Enzymatic assays demonstrate that LpxJ and homologues in Campylobacter jejuni and Wolinella succinogenes can act before the 2' secondary acyltransferase, LpxL, as well as the 3-deoxy-d-manno-octulosonic acid (Kdo) transferase, KdtA. Ultimately, LpxJ is one member of a large class of acyltransferases found in a diverse range of organisms that lack an E. coli LpxM homologue, suggesting that LpxJ participates in lipid A biosynthesis in place of an LpxM homologue.

Tafazzin splice variants and mutations in Barth syndrome

Barth syndrome is caused by mutations in the TAZ (tafazzin) gene on human chromosome Xq28. The human tafazzin gene produces four major mRNA splice variants; two of which have been shown to be functional (TAZ lacking exon 5 and full-length) in complementation studies with yeast and Drosophila. This study characterizes the multiple alternative splice variants of TAZ mRNA and their proportions in blood samples from a cohort of individuals with Barth syndrome (BTHS). Because it has been reported that collection and processing methods can affect the expression of various genes, we tested and chose a stabilizing medium for collecting, shipping and processing of the blood samples of these individuals. In both healthy controls and in BTHS individuals, we found a greater variety of alternatively spliced forms than previously described, with a sizeable proportion of minor splice variants besides the four dominant isoforms. Individuals with certain exonic and intronic splice mutations produce additional mutant mRNAs that could be translated into two or more proteins with different amino acid substitutions in a single individual. A fraction of the minor splice variants is predicted to be non-productive.

Metabolic engineering of lipid catabolism increases microalgal lipid accumulation without compromising growth

Biologically derived fuels are viable alternatives to traditional fossil fuels, and microalgae are a particularly promising source, but improvements are required throughout the production process to increase productivity and reduce cost. Metabolic engineering to increase yields of biofuel-relevant lipids in these organisms without compromising growth is an important aspect of advancing economic feasibility. We report that the targeted knockdown of a multifunctional lipase/phospholipase/acyltransferase increased lipid yields without affecting growth in the diatom Thalassiosira pseudonana. Antisense-expressing knockdown strains 1A6 and 1B1 exhibited wild-type-like growth and increased lipid content under both continuous light and alternating light/dark conditions. Strains 1A6 and 1B1, respectively, contained 2.4- and 3.3-fold higher lipid content than wild-type during exponential growth, and 4.1- and 3.2-fold higher lipid content than wild-type after 40 h of silicon starvation. Analyses of fatty acids, lipid classes, and membrane stability in the transgenic strains suggest a role for this enzyme in membrane lipid turnover and lipid homeostasis. These results demonstrate that targeted metabolic manipulations can be used to increase lipid accumulation in eukaryotic microalgae without compromising growth.

Minotaur is critical for primary piRNA biogenesis

Piwi proteins and their associated small RNAs are essential for fertility in animals. In part, this is due to their roles in guarding germ cell genomes against the activity of mobile genetic elements. piRNA populations direct Piwi proteins to silence transposon targets and, as such, form a molecular code that discriminates transposons from endogenous genes. Information ultimately carried by piRNAs is encoded within genomic loci, termed piRNA clusters. These give rise to long, single-stranded, primary transcripts that are processed into piRNAs. Despite the biological importance of this pathway, neither the characteristics that define a locus as a source of piRNAs nor the mechanisms that catalyze primary piRNA biogenesis are well understood. We searched an EMS-mutant collection annotated for fertility phenotypes for genes involved in the piRNA pathway. Twenty-seven homozygous sterile strains showed transposon-silencing defects. One of these, which strongly impacted primary piRNA biogenesis, harbored a causal mutation in CG5508, a member of the Drosophila glycerol-3-phosphate O-acetyltransferase (GPAT) family. These enzymes catalyze the first acylation step on the path to the production of phosphatidic acid (PA). Though this pointed strongly to a function for phospholipid signaling in the piRNA pathway, a mutant form of CG5508, which lacks the GPAT active site, still functions in piRNA biogenesis. We have named this new biogenesis factor Minotaur.

Acyltransferases in bacteria

Long-chain-length hydrophobic acyl residues play a vital role in a multitude of essential biological structures and processes. They build the inner hydrophobic layers of biological membranes, are converted to intracellular storage compounds, and are used to modify protein properties or function as membrane anchors, to name only a few functions. Acyl thioesters are transferred by acyltransferases or transacylases to a variety of different substrates or are polymerized to lipophilic storage compounds. Lipases represent another important enzyme class dealing with fatty acyl chains; however, they cannot be regarded as acyltransferases in the strict sense. This review provides a detailed survey of the wide spectrum of bacterial acyltransferases and compares different enzyme families in regard to their catalytic mechanisms. On the basis of their studied or assumed mechanisms, most of the acyl-transferring enzymes can be divided into two groups. The majority of enzymes discussed in this review employ a conserved acyltransferase motif with an invariant histidine residue, followed by an acidic amino acid residue, and their catalytic mechanism is characterized by a noncovalent transition state. In contrast to that, lipases rely on completely different mechanism which employs a catalytic triad and functions via the formation of covalent intermediates. This is, for example, similar to the mechanism which has been suggested for polyester synthases. Consequently, although the presented enzyme types neither share homology nor have a common three-dimensional structure, and although they deal with greatly varying molecule structures, this variety is not reflected in their mechanisms, all of which rely on a catalytically active histidine residue.

