Lysophosphatidylcholine metabolism in Saccharomyces cerevisiae: the role of P-type ATPases in transport and a broad specificity acyltransferase in acylation

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.

Membranes that make fat: roles of membrane lipids as acyl donors for triglyceride synthesis and organelle function

Triglycerides constitute an inert storage form for fatty acids deposited in lipid droplets and are mobilized to provide metabolic energy or membrane building blocks. The biosynthesis of triglycerides is highly conserved within eukaryotes and normally involves the sequential esterification of activated fatty acids with a glycerol backbone. Some eukaryotes, however, can also use cellular membrane lipids as direct fatty acid donors for triglyceride synthesis. The biological significance of a pathway that generates triglycerides at the expense of organelle membranes has remained elusive. Here we review current knowledge on how cells use membrane lipids as fatty acid donors for triglyceride synthesis and discuss the hypothesis that a primary function of this pathway is to regulate membrane lipid remodeling and organelle function.

Flipping the script: Advances in understanding how and why P4-ATPases flip lipid across membranes

Type IV P-type ATPases (P4-ATPases) are a family of transmembrane enzymes that translocate lipid substrates from the outer to the inner leaflet of biological membranes and thus create an asymmetrical distribution of lipids within membranes. On the cellular level, this asymmetry is essential for maintaining the integrity and functionality of biological membranes, creating platforms for signaling events and facilitating vesicular trafficking. On the organismal level, this asymmetry has been shown to be important in maintaining blood homeostasis, liver metabolism, neural development, and the immune response. Indeed, dysregulation of P4-ATPases has been linked to several diseases; including anemia, cholestasis, neurological disease, and several cancers. This review will discuss the evolutionary transition of P4-ATPases from cation pumps to lipid flippases, the new lipid substrates that have been discovered, the significant advances that have been achieved in recent years regarding the structural mechanisms underlying the recognition and flipping of specific lipids across biological membranes, and the consequences of P4-ATPase dysfunction on cellular and physiological functions. Additionally, we emphasize the requirement for additional research to comprehensively understand the involvement of flippases in cellular physiology and disease and to explore their potential as targets for therapeutics in treating a variety of illnesses. The discussion in this review will primarily focus on the budding yeast, C. elegans, and mammalian P4-ATPases.

MBOAT7 in liver and extrahepatic diseases

MBOAT7 is a protein anchored to endomembranes by several transmembrane domains. It has a catalytic dyad involved in remodelling of phosphatidylinositol with polyunsaturated fatty acids. Genetic variants in the MBOAT7 gene have been associated with the entire spectrum of non-alcoholic fatty liver (NAFLD), recently redefined as metabolic dysfunction-associated fatty liver disease (MAFLD) and, lately, steatotic liver disease (SLD), and to an increasing number of extrahepatic conditions. In this review, we will (a) elucidate the molecular mechanisms by which MBOAT7 loss-of-function predisposes to MAFLD and neurodevelopmental disorders and (b) discuss the growing number of genetic studies linking MBOAT7 to hepatic and extrahepatic diseases. MBOAT7 complete loss of function causes severe changes in brain development resulting in several neurological manifestations. Lower MBOAT7 hepatic expression at both the mRNA and protein levels, due to missense nucleotide polymorphisms (SNPs) in the locus containing the MBOAT7 gene, affects specifically metabolic and viral diseases in the liver from simple steatosis to hepatocellular carcinoma, and potentially COVID-19 disease. This body of evidence shows that phosphatidylinositol remodelling is a key factor for human health.

The acyltransferase Gpc1 is both a target and an effector of the unfolded protein response in Saccharomyces cerevisiae

The unfolded protein response (UPR) is sensitive to proteotoxic and membrane bilayer stress, both of which are sensed by the ER protein Ire1. When activated, Ire1 splices HAC1 mRNA, producing a transcription factor that targets genes involved in proteostasis and lipid metabolism, among others. The major membrane lipid phosphatidylcholine (PC) is subject to phospholipase-mediated deacylation, producing glycerophosphocholine (GPC), followed by reacylation of GPC through the PC deacylation/reacylation pathway (PC-DRP). The reacylation events occur via a two-step process catalyzed first by the GPC acyltransferase Gpc1, followed by acylation of the lyso-PC molecule by Ale1. However, whether Gpc1 is critical for ER bilayer homeostasis is unclear. Using an improved method for C14-choline-GPC radiolabeling, we first show that loss of Gpc1 results in abrogation of PC synthesis through PC-DRP and that Gpc1 colocalizes with the ER. We then probe the role of Gpc1 as both a target and an effector of the UPR. Exposure to the UPR-inducing compounds tunicamycin, DTT, and canavanine results in a Hac1-dependent increase in GPC1 message. Further, cells lacking Gpc1 exhibit increased sensitivity to those proteotoxic stressors. Inositol limitation, known to induce the UPR via bilayer stress, also induces GPC1 expression. Finally, we show that loss of GPC1 induces the UPR. A gpc1Δ mutant displays upregulation of the UPR in strains expressing a mutant form of Ire1 that is unresponsive to unfolded proteins, indicating that bilayer stress is responsible for the observed upregulation. Collectively, our data indicate an important role for Gpc1 in yeast ER bilayer homeostasis.

Functional Analysis of the P-Type ATPases Apt2-4 from Cryptococcus neoformans by Heterologous Expression in Saccharomyces cerevisiae

Lipid flippases of the P4-ATPase family actively transport phospholipids across cell membranes, an activity essential for key cellular processes such as vesicle budding and membrane trafficking. Members of this transporter family have also been implicated in the development of drug resistance in fungi. The encapsulated fungal pathogen Cryptococcus neoformans contains four P4-ATPases, among which Apt2-4p are poorly characterized. Using heterologous expression in the flippase-deficient S. cerevisiae strain dnf1Δdnf2Δdrs2Δ, we tested their lipid flippase activity in comparison to Apt1p using complementation tests and fluorescent lipid uptake assays. Apt2p and Apt3p required the co-expression of the C. neoformans Cdc50 protein for activity. Apt2p/Cdc50p displayed a narrow substrate specificity, limited to phosphatidylethanolamine and -choline. Despite its inability to transport fluorescent lipids, the Apt3p/Cdc50p complex still rescued the cold-sensitive phenotype of dnf1Δdnf2Δdrs2Δ, suggesting a functional role for the flippase in the secretory pathway. Apt4p, the closest homolog to Saccharomyces Neo1p, which does not require a Cdc50 protein, was unable to complement several flippase-deficient mutant phenotypes, neither in the presence nor absence of a β-subunit. These results identify C. neoformans Cdc50 as an essential subunit for Apt1-3p and provide a first insight into the molecular mechanisms underlying their physiological functions.

Capture Hi-C reveals the influence on dynamic three-dimensional chromosome organization perturbed by genetic variation or vanillin stress in Saccharomyces cerevisiae

Studying the mechanisms of resistance to vanillin in microorganisms, which is derived from lignin and blocks a major pathway of DNA double-strand break repair in yeast, will benefit the design of robust cell factories that produce biofuels and chemicals using lignocellulosic materials. A high vanillin-tolerant Saccharomyces cerevisiae strain EMV-8 carrying site mutations compared to its parent strain NAN-27 was selected for the analyses. The dynamics of the chromatin structure of eukaryotic cells play a critical role in transcription and the regulation of gene expression and thus the phenotype. Consequently, Hi-C and transcriptome analyses were conducted in EMV-8 and NAN-27 in the log phase with or without vanillin stress to determine the effects of mutations and vanillin disturbance on the dynamics of three-dimensional chromosome organization and the influence of the organization on the transcriptome. The outcomes indicated that the chromosome interaction pattern disturbed by vanillin stress or genetic mutations in the log phase was similar to that in mouse cells. The short chromosomes contact the short chromosomes, and the long chromosomes contact the long chromosomes. In response to vanillin stress, the boundaries of the topologically associating domain (TAD) in the vanillin-tolerant strain EMV-8 were more stable than those in its parent strain NAN-27. The motifs of SFL1, STB3, and NHP6A/B were enriched at TAD boundaries in both EMV-8 and NAN-27 with or without vanillin, indicating that these four genes were probably related to TAD formation. The Indel mutation of YRR1, whose absence was confirmed to benefit vanillin tolerance in EMV-8, caused two new interaction sites that contained three genes, WTM2, PUP1, and ALE1, whose overexpression did not affect vanillin resistance in yeast. Overall, our results revealed that in the log phase, genetic mutations and vanillin disturbance have a negligible effect on three-dimensional chromosome organization, and the reformation or disappearance of TAD boundaries did not show an association with gene expression, which provides an example for studying yeast chromatin structure during stress tolerance using Hi-C technology.

Autoinhibition and regulation by phosphoinositides of ATP8B1, a human lipid flippase associated with intrahepatic cholestatic disorders

P4-ATPases flip lipids from the exoplasmic to the cytosolic leaflet, thus maintaining lipid asymmetry in eukaryotic cell membranes. Mutations in several human P4-ATPase genes are associated with severe diseases, for example in ATP8B1 causing progressive familial intrahepatic cholestasis, a rare inherited disorder progressing toward liver failure. ATP8B1 forms a binary complex with CDC50A and displays a broad specificity to glycerophospholipids, but regulatory mechanisms are unknown. Here, we report functional studies and the cryo-EM structure of the human lipid flippase ATP8B1-CDC50A at 3.1 Å resolution. We find that ATP8B1 is autoinhibited by its N- and C-terminal tails, which form extensive interactions with the catalytic sites and flexible domain interfaces. Consistently, ATP hydrolysis is unleashed by truncation of the C-terminus, but also requires phosphoinositides, most markedly phosphatidylinositol-3,4,5-phosphate (PI(3,4,5)P₃), and removal of both N- and C-termini results in full activation. Restored inhibition of ATP8B1 truncation constructs with a synthetic peptide mimicking the C-terminal segment further suggests molecular communication between N- and C-termini in the autoinhibition and demonstrates that the regulatory mechanism can be interfered with by exogenous compounds. A recurring (G/A)(Y/F)AFS motif of the C-terminal segment suggests that this mechanism is employed widely across P4-ATPase lipid flippases in plasma membrane and endomembranes.

P-Type ATPase Apt1 of the Fungal Pathogen Cryptococcus neoformans Is a Lipid Flippase of Broad Substrate Specificity

Lipid flippases of the P4-ATPase family are ATP-driven transporters that translocate lipids from the exoplasmic to the cytosolic leaflet of biological membranes. In the encapsulated fungal pathogen Cryptococcus neoformans, the P4-ATPase Apt1p is an important regulator of polysaccharide secretion and pathogenesis, but its biochemical characterization is lacking. Phylogenetic analysis revealed that Apt1p belongs to the subclade of P4A-ATPases characterized by the common requirement for a β-subunit. Using heterologous expression in S. cerevisiae, we demonstrate that Apt1p forms a heterodimeric complex with the C. neoformans Cdc50 protein. This association is required for both localization and activity of the transporter complex. Lipid flippase activity of the heterodimeric complex was assessed by complementation tests and uptake assays employing fluorescent lipids and revealed a broad substrate specificity, including several phospholipids, the alkylphospholipid miltefosine, and the glycolipids glucosyl- and galactosylceramide. Our results suggest that transbilayer lipid transport in C. neoformans is finely regulated to promote fungal virulence, which reinforces the potential of Apt1p as a target for antifungal drug development.

Lipid Transporters Beam Signals from Cell Membranes

Lipid composition in cellular membranes plays an important role in maintaining the structural integrity of cells and in regulating cellular signaling that controls functions of both membrane-anchored and cytoplasmic proteins. ATP-dependent ABC and P4-ATPase lipid transporters, two integral membrane proteins, are known to contribute to lipid translocation across the lipid bilayers on the cellular membranes. In this review, we will highlight current knowledge about the role of cholesterol and phospholipids of cellular membranes in regulating cell signaling and how lipid transporters participate this process.

LPIAT1/MBOAT7 contains a catalytic dyad transferring polyunsaturated fatty acids to lysophosphatidylinositol

Human membrane bound O-acyltransferase domain-containing 7 (MBOAT7), also known as lysophosphatidylinositol acyltransferase 1 (LPIAT1), is an enzyme involved in the acyl-chain remodeling of phospholipids via the Lands' cycle. The MBOAT7 rs641738 variant has been associated with the entire spectrum of fatty liver disease (FLD) and neurodevelopmental disorders, but the exact enzymatic activity and the catalytic site of the protein are still unestablished. Human wild type MBOAT7 and three MBOAT7 mutants missing in the putative catalytic residues (N321A, H356A, N321A + H356A) were produced into Pichia pastoris, and purified using Ni-affinity chromatography. The enzymatic activity of MBOAT7 wild type and mutants was assessed measuring the incorporation of radiolabeled fatty acids into lipid acceptors. MBOAT7 preferentially transferred 20:4 and 20:5 polyunsaturated fatty acids (PUFAs) to lysophosphatidylinositol (LPI). On the contrary, MBOAT7 showed weak enzymatic activity for transferring saturated and unsaturated fatty acids, regardless the lipid substrate. Missense mutations in the putative catalytic residues (N321A, H356A, N321A + H356A) result in a loss of O-acyltransferase activity. Thus, MBOAT7 catalyzes the transfer of PUFAs to lipid acceptors. MBOAT7 shows the highest affinity for LPI, and missense mutations at the MBOAT7 putative catalytic dyad inhibit the O-acyltransferase activity of the protein. Our findings support the hypothesis that the association between the MBOAT7 rs641738 variant and the increased risk of NAFLD is mediated by changes in the hepatic phosphatidylinositol acyl-chain remodeling. Taken together, the increased knowledge of the enzymatic activity of MBOAT7 gives insights into the understanding on the basis of FLD.

Lipid flippases in polarized growth

Polarized growth is required in eukaryotic cells for processes such as cell division, morphogenesis and motility, which involve conserved and interconnected signalling pathways controlling cell cycle progression, cytoskeleton reorganization and secretory pathway functioning. While many of the factors involved in polarized growth are known, it is not yet clear how they are coordinated both spatially and temporally. Several lines of evidence point to the important role of lipid flippases in polarized growth events. Lipid flippases, which mainly belong to the P4 subfamily of P-type ATPases, are active transporters that move different lipids to the cytosolic side of biological membranes at the expense of ATP. The involvement of the Saccharomyces cerevisiae plasma membrane P4 ATPases Dnf1p and Dnf2p in polarized growth and their activation by kinase phosphorylation were established some years ago. However, these two proteins do not seem to be responsible for the phosphatidylserine internalization required for early recruitment of proteins to the plasma membrane during yeast mating and budding. In a recent publication, we demonstrated that the Golgi-localized P4 ATPase Dnf3p has a preference for PS as a substrate, can reach the plasma membrane in a cell cycle-dependent manner, and is regulated by the same kinases that activate Dnf1p and Dnf2p. This finding solves a long-lasting enigma in the field of lipid flippases and suggests that tight and heavily coordinated spatiotemporal control of lipid translocation at the plasma membrane is important for proper polarized growth.

Aminoglycerophospholipid flipping and P4-ATPases in Toxoplasma gondii

Lipid flipping in the membrane bilayers is a widespread eukaryotic phenomenon that is catalyzed by assorted P4-ATPases. Its occurrence, mechanism, and importance in apicomplexan parasites have remained elusive, however. Here we show that Toxoplasma gondii, an obligate intracellular parasite with high clinical relevance, can salvage phosphatidylserine (PtdSer) and phosphatidylethanolamine (PtdEtn) but not phosphatidylcholine (PtdCho) probes from its milieu. Consistently, the drug analogs of PtdCho are broadly ineffective in the parasite culture. NBD-PtdSer imported to the parasite interior is decarboxylated to NBD-PtdEtn, while the latter is not methylated to yield PtdCho, which confirms the expression of PtdSer decarboxylase but a lack of PtdEtn methyltransferase activity and suggests a role of exogenous lipids in membrane biogenesis of T. gondii. Flow cytometric quantitation of NBD-probes endorsed the selectivity of phospholipid transport and revealed a dependence of the process on energy and protein. Accordingly, our further work identified five P4-ATPases (TgP4-ATPase1-5), all of which harbor the signature residues and motifs required for phospholipid flipping. Of the four proteins expressed during the lytic cycle, TgP4-ATPase1 is present in the apical plasmalemma; TgP4-ATPase3 resides in the Golgi network along with its noncatalytic partner Ligand Effector Module 3 (TgLem3), whereas TgP4-ATPase2 and TgP4-ATPase5 localize in the plasmalemma as well as endo/cytomembranes. Last but not least, auxin-induced degradation of TgP4-ATPase1-3 impaired the parasite growth in human host cells, disclosing their crucial roles during acute infection. In conclusion, we show selective translocation of PtdEtn and PtdSer at the parasite surface and provide the underlying mechanistic and physiological insights in a model eukaryotic pathogen.

Exofacial membrane composition and lipid metabolism regulates plasma membrane P4-ATPase substrate specificity

The plasma membrane of a cell is characterized by an asymmetric distribution of lipid species across the exofacial and cytofacial aspects of the bilayer. Regulation of membrane asymmetry is a fundamental characteristic of membrane biology and is crucial for signal transduction, vesicle transport, and cell division. The type IV family of P-ATPases, or P4-ATPases, establishes membrane asymmetry by selection and transfer of a subset of membrane lipids from the lumenal or exofacial leaflet to the cytofacial aspect of the bilayer. It is unclear how P4-ATPases sort through the spectrum of membrane lipids to identify their desired substrate(s) and how the membrane environment modulates this activity. Therefore, we tested how the yeast plasma membrane P4-ATPase, Dnf2, responds to changes in membrane composition induced by perturbation of endogenous lipid biosynthetic pathways or exogenous application of lipid. The primary substrates of Dnf2 are glucosylceramide (GlcCer) and phosphatidylcholine (PC, or their lyso-lipid derivatives), and we find that these substrates compete with each other for transport. Acutely inhibiting sphingolipid synthesis using myriocin attenuates transport of exogenously applied GlcCer without perturbing PC transport. Deletion of genes controlling later steps of glycosphingolipid production also perturb GlcCer transport to a greater extent than PC transport. In contrast, perturbation of ergosterol biosynthesis reduces PC and GlcCer transport equivalently. Surprisingly, application of lipids that are poor transport substrates differentially affects PC and GlcCer transport by Dnf2, thus altering substrate preference. Our data indicate that Dnf2 exhibits exquisite sensitivity to the membrane composition, thus providing feedback onto the function of the P4-ATPases.

An optogenetic system to control membrane phospholipid asymmetry through flippase activation in budding yeast

Lipid asymmetry in biological membranes is essential for various cell functions, such as cell polarity, cytokinesis, and apoptosis. P4-ATPases (flippases) are involved in the generation of such asymmetry. In Saccharomyces cerevisiae, the protein kinases Fpk1p/Fpk2p activate the P4-ATPases Dnf1p/Dnf2p by phosphorylation. Previously, we have shown that a blue-light-dependent protein kinase, phototropin from Chlamydomonas reinhardtii (CrPHOT), complements defects in an fpk1Δ fpk2Δ mutant. Herein, we investigated whether CrPHOT optically regulates P4-ATPase activity. First, we demonstrated that the translocation of NBD-labelled phospholipids to the cytoplasmic leaflet via P4-ATPases was promoted by blue-light irradiation in fpk1Δ fpk2Δ cells with CrPHOT. In addition, blue light completely suppressed the defects in membrane functions (such as endocytic recycling, actin depolarization, and apical-isotropic growth switching) caused by fpk1Δ fpk2Δ mutations. All responses required the kinase activity of CrPHOT. Hence, these results indicate the utility of CrPHOT as a powerful and first tool for optogenetic manipulation of P4-ATPase activity.

Osh6 requires Ist2 for localization to ER-PM contacts and efficient phosphatidylserine transport in budding yeast

Osh6 and Osh7 are lipid transfer proteins (LTPs) that move phosphatidylserine (PS) from the endoplasmic reticulum (ER) to the plasma membrane (PM). High PS levels at the PM are key for many cellular functions. Intriguingly, Osh6 and Osh7 localize to ER-PM contact sites, although they lack membrane-targeting motifs, in contrast to multidomain LTPs that both bridge membranes and convey lipids. We show that Osh6 localization to contact sites depends on its interaction with the cytosolic tail of the ER-PM tether Ist2, a homolog of TMEM16 proteins. We identify a motif in the Ist2 tail, conserved in yeasts, as the Osh6-binding region, and we map an Ist2-binding surface on Osh6. Mutations in the Ist2 tail phenocopy osh6Δ osh7Δ deletion: they decrease cellular PS levels and block PS transport to the PM. Our study unveils an unexpected partnership between a TMEM16-like protein and a soluble LTP, which together mediate lipid transport at contact sites.This article has an associated First Person interview with the first author of the paper.

Disruption in phosphate transport affects membrane lipid and lipid droplet homeostasis in Saccharomyces cerevisiae

Phosphate plays a crucial role in phospholipid metabolism and it is transported by the phosphate (Pi) transporters. Phospholipids are building blocks of the cell membrane, and essential for cell growth; however, the role of phosphate transporters in lipid metabolism remains elusive. The present study shows that the deletion of Pi transporters exhibited an increase in both phospholipid and neutral lipid levels when compared to wild type. The mRNA expressions of genes involved in phospholipid synthesis (CKI1, EKI1, CHO2, and OPI3) were increased due to de-repression of the transcription factors (INO2 and INO4). Neutral lipid levels (triacylglycerol and sterol ester) and their synthesizing genes (LRO1, ARE2, ACC1, and FAS1) were also increased, resulting in lipid droplet accumulation in Pi transporter mutants. Interestingly, phospholipase (PLC1) and histone acetyltransferase genes (ESA1, EAF1, YNG1, YNG2, and GCN5) were also found to be significantly increased, leading to dysregulation of lipid levels in Pi transporter mutants. In summary, our results suggest that the Pi transporters are involved in lipid droplet and membrane lipid homeostasis.

How is the acyl chain composition of phosphoinositides created and does it matter?

The phosphoinositide (PIPn) family of signalling phospholipids are central regulators in membrane cell biology. Their varied functions are based on the phosphorylation pattern of their inositol ring, which can be recognized by selective binding domains in their effector proteins and be modified by a series of specific PIPn kinases and phosphatases, which control their interconversion in a spatial and temporal manner. Yet, a unique feature of PIPns remains largely unexplored: their unusually uniform acyl chain composition. Indeed, while most phospholipids present a range of molecular species comprising acyl chains of diverse length and saturation, PIPns in several organisms and tissues show the predominance of a single hydrophobic backbone, which in mammals is composed of arachidonoyl and stearoyl chains. Despite evolution having favoured this specific PIPn configuration, little is known regarding the mechanisms and functions behind it. In this review, we explore the metabolic pathways that could control the acyl chain composition of PIPns as well as the potential roles of this selective enrichment. While our understanding of this phenomenon has been constrained largely by the technical limitations in the methods traditionally employed in the PIPn field, we believe that the latest developments in PIPn analysis should shed light onto this old question.

Phospholipid turnover and acyl chain remodeling in the yeast ER

The turnover of phospholipids plays an essential role in membrane lipid homeostasis by impacting both lipid head group and acyl chain composition. This review focusses on the degradation and acyl chain remodeling of the major phospholipid classes present in the ER membrane of the reference eukaryote Saccharomyces cerevisiae, i.e. phosphatidylcholine (PC), phosphatidylinositol (PI) and phosphatidylethanolamine (PE). Phospholipid turnover reactions are introduced, and the occurrence and important functions of phospholipid remodeling in higher eukaryotes are briefly summarized. After presenting an inventory of established mechanisms of phospholipid acyl chain exchange, current knowledge of phospholipid degradation and remodeling by phospholipases and acyltransferases localized to the yeast ER is summarized. PC is subject to the PC deacylation-reacylation remodeling pathway (PC-DRP) involving a phospholipase B, the recently identified glycerophosphocholine acyltransferase Gpc1p, and the broad specificity acyltransferase Ale1p. PI is post-synthetically enriched in C18:0 acyl chains by remodeling reactions involving Cst26p. PE may undergo turnover by the phospholipid: diacylglycerol acyltransferase Lro1p as first step in acyl chain remodeling. Clues as to the functions of phospholipid acyl chain remodeling are discussed.

Phosphatidate-mediated regulation of lipid synthesis at the nuclear/endoplasmic reticulum membrane

In yeast and higher eukaryotes, phospholipids and triacylglycerol are derived from phosphatidate at the nuclear/endoplasmic reticulum membrane. In de novo biosynthetic pathways, phosphatidate is channeled into membrane phospholipids via its conversion to CDP-diacylglycerol. Its dephosphorylation to diacylglycerol is required for the synthesis of triacylglycerol as well as for the synthesis of phosphatidylcholine and phosphatidylethanolamine via the Kennedy pathway. In addition to the role of phosphatidate as a precursor, it is a regulatory molecule in the transcriptional control of phospholipid synthesis genes via the Henry regulatory circuit. Pah1 phosphatidate phosphatase and Dgk1 diacylglycerol kinase are key players that function counteractively in the control of the phosphatidate level at the nuclear/endoplasmic reticulum membrane. Loss of Pah1 phosphatidate phosphatase activity not only affects triacylglycerol synthesis but also disturbs the balance of the phosphatidate level, resulting in the alteration of lipid synthesis and related cellular defects. The pah1Δ phenotypes requiring Dgk1 diacylglycerol kinase exemplify the importance of the phosphatidate level in the misregulation of cellular processes. The catalytic function of Pah1 requires its translocation from the cytoplasm to the nuclear/endoplasmic reticulum membrane, which is regulated through its phosphorylation in the cytoplasm by multiple protein kinases as well as through its dephosphorylation by the membrane-associated Nem1-Spo7 protein phosphatase complex. This article is part of a Special Issue entitled Endoplasmic reticulum platforms for lipid dynamics edited by Shamshad Cockcroft and Christopher Stefan.

Conserved mechanism of phospholipid substrate recognition by the P4-ATPase Neo1 from Saccharomyces cerevisiae

The type IV P-type ATPases (P4-ATPases) thus far characterized are lipid flippases that transport specific substrates, such as phosphatidylserine (PS) and phosphatidylethanolamine (PE), from the exofacial leaflet to the cytofacial leaflet of membranes. This transport activity generates compositional asymmetry between the two leaflets important for signal transduction, cytokinesis, vesicular transport, and host-pathogen interactions. Most P4-ATPases function as a heterodimer with a β-subunit from the Cdc50 protein family, but Neo1 from Saccharomyces cerevisiae and its metazoan orthologs lack a β-subunit requirement and it is unclear how these proteins transport substrate. Here we tested if residues linked to lipid substrate recognition in other P4-ATPases also contribute to Neo1 function in budding yeast. Point mutations altering entry gate residues in the first (Q209A) and fourth (S457Q) transmembrane segments of Neo1, where phospholipid substrate would initially be selected, disrupt PS and PE membrane asymmetry, but do not perturb growth of cells. Mutation of both entry gate residues inactivates Neo1, and cells expressing this variant are inviable. We also identified a gain-of-function mutation in the second transmembrane segment of Neo1 (Neo1[Y222S]), predicted to help form the entry gate, that substantially enhances Neo1's ability to replace the function of a well characterized phospholipid flippase, Drs2, in establishing PS and PE asymmetry. These results suggest a common mechanism for substrate recognition in widely divergent P4-ATPases.

Role of lipid transporters in fungal physiology and pathogenicity

The fungal cell wall and membrane are the most common targets of antifungal agents, but the potential of membrane lipid organization in regulating drug-target interactions has yet to be investigated. Energy-dependent lipid transporters have been recently associated with virulence and drug resistance in many pathogenic fungi. To illustrate this view, we discuss (i) the structural and biological aspects of ATP-driven lipid transporters, comprising P-type ATPases and ATP-binding cassette transporters, (ii) the role of these transporters in fungal physiology and virulence, and (iii) the potential of lipid transporters as targets for the development of novel antifungals. These recent observations indicate that the lipid-trafficking machinery in fungi is a promising target for studies on physiology, pathogenesis and drug development.

Following Anterograde Transport of Phosphatidylserine in Yeast in Real Time

In order to understand how lipids are sorted between cellular compartments, kinetic assays are required to selectively follow the transport of lipid species in cells. We present here a microfluidics-based protocol to follow the transport of phosphatidylserine (PS) in yeast cells from the site of its synthesis, the endoplasmic reticulum (ER), to downstream compartments, primarily the plasma membrane under our conditions. This assay takes advantage of yeast cells lacking Cho1, the enzyme responsible for PS synthesis. Lyso-PS can be added exogenously and is taken up by the cells and converted to PS. Because acylation of lyso-PS to PS appears to occur at the ER, anterograde transport of PS from the ER can then be followed by fluorescent microscopy using the specific PS reporter C2_Lact-GFP. We describe the construction of the required cho1Δ yeast strain and the preparation of lyso-PS. We present an example of the use of this assay to follow the activity of the yeast PS transport proteins Osh6 and Osh7.

MBOAT7 is anchored to endomembranes by six transmembrane domains

Membrane bound O-acyltransferase domain- containing 7 (MBOAT7, also known as LPIAT1) is a protein involved in the acyl chain remodeling of phospholipids via the Lands' cycle. The MBOAT7 is a susceptibility risk genetic locus for non-alcoholic fatty liver disease (NAFLD) and mental retardation. Although it has been shown that MBOAT7 is associated to membranes, the MBOAT7 topology remains unknown. To solve the topological organization of MBOAT7, we performed: A) solubilization of the total membrane fraction of cells overexpressing the recombinant MBOAT7-V5, which revealed MBOAT7 is an integral protein strongly attached to endomembranes; B) in silico analysis by using 22 computational methods, which predicted the number and localization of transmembrane domains of MBOAT7 with a range between 5 and 12; C) in vitro analysis of living cells transfected with GFP-tagged MBOAT7 full length and truncated forms, using a combination of Western Blotting, co-immunofluorescence and Fluorescence Protease Protection (FPP) assay; D) in vitro analysis of living cells transfected with FLAG-tagged MBOAT7 full length forms, using a combination of Western Blotting, selective membrane permeabilization followed by indirect immunofluorescence. All together, these data revealed that MBOAT7 is a multispanning transmembrane protein with six transmembrane domains. Based on our model, the predicted catalytic dyad of the protein, composed of the conserved asparagine in position 321 (Asn-321) and the preserved histidine in position 356 (His-356), has a lumenal localization. These data are compatible with the role of MBOAT7 in remodeling the acyl chain composition of endomembranes.

Genetically Compromising Phospholipid Metabolism Limits Candida albicans' Virulence

Perturbing ergosterol synthesis has been previously shown to reduce the virulence of Candida albicans. We tested the hypothesis that further altering cell membrane composition by limiting phospholipid synthesis or remodeling will have the same effect. To model partial inhibition, C. albicans strains independently harboring heterozygous deletion of four genes that encode for enzymes that mediate phospholipid synthesis or modification were generated. Quantitative PCR determined that heterozygous deletion routinely caused a nearly 50% reduction in the respective gene's transcript abundance. Compensatory increased transcript abundance was only found with the deletion of LRO1, a homolog of phospholipid diacylglycerol acyltransferases. Virulence of the mutants was assayed in a Caenorhabditis elegans host model. Even modestly reduced expression of LRO1, phosphatidylserine synthase (CHO1), and lysophospholipid acyltransferase (LPT1) significantly reduced virulence by 23-38%. Reintroducing a second functional allele, respectively, to all three mutants restored virulence. Heterozygous deletion of SLC1, a homolog of 1-acylglycerol-3-phosphate O-acyltransferases, did not significantly reduce virulence. Electrospray ionization tandem mass spectrometry analysis of phospholipid composition followed by principal component analysis identified comprehensive changes in the LRO1 and CHO1 deletion heterozygotes. Strikingly (p < 0.001), univariate comparisons found that both deletion heterozygotes had 20% more phosphatidylinositol, 75% less lysophosphatidylcholine, and 35% less lysophosphatidylethanolamine compared to wild type. Heterozygous deletion of LPT1 also significantly increased phosphatidylinositol abundance. No growth phenotype, including filamentation, was affected by any mutation. Together, these data predict that even partial pharmacological inhibition of Lro1p, Cho1p, and Lpt1p will limit C. albicans virulence through altering phospholipid composition.

The glycerophosphocholine acyltransferase Gpc1 is part of a phosphatidylcholine (PC)-remodeling pathway that alters PC species in yeast

Phospholipase B-mediated hydrolysis of phosphatidylcholine (PC) results in the formation of free fatty acids and glycerophosphocholine (GPC) in the yeast Saccharomyces cerevisiae GPC can be reacylated by the glycerophosphocholine acyltransferase Gpc1, which produces lysophosphatidylcholine (LPC), and LPC can be converted to PC by the lysophospholipid acyltransferase Ale1. Here, we further characterized the regulation and function of this distinct PC deacylation/reacylation pathway in yeast. Through in vitro and in vivo experiments, we show that Gpc1 and Ale1 are the major cellular GPC and LPC acyltransferases, respectively. Importantly, we report that Gpc1 activity affects the PC species profile. Loss of Gpc1 decreased the levels of monounsaturated PC species and increased those of diunsaturated PC species, whereas Gpc1 overexpression had the opposite effects. Of note, Gpc1 loss did not significantly affect phosphatidylethanolamine, phosphatidylinositol, and phosphatidylserine profiles. Our results indicate that Gpc1 is involved in postsynthetic PC remodeling that produces more saturated PC species. qRT-PCR analyses revealed that GPC1 mRNA abundance is regulated coordinately with PC biosynthetic pathways. Inositol availability, which regulates several phospholipid biosynthetic genes, down-regulated GPC1 expression at the mRNA and protein levels and, as expected, decreased levels of monounsaturated PC species. Finally, loss of GPC1 decreased stationary phase viability in inositol-free medium. These results indicate that Gpc1 is part of a postsynthetic PC deacylation/reacylation remodeling pathway (PC-DRP) that alters the PC species profile, is regulated in coordination with other major lipid biosynthetic pathways, and affects yeast growth.

Role of phospholipid synthesis in the development and differentiation of malaria parasites in the blood

The life cycle of malaria parasites in both their mammalian host and mosquito vector consists of multiple developmental stages that ensure proper replication and progeny survival. The transition between these stages is fueled by nutrients scavenged from the host and fed into specialized metabolic pathways of the parasite. One such pathway is used by Plasmodium falciparum, which causes the most severe form of human malaria, to synthesize its major phospholipids, phosphatidylcholine, phosphatidylethanolamine, and phosphatidylserine. Much is known about the enzymes involved in the synthesis of these phospholipids, and recent advances in genetic engineering, single-cell RNA-Seq analyses, and drug screening have provided new perspectives on the importance of some of these enzymes in parasite development and sexual differentiation and have identified targets for the development of new antimalarial drugs. This Minireview focuses on two phospholipid biosynthesis enzymes of P. falciparum that catalyze phosphoethanolamine transmethylation (PfPMT) and phosphatidylserine decarboxylation (PfPSD) during the blood stages of the parasite. We also discuss our current understanding of the biochemical, structural, and biological functions of these enzymes and highlight efforts to use them as antimalarial drug targets.

Phospholipid flippases and Sfk1p, a novel regulator of phospholipid asymmetry, contribute to low permeability of the plasma membrane

Phospholipid flippase (type 4 P-type ATPase) plays a major role in the generation of phospholipid asymmetry in eukaryotic cell membranes. Loss of Lem3p-Dnf1/2p flippases leads to the exposure of phosphatidylserine (PS) and phosphatidylethanolamine (PE) on the cell surface in yeast, resulting in sensitivity to PS- or PE-binding peptides. We isolated Sfk1p, a conserved membrane protein in the TMEM150/FRAG1/DRAM family, as a multicopy suppressor of this sensitivity. Overexpression of SFK1 decreased PS/PE exposure in lem3Δ mutant cells. Consistent with this, lem3Δ sfk1Δ double mutant cells exposed more PS/PE than the lem3Δ mutant. Sfk1p was previously implicated in the regulation of the phosphatidylinositol-4 kinase Stt4p, but the effect of Sfk1p on PS/PE exposure in lem3Δ was independent of Stt4p. Surprisingly, Sfk1p did not facilitate phospholipid flipping but instead repressed it, even under ATP-depleted conditions. We propose that Sfk1p negatively regulates transbilayer movement of phospholipids irrespective of directions. In addition, we showed that the permeability of the plasma membrane was dramatically elevated in the lem3Δ sfk1Δ double mutant in comparison with the corresponding single mutants. Interestingly, total ergosterol was decreased in the lem3Δ sfk1Δ mutant. Our results suggest that phospholipid asymmetry is required for the maintenance of low plasma membrane permeability.

PS, It's Complicated: The Roles of Phosphatidylserine and Phosphatidylethanolamine in the Pathogenesis of Candida albicans and Other Microbial Pathogens

The phospholipids phosphatidylserine (PS) and phosphatidylethanolamine (PE) play important roles in the virulence of Candida albicans and loss of PS synthesis or synthesis of PE from PS (PS decarboxylase) severely compromises virulence in C. albicans in a mouse model of systemic candidiasis. This review discusses synthesis of PE and PS in C. albicans and mechanisms by which these lipids impact virulence in this fungus. This is further compared to how PS and PE synthesis impact virulence in other fungi, parasites and bacteria. Furthermore, the impact of PS asymmetry on virulence and extracellular vesicle formation in several microbes is reviewed. Finally, the potential for PS and PE synthases as drug targets in these various kingdoms is also examined.

Evolution of the Phosphatidylcholine Biosynthesis Pathways in Green Algae: Combinatorial Diversity of Methyltransferases

Phosphatidylcholine (PC) is one of the most common phospholipids in eukaryotes, although some green algae such as Chlamydomonas reinhardtii are known to lack PC. Recently, we detected PC in four species in the genus Chlamydomonas: C. applanata NIES-2202, C. asymmetrica NIES-2207, C. debaryana NIES-2212, and C. sphaeroides NIES-2242. To reveal the PC biosynthesis pathways in green algae and the evolutionary scenario involved in their diversity, we analyzed the PC biosynthesis genes in these four algae using draft genome sequences. Homology searches suggested that PC in these species is synthesized by phosphoethanolamine-N-methyltransferase (PEAMT) and/or phosphatidylethanolamine-N-methyltransferase (PEMT), both of which are absent in C. reinhardtii. Recombinant PEAMTs from these algae showed methyltransferase activity for phosphoethanolamine but not for monomethyl phosphoethanolamine in vitro, in contrast to land plant PEAMT, which catalyzes the three methylations from phosphoethanolamine to phosphocholine. This suggested an involvement of other methyltransferases in PC biosynthesis. Here, we characterized the putative phospholipid-N-methyltransferase (PLMT) genes of these species by genetic and phylogenetic analysis. Complementation assays using a PC biosynthesis-deficient yeast suggested that the PLMTs of these algae can synthesize PC from phosphatidylethanolamine. These results indicated that the PC biosynthesis pathways in green algae differ from those of land plants, although the enzymes involved are homologous. Phylogenetic analysis suggested that the PEAMTs and PLMTs in these algae were inherited from the common ancestor of green algae. The absence of PC biosynthesis in many Chlamydomonas species is likely a result of parallel losses of PEAMT and PLMT in this genus.

The role of phospholipid molecular species in determining the physical properties of yeast membranes

In most eukaryotes, including Saccharomyces cerevisiae, glycerophospholipids are the main membrane lipid constituents. Besides serving as general membrane ‘building blocks’, glycerophospholipids play an important role in determining the physical properties of the membrane, which are crucial for proper membrane function. To ensure optimal physical properties, membrane glycerophospholipid composition and synthesis are tightly regulated. This review will summarize our current knowledge of factors and processes determining the membrane glycerophospholipid composition of the reference eukaryote S. cerevisiae at the level of molecular species. Extrapolating from relevant model membrane data, we also discuss how modulation of the molecular species composition can regulate membrane physical properties.

Ethanolamine and Phosphatidylethanolamine: Partners in Health and Disease

Phosphatidylethanolamine (PE) is the second most abundant phospholipid in mammalian cells. PE comprises about 15–25% of the total lipid in mammalian cells; it is enriched in the inner leaflet of membranes, and it is especially abundant in the inner mitochondrial membrane. PE has quite remarkable activities: it is a lipid chaperone that assists in the folding of certain membrane proteins, it is required for the activity of several of the respiratory complexes, and it plays a key role in the initiation of autophagy. In this review, we focus on PE's roles in lipid-induced stress in the endoplasmic reticulum (ER), Parkinson's disease (PD), ferroptosis, and cancer.

Four Acyltransferases Uniquely Contribute to Phospholipid Heterogeneity in Saccharomyces cerevisiae

Diverse acyl-CoA species and acyltransferase isoenzymes are components of a complex system that synthesizes glycerophospholipids and triacylglycerols. Saccharomyces cerevisiae has four main acyl-CoA species, two main glycerol-3-phosphate 1-O-acyltransferases (Gat1p, Gat2p), and two main 1-acylglycerol-3-phosphate O-acyltransferases (Lpt1p, Slc1p). The in vivo contribution of these isoenzymes to phospholipid heterogeneity was determined using haploids with compound mutations: gat1Δlpt1Δ, gat2Δlpt1Δ, gat1Δslc1Δ, and gat2Δslc1Δ. All mutations mildly reduced [3H]palmitic acid incorporation into phospholipids relative to triacylglycerol. Electrospray ionization tandem mass spectrometry identified few differences from wild type in gat1Δlpt1Δ, dramatic differences in gat2Δslc1Δ, and intermediate changes in gat2Δlpt1Δ and gat1Δslc1Δ. Yeast expressing Gat1p and Lpt1p had phospholipids enriched with acyl chains that were unsaturated, 18 carbons long, and paired for length. These alterations prevented growth at 18.5°C and in 10% ethanol. Therefore, Gat2p and Slc1p dictate phospholipid acyl chain composition in rich media at 30°C. Slc1p selectively pairs acyl chains of different lengths.

Decoding P4-ATPase substrate interactions

Cellular membranes display a diversity of functions that are conferred by the unique composition and organization of their proteins and lipids. One important aspect of lipid organization is the asymmetric distribution of phospholipids (PLs) across the plasma membrane. The unequal distribution of key PLs between the cytofacial and exofacial leaflets of the bilayer creates physical surface tension that can be used to bend the membrane; and like Ca2+, a chemical gradient that can be used to transduce biochemical signals. PL flippases in the type IV P-type ATPase (P4-ATPase) family are the principle transporters used to set and repair this PL gradient and the asymmetric organization of these membranes are encoded by the substrate specificity of these enzymes. Thus, understanding the mechanisms of P4-ATPase substrate specificity will help reveal their role in membrane organization and cell biology. Further, decoding the structural determinants of substrate specificity provides investigators the opportunity to mutationally tune this specificity to explore the role of particular PL substrates in P4-ATPase cellular functions. This work reviews the role of P4-ATPases in membrane biology, presents our current understanding of P4-ATPase substrate specificity, and discusses how these fundamental aspects of P4-ATPase enzymology may be used to enhance our knowledge of cellular membrane biology.

Lipid somersaults: Uncovering the mechanisms of protein-mediated lipid flipping

Membrane lipids diffuse rapidly in the plane of the membrane but their ability to flip spontaneously across a membrane bilayer is hampered by a significant energy barrier. Thus spontaneous flip-flop of polar lipids across membranes is very slow, even though it must occur rapidly to support diverse aspects of cellular life. Here we discuss the mechanisms by which rapid flip-flop occurs, and what role lipid flipping plays in membrane homeostasis and cell growth. We focus on conceptual aspects, highlighting mechanistic insights from biochemical and in silico experiments, and the recent, ground-breaking identification of a number of lipid scramblases.

Lipid Acyl Chain Remodeling in Yeast

Membrane lipid homeostasis is maintained by de novo synthesis, intracellular transport, remodeling, and degradation of lipid molecules. Glycerophospholipids, the most abundant structural component of eukaryotic membranes, are subject to acyl chain remodeling, which is defined as the post-synthetic process in which one or both acyl chains are exchanged. Here, we review studies addressing acyl chain remodeling of membrane glycerophospholipids in Saccharomyces cerevisiae, a model organism that has been successfully used to investigate lipid synthesis and its regulation. Experimental evidence for the occurrence of phospholipid acyl chain exchange in cardiolipin, phosphatidylcholine, phosphatidylinositol, and phosphatidylethanolamine is summarized, including methods and tools that have been used for detecting remodeling. Progress in the identification of the enzymes involved is reported, and putative functions of acyl chain remodeling in yeast are discussed.

Phosphatidylethanolamine Metabolism in Health and Disease

Phosphatidylethanolamine (PE) is the second most abundant glycerophospholipid in eukaryotic cells. The existence of four only partially redundant biochemical pathways that produce PE, highlights the importance of this essential phospholipid. The CDP-ethanolamine and phosphatidylserine decarboxylase pathways occur in different subcellular compartments and are the main sources of PE in cells. Mammalian development fails upon ablation of either pathway. Once made, PE has diverse cellular functions that include serving as a precursor for phosphatidylcholine and a substrate for important posttranslational modifications, influencing membrane topology, and promoting cell and organelle membrane fusion, oxidative phosphorylation, mitochondrial biogenesis, and autophagy. The importance of PE metabolism in mammalian health has recently emerged following its association with Alzheimer's disease, Parkinson's disease, nonalcoholic liver disease, and the virulence of certain pathogenic organisms.

Phosphatidylserine translocation at the yeast trans-Golgi network regulates protein sorting into exocytic vesicles

Protein sorting into exocytic vesicles at the yeast trans-Golgi network is believed to be mediated by their coalescence with specific lipids, but how this event is regulated is poorly understood. It is shown that phosphatidylserine flip by Drs2 is required for efficient sorting of the plasma membrane proteins Pma1 and Can1 into exocytic vesicles.

Sorting of plasma membrane proteins into exocytic vesicles at the yeast trans-Golgi network (TGN) is believed to be mediated by their coalescence with specific lipids, but how these membrane-remodeling events are regulated is poorly understood. Here we show that the ATP-dependent phospholipid flippase Drs2 is required for efficient segregation of cargo into exocytic vesicles. The plasma membrane proteins Pma1 and Can1 are missorted from the TGN to the vacuole in drs2∆ cells. We also used a combination of flippase mutants that either gain or lose the ability to flip phosphatidylserine (PS) to determine that PS flip by Drs2 is its critical function in this sorting event. The primary role of PS flip at the TGN appears to be to control the oxysterol-binding protein homologue Kes1/Osh4 and regulate ergosterol subcellular distribution. Deletion of KES1 suppresses plasma membrane–missorting defects and the accumulation of intracellular ergosterol in drs2 mutants. We propose that PS flip is part of a homeostatic mechanism that controls sterol loading and lateral segregation of protein and lipid domains at the TGN.

A phospholipid uptake system in the model plant Arabidopsis thaliana

Plants use solar energy to produce lipids directly from inorganic elements and are not thought to require molecular systems for lipid uptake from the environment. Here we show that Arabidopsis thaliana Aminophospholipid ATPase10 (ALA10) is a P4-type ATPase flippase that internalizes exogenous phospholipids across the plasma membrane, after which they are rapidly metabolized. ALA10 expression and phospholipid uptake are high in the epidermal cells of the root tip and in guard cells, the latter of which regulate the size of stomatal apertures to modulate gas exchange. ALA10-knockout mutants exhibit reduced phospholipid uptake at the root tips and guard cells and are affected in growth and transpiration. The presence of a phospholipid uptake system in plants is surprising. Our results suggest that one possible physiological role of this system is to internalize lysophosphatidylcholine, a signalling lipid involved in root development and stomatal control.

P4-ATPases: lipid flippases in cell membranes

Cellular membranes, notably eukaryotic plasma membranes, are equipped with special proteins that actively translocate lipids from one leaflet to the other and thereby help generate membrane lipid asymmetry. Among these ATP-driven transporters, the P4 subfamily of P-type ATPases (P4-ATPases) comprises lipid flippases that catalyze the translocation of phospholipids from the exoplasmic to the cytosolic leaflet of cell membranes. While initially characterized as aminophospholipid translocases, recent studies of individual P4-ATPase family members from fungi, plants, and animals show that P4-ATPases differ in their substrate specificities and mediate transport of a broader range of lipid substrates, including lysophospholipids and synthetic alkylphospholipids. At the same time, the cellular processes known to be directly or indirectly affected by this class of transporters have expanded to include the regulation of membrane traffic, cytoskeletal dynamics, cell division, lipid metabolism, and lipid signaling. In this review, we will summarize the basic features of P4-ATPases and the physiological implications of their lipid transport activity in the cell.

Insight into yeast: A study model of lipid metabolism and terpenoid biosynthesis

With the development of transcriptomics, metabolomics, proteomics, and mathematical modeling, yeast Saccharomyces cerevisiae is recently considered as a model studying strain by biologists who try to reveal the mystery of microorganic metabolism or develop heterologous pharmaceutical and economic products. Among S. cerevisiae metabolic research, lipid metabolism always attracts great interest because of its dominant role in cell physiology. Related researchers have developed multiple functions from cell membrane component such as adjustment to changing environment and impact on protein folding. Nowadays, many common human diseases such as diabetes mellitus, Alzheimer's disease, obesity, and atherosclerosis are related to lipid metabolism, which makes the study of lipids a desperate need. In addition to lipid metabolism, the study of the native mevalonic acid (MVA) pathway in S. cerevisiae has increased exponentially because of its huge potential to produce economically important products terpenoids. With the progress of technology in gene engineering and metabolic engineering, more and more biosynthetic pathways will be developed and put into industrial application.

Role of flippases, scramblases and transfer proteins in phosphatidylserine subcellular distribution

It is well known that lipids are heterogeneously distributed throughout the cell. Most lipid species are synthesized in the endoplasmic reticulum (ER) and then distributed to different cellular locations in order to create the distinct membrane compositions observed in eukaryotes. However, the mechanisms by which specific lipid species are trafficked to and maintained in specific areas of the cell are poorly understood and constitute an active area of research. Of particular interest is the distribution of phosphatidylserine (PS), an anionic lipid that is enriched in the cytosolic leaflet of the plasma membrane. PS transport occurs by both vesicular and non-vesicular routes, with members of the oxysterol-binding protein family (Osh6 and Osh7) recently implicated in the latter route. In addition, the flippase activity of P4-ATPases helps build PS membrane asymmetry by preferentially translocating PS to the cytosolic leaflet. This asymmetric PS distribution can be used as a signaling device by the regulated activation of scramblases, which rapidly expose PS on the extracellular leaflet and play important roles in blood clotting and apoptosis. This review will discuss recent advances made in the study of phospholipid flippases, scramblases and PS-specific lipid transfer proteins, as well as how these proteins contribute to subcellular PS distribution.