Phospholipid transfer proteins in perspective

A Membrane-Assisted Mechanism for the Release of Ceramide from the CERT START Domain

Ceramide transfer protein CERT is the mediator of nonvesicular transfer of ceramide from the ER to Golgi. In CERT, START is the domain responsible for the binding and transport of ceramide. A wealth of structural data has revealed a helix-grip fold surrounding a large hydrophobic cavity holding the ceramide. Yet, little is known about the mechanisms by which START releases the ceramide through the polar region and into the packed environment of cellular membranes. As such events do not lend themselves easily to experimental investigations, we used multiple unbiased microsecond-long molecular simulations. We propose a membrane-assisted mechanism in which the membrane acts as an allosteric effector initiating the release of ceramide and where the passage of the ceramide acyl chains is facilitated by the intercalation of a single phosphatidylcholine lipid in the cavity, practically greasing the ceramide way out. We verify using free energy calculation and experimental lipidomics data that CERT forms stable complexes with phosphatidylcholine lipids, in addition to ceramide, thus providing validation for the proposed mechanism.

SCP2 mediates the transport of lipid hydroperoxides to mitochondria in chondrocyte ferroptosis

Sterol carrier protein 2 (SCP2) is highly expressed in human osteoarthritis (OA) cartilage, accompanied by ferroptosis hallmarks, especially the accumulation of lipid hydroperoxides (LPO). However, the role of SCP2 in chondrocyte ferroptosis remains unexplored. Here, we identify that SCP2 transports cytoplasmic LPO to mitochondria in RSL3-induced chondrocyte ferroptosis, resulting in mitochondrial membrane damage and release of reactive oxygen species (ROS). The localization of SCP2 on mitochondria is associated with mitochondrial membrane potential, but independent of microtubules transport or voltage-dependent anion channel. Moreover, SCP2 promotes lysosomal LPO increase and lysosomal membrane damage through elevating ROS. However, SCP2 is not directly involved in the cell membrane rupture caused by RSL3. Inhibition of SCP2 markedly protects mitochondria and reduces LPO levels, attenuating chondrocyte ferroptosis in vitro and alleviating the progression of OA in rats. Our study demonstrates that SCP2 mediates the transport of cytoplasmic LPO to mitochondria and the spread of intracellular LPO, accelerating chondrocyte ferroptosis.

Establishment of mouse line showing inducible priapism-like phenotypes

Purpose

Penile research is expected to reveal new targets for treatment and prevention of the complex mechanisms of its disorder including erectile dysfunction (ED). Thus, analyses of the molecular processes of penile ED and continuous erection as priapism are essential issues of reproductive medicine.

Methods

By performing mouse N-ethyl-N-nitrosourea mutagenesis and exome sequencing, we established a novel mouse line displaying protruded genitalia phenotype (PGP; priapism-like phenotype) and identified a novel Pitpna gene mutation for PGP. Extensive histological analyses on the Pitpna mutant and intracavernous pressure measurement (ICP) and liquid chromatography-electrospray ionization tandem mass spectrometry (LC-ESI/MS)/MS analyses were performed.

Results

We evaluated the role of phospholipids during erection for the first time and showed the mutants of inducible phenotypes of priapism. Moreover, quantitative analysis using LC-ESI/MS/MS revealed that the level of phosphatidylinositol (PI) was significantly lower in the mutant penile samples. These results imply that PI may contribute to penile erection by PITPα.

Conclusions

Our findings suggest that the current mutant is a mouse model for priapism and abnormalities in PI signaling pathways through PITPα may lead to priapism providing an attractive novel therapeutic target in its treatment.

Phosphatidylserine biosynthesis pathways in lipid homeostasis: Toward resolution of the pending central issue for decades

Enzymatic control of lipid homeostasis in the cell is a vital element in the complex organization of life. Phosphatidylserine (PS) is an essential anionic phospholipid of cell membranes, and conducts numerous roles for their structural and functional integrity. In mammalian cells, two distinct enzymes phosphatidylserine synthases-1 (PSS1) and -2 (PSS2) in the mitochondria-associated membrane (MAM) in the ER perform de novo synthesis of PS. It is based on base-exchange reactions of the preexisting dominant phospholipids phosphatidylcholine (PC) and phosphatidylethanolamine (PE). While PSS2 specifically catalyzes the reaction "PE → PS," whether or not PSS1 is responsible for the same reaction along with the reaction "PC → PS" remains unsettled despite its fundamental impact on the major stoichiometry. We propose here that a key but the only report that appeared to have put scientists on hold for decades in answering to this issue may be viewed consistently with other available research reports; PSS1 utilizes the two dominant phospholipid classes at a similar intrinsic rate. In this review, we discuss the issue in view of the current information for the enzyme machineries, membrane structure and dynamics, intracellular network of lipid transport, and PS synthesis in health and disease. Resolution of the pending issue is thus critical in advancing our understanding of roles of the essential anionic lipid in biology, health, and disease.

Membrane Curvature, Trans-Membrane Area Asymmetry, Budding, Fission and Organelle Geometry

In biology, the modern scientific fashion is to mostly study proteins. Much less attention is paid to lipids. However, lipids themselves are extremely important for the formation and functioning of cellular membrane organelles. Here, the role of the geometry of the lipid bilayer in regulation of organelle shape is analyzed. It is proposed that during rapid shape transition, the number of lipid heads and their size (i.e., due to the change in lipid head charge) inside lipid leaflets modulates the geometrical properties of organelles, in particular their membrane curvature. Insertion of proteins into a lipid bilayer and the shape of protein trans-membrane domains also affect the trans-membrane asymmetry between surface areas of luminal and cytosol leaflets of the membrane. In the cases where lipid molecules with a specific shape are not predominant, the shape of lipids (cylindrical, conical, or wedge-like) is less important for the regulation of membrane curvature, due to the flexibility of their acyl chains and their high ability to diffuse.

Ins and Outs of Interpreting Lipidomic Results

Membrane lipids are essential for life; however, research on how cells regulate cell lipid composition has been falling behind for quite some time. One reason was the difficulty in establishing analytical methods able to cope with the cell lipid repertoire. Development of a diversity of mass spectrometry-based technologies, including imaging mass spectrometry, has helped to demonstrate beyond doubt that the cell lipidome is not only greatly cell type dependent but also highly sensitive to any pathophysiological alteration such as differentiation or tumorigenesis. Interestingly, the current popularization of metabolomic studies among numerous disciplines has led many researchers to rediscover lipids. Hence, it is important to underscore the peculiarities of these metabolites and their metabolism, which are both radically different from protein and nucleic acid metabolism. Once differences in lipid composition have been established, researchers face a rather complex scenario, to investigate the signaling pathways and molecular mechanisms accounting for their results. Thus, a detail often overlooked, but of crucial relevance, is the complex networks of enzymes involved in controlling the level of each one of the lipid species present in the cell. In most cases, these enzymes are redundant and promiscuous, complicating any study on lipid metabolism, since the modification of one particular lipid enzyme impacts simultaneously on many species. Altogether, this review aims to describe the difficulties in delving into the regulatory mechanisms tailoring the lipidome at the activity, genetic, and epigenetic level, while conveying the numerous, stimulating, and sometimes unexpected research opportunities afforded by this type of studies.

Regulation and Essentiality of the StAR-related Lipid Transfer (START) Domain-containing Phospholipid Transfer Protein PFA0210c in Malaria Parasites

StAR-related lipid transfer (START) domains are phospholipid- or sterol-binding modules that are present in many proteins. START domain-containing proteins (START proteins) play important functions in eukaryotic cells, including the redistribution of phospholipids to subcellular compartments and delivering sterols to the mitochondrion for steroid synthesis. How the activity of the START domain is regulated remains unknown for most of these proteins. The Plasmodium falciparum START protein PFA0210c (PF3D7_0104200) is a broad-spectrum phospholipid transfer protein that is conserved in all sequenced Plasmodium species and is most closely related to the mammalian START proteins STARD2 and STARD7. PFA0210c is unusual in that it contains a signal sequence and a PEXEL export motif that together mediate transfer of the protein from the parasite to the host erythrocyte. The protein also contains a C-terminal extension, which is very uncommon among mammalian START proteins. Whereas the biochemical properties of PFA0210c have been characterized, the function of the protein remains unknown. Here, we provide evidence that the unusual C-terminal extension negatively regulates phospholipid transfer activity. Furthermore, we use the genetically tractable Plasmodium knowlesi model and recently developed genetic technology in P. falciparum to show that the protein is essential for growth of the parasite during the clinically relevant asexual blood stage life cycle. Finally, we show that the regulation of phospholipid transfer by PFA0210c is required in vivo, and we identify a potential second regulatory domain. These findings provide insight into a novel mechanism of regulation of phospholipid transfer in vivo and may have important implications for the interaction of the malaria parasite with its host cell.

Sec14-like phosphatidylinositol transfer proteins and the biological landscape of phosphoinositide signaling in plants

Phosphoinositides and soluble inositol phosphates are essential components of a complex intracellular chemical code that regulates major aspects of lipid signaling in eukaryotes. These involvements span a broad array of biological outcomes and activities, and cells are faced with the problem of how to compartmentalize and organize these various signaling events into a coherent scheme. It is in the arena of how phosphoinositide signaling circuits are integrated and, and how phosphoinositide pools are functionally defined and channeled to privileged effectors, that phosphatidylinositol (PtdIns) transfer proteins (PITPs) are emerging as critical players. As plant systems offer some unique advantages and opportunities for study of these proteins, we discuss herein our perspectives regarding the progress made in plant systems regarding PITP function. We also suggest interesting prospects that plant systems hold for interrogating how PITPs work, particularly in multi-domain contexts, to diversify the biological outcomes for phosphoinositide signaling. This article is part of a Special Issue entitled: Plant Lipid Biology edited by Kent D. Chapman and Ivo Feussner.

Global gene expression profiling using heterologous DNA microarrays to analyze alterations in the transcriptome of Mus spretus mice living in a heavily polluted environment

Microarray platforms are a good approach for assessing biological responses to pollution as they enable the simultaneous analyses of changes in the expression of thousands of genes. As an omic and non-targeted methodology, this technique is open to unforeseen responses under particular environmental conditions. In this study, we successfully apply a commercial oligonucleotide microarray containing Mus musculus whole-genome probes to compare and assess the biological effects of living in a heavily polluted settlement, the Domingo Rubio stream (DRS), at the Huelva Estuary (SW Spain), on inhabitant free-living Mus spretus mice. Our microarray results show that mice living in DRS suffer dramatic changes in gene and protein expression compared with reference specimens. DRS mice showed alteration in the oxidative status of hepatocytes, with activation of both the innate and the acquired immune responses and the induction of chronic inflammation, accompanied by metabolic alterations that imply the accumulation of lipids in the liver (hepatic steatosis). The identified deregulated genes may be useful as biomarkers of environmental pollution.

Phosphatidylinositol transfer proteins and instructive regulation of lipid kinase biology

Phosphatidylinositol is a metabolic precursor of phosphoinositides and soluble inositol phosphates. Both sets of molecules represent versatile intracellular chemical signals in eukaryotes. While much effort has been invested in understanding the enzymes that produce and consume these molecules, central aspects for how phosphoinositide production is controlled and functionally partitioned remain unresolved and largely unappreciated. It is in this regard that phosphatidylinositol (PtdIns) transfer proteins (PITPs) are emerging as central regulators of the functional channeling of phosphoinositide pools produced on demand for specific signaling purposes. The physiological significance of these proteins is amply demonstrated by the consequences that accompany deficits in individual PITPs. Although the biological problem is fascinating, and of direct relevance to disease, PITPs remain largely uncharacterized. Herein, we discuss our perspectives regarding what is known about how PITPs work as molecules, and highlight progress in our understanding of how PITPs are integrated into cellular physiology. This article is part of a Special Issue entitled Phosphoinositides.

Rho regulation: DLC proteins in space and time

Rho GTPases function as molecular switches that connect changes of the external environment to intracellular signaling pathways. They are active at various subcellular sites and require fast and tight regulation to fulfill their role as transducers of extracellular stimuli. New imaging technologies visualizing the active states of Rho proteins in living cells elucidated the necessity of precise spatiotemporal activation of the GTPases. The local regulation of Rho proteins is coordinated by the interaction with different guanine nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs) that turn on and off GTPase signaling to downstream effectors. GEFs and GAPs thus serve as critical signaling nodes that specify the amplitude and duration of a particular Rho signaling pathway. Despite their importance in Rho regulation, the molecular aspects underlying the spatiotemporal control of the regulators themselves are still largely elusive. In this review we will focus on the Deleted in Liver Cancer (DLC) family of RhoGAP proteins and summarize the evidence gathered over the past years revealing their different subcellular localizations that might account for isoform-specific functions. We will also highlight the importance of their tightly controlled expression in the context of neoplastic transformation.

Sphingolipid metabolism and interorganellar transport: localization of sphingolipid enzymes and lipid transfer proteins

In recent decades, many sphingolipid enzymes, sphingolipid-metabolism regulators and sphingolipid transfer proteins have been isolated and characterized. This review will provide an overview of the intracellular localization and topology of sphingolipid enzymes in mammalian cells to highlight the locations where respective sphingolipid species are produced. Interestingly, three sphingolipids that reside or are synthesized in cytosolic leaflets of membranes (ceramide, glucosylceramide and ceramide-1-phosphate) all have cytosolic lipid transfer proteins (LTPs). These LTPs consist of ceramide transfer protein (CERT), four-phosphate adaptor protein 2 (FAPP2) and ceramide-1-phosphate transfer protein (CPTP), respectively. These LTPs execute functions that affect both the location and metabolism of the lipids they bind. Molecular details describing the mechanisms of regulation of LTPs continue to emerge and reveal a number of critical processes, including competing phosphorylation and dephosphorylation reactions and binding interactions with regulatory proteins and lipids that influence the transport, organelle distribution and metabolism of sphingolipids.

A new TIPE of phosphoinositide regulator in cancer

Specific phosphoinositide lipids promote cell growth and cancer. In this issue of Cancer Cell, Fayngerts and colleagues demonstrate that the TIPE3 protein enhances PtdIns(4,5)P2 and PtdIns(3,4,5)P3, is overexpressed in certain cancers, and promotes tumorigenesis. TIPE3 can act as a lipid transfer protein and may constitute a novel phosphoinositide metabolism regulator.

Sterol carrier protein 2 regulates proximal tubule size in the Xenopus pronephric kidney by modulating lipid rafts

The kidney is a homeostatic organ required for waste excretion and reabsorption of water, salts and other macromolecules. To this end, a complex series of developmental steps ensures the formation of a correctly patterned and properly proportioned organ. While previous studies have mainly focused on the individual signaling pathways, the formation of higher order receptor complexes in lipid rafts is an equally important aspect. These membrane platforms are characterized by differences in local lipid and protein compositions. Indeed, the cells in the Xenopus pronephric kidney were positive for the lipid raft markers ganglioside GM1 and Caveolin-1. To specifically interfere with lipid raft function in vivo, we focused on the Sterol Carrier Protein 2 (scp2), a multifunctional protein that is an important player in remodeling lipid raft composition. In Xenopus, scp2 mRNA was strongly expressed in differentiated epithelial structures of the pronephric kidney. Knockdown of scp2 did not interfere with the patterning of the kidney along its proximo-distal axis, but dramatically decreased the size of the kidney, in particular the proximal tubules. This phenotype was accompanied by a reduction of lipid rafts, but was independent of the peroxisomal or transcriptional activities of scp2. Finally, disrupting lipid micro-domains by inhibiting cholesterol synthesis using Mevinolin phenocopied the defects seen in scp2 morphants. Together these data underscore the importance for localized signaling platforms in the proper formation of the Xenopus kidney.

Role of Phospholipids in Endocytosis, Phagocytosis, and Macropinocytosis

Endocytosis, phagocytosis, and macropinocytosis are fundamental processes that enable cells to sample their environment, eliminate pathogens and apoptotic bodies, and regulate the expression of surface components. While a great deal of effort has been devoted over many years to understanding the proteins involved in these processes, the important contribution of phospholipids has only recently been appreciated. This review is an attempt to collate and analyze the rapidly emerging evidence documenting the role of phospholipids in clathrin-mediated endocytosis, phagocytosis, and macropinocytosis. A primer on phospholipid biosynthesis, catabolism, subcellular distribution, and transport is presented initially, for reference, together with general considerations of the effects of phospholipids on membrane curvature and charge. This is followed by a detailed analysis of the critical functions of phospholipids in the internalization processes and in the maturation of the resulting vesicles and vacuoles as they progress along the endo-lysosomal pathway.

Nonvesicular lipid transfer from the endoplasmic reticulum

The transport of lipids from their synthesis site at the endoplasmic reticulum (ER) to different target membranes could be mediated by both vesicular and nonvesicular transport mechanisms. Nonvesicular lipid transport appears to be the major transport route of certain lipid species, and could be mediated by either spontaneous lipid transport or by lipid-transfer proteins (LTPs). Although nonvesicular lipid transport has been extensively studied for more than four decades, its underlying mechanism, advantage and regulation, have not been fully explored. In particular, the function of LTPs and their involvement in intracellular lipid movement remain largely controversial. In this article, we describe the pathways by which lipids are synthesized at the ER and delivered to different cellular membranes, and discuss the role of LTPs in lipid transport both in vitro and in intact cells.

Synthesis and biosynthetic trafficking of membrane lipids

Eukaryotic cells can synthesize thousands of different lipid molecules that are incorporated into their membranes. This involves the activity of hundreds of enzymes with the task of creating lipid diversity. In addition, there are several, typically redundant, mechanisms to transport lipids from their site of synthesis to other cellular membranes. Biosynthetic lipid transport helps to ensure that each cellular compartment will have its characteristic lipid composition that supports the functions of the associated proteins. In this article, we provide an overview of the biosynthesis of the major lipid constituents of cell membranes, that is, glycerophospholipids, sphingolipids, and sterols, and discuss the mechanisms by which these newly synthesized lipids are delivered to their target membranes.

Specificity of intramembrane protein-lipid interactions

Our concept of biological membranes has markedly changed, from the fluid mosaic model to the current model that lipids and proteins have the ability to separate into microdomains, differing in their protein and lipid compositions. Since the breakthrough in crystallizing membrane proteins, the most powerful method to define lipid-binding sites on proteins has been X-ray and electron crystallography. More recently, chemical biology approaches have been developed to analyze protein-lipid interactions. Such methods have the advantage of providing highly specific cellular probes. With the advent of novel tools to study functions of individual lipid species in membranes together with structural analysis and simulations at the atomistic resolution, a growing number of specific protein-lipid complexes are defined and their functions explored. In the present article, we discuss the various modes of intramembrane protein-lipid interactions in cellular membranes, including examples for both annular and nonannular bound lipids. Furthermore, we will discuss possible functional roles of such specific protein-lipid interactions as well as roles of lipids as chaperones in protein folding and transport.

Pulmonary surfactant pathophysiology: current models and open questions

Pulmonary surfactant is an essential lipid-protein complex that stabilizes the respiratory units (alveoli) involved in gas exchange. Quantitative or qualitative derangements in surfactant are associated with severe respiratory pathologies. The integrated regulation of surfactant synthesis, secretion, and metabolism is critical for air breathing and, ultimately, survival. The goal of this review is to summarize our current understanding and highlight important knowledge gaps in surfactant homeostatic mechanisms.