Cloning and characterization of GPAT gene from Lepidium latifolium L.: a step towards translational research in agri-genomics for food and fuel

Glycerol-3-phosphate acyltransferase (GPAT) catalyzes first and the rate limiting step in glycerolipid synthesis pathway, which in turn contribute to stabilization of plasma membrane structure and oil lipid synthesis in plant cells. Here, we report cloning and characterization of GPAT gene from Lepidium latifolium (LlaGPAT). The cDNA sequence (1,615 bp) of LlaGPAT gene consisted of 1,113 bp ORF encoding a protein of 370 aa residues, with deduced mass of 41.2 kDa and four acyltransferase (AT) motifs having role in catalysis and in glycerol-3-phosphate binding. Southern blot analysis suggested presence of a single copy of the gene in the genome. Tissue specific expression of the gene was seen more abundantly in aerial parts, compared to the roots. Quantitative real-time PCR indicated down-regulation of the gene by cold (4 °C), drought (PEG6000), salt (300 mM NaCl) and ABA (100 μM) treatments. Considering the vitality of the function of encoded enzyme, LlaGPAT can be considered a potential candidate gene for genetic engineering of oil yields and abiotic stress management in food as well as fuel crops.

Identification and characterization of a gene encoding a putative lysophosphatidyl acyltransferase from Arachis hypogaea

Lysophosphatidyl acyltransferase (LPAT) is the important enzyme responsible for the acylation of lysophosphatidic acid (LPA), leading to the generation of phosphatidic acid (PA) in plant. Its encoding gene is an essential candidate for oil crops to improve oil composition and increase seed oil content through genetic engineering. In this study, a full length AhLPAT4 gene was isolated via cDNA library screening and rapid amplification of cDNA ends (RACE); our data demonstrated that AhLPAT4 had 1631 nucleotides, encoding a putative 43.8 kDa protein with 383 amino acid residues. The deduced protein included a conserved acyltransferase domain and four motifs (I–IV) with putative LPA and acyl-CoA catalytic and binding sites. Bioinformatic analysis indicated that AhLPAT4 contained four transmembrane domains (TMDs), localized to the endoplasmic reticulum (ER) membrane; detailed analysis indicated that motif I and motifs II–III in AhLPAT4 were separated by the third TMD, which located on cytosolic and ER luminal side respectively, and hydrophobic residues on the surface of AhLPAT4 protein fold to form a hydrophobic tunnel to accommodate the acyl chain. Subcellular localization analysis confirmed that AhLPAT4 was a cytoplasm protein.Phylogenetic analysis revealed that AhLPAT4 had a high homology (63.7–78.3%) with putative LPAT4 proteins from Glycine max, Arabidopsis thaliana and Ricinus communis. AhLPAT4 was ubiquitously expressed in diverse tissues except in flower, which is almost undetectable. The expression analysis in different developmental stages in peanut seeds indicated that AhLPAT4 did not coincide with oil accumulation.

The multigene family of lysophosphatidate acyltransferase (LPAT)-related enzymes in Ricinus communis: cloning and molecular characterization of two LPAT genes that are expressed in castor seeds

The multigene family encoding proteins related to lysophosphatidyl-acyltransferases (LPATs) has been analyzed in the castor plant Ricinus communis. Among them, two genes designated RcLPAT2 and RcLPATB, encoding proteins with LPAT activity and expressed in the developing seed, have been cloned and characterized in some detail. RcLPAT2 groups with well characterized members of the so-called A-class LPATs and it shows a generalized expression pattern in the plant and along seed development. Enzymatic assays of RcLPAT2 indicate a preference for ricinoleoyl-CoA over other fatty acid thioesters when ricinoleoyl-LPA is used as the acyl acceptor, while oleoyl-CoA is the preferred substrate when oleoyl-LPA is employed. RcLPATB groups with B-class LPAT enzymes described as seed specific and selective for unusual fatty acids. However, RcLPATB exhibit a broad specificity on the acyl-CoAs, with saturated fatty acids (12:0-16:0) being the preferred substrates. RcLPATB is upregulated coinciding with seed triacylglycerol accumulation, but its expression is not restricted to the seed. These results are discussed in the light of a possible role for LPAT isoenzymes in the channelling of ricinoleic acid into castor bean triacylglycerol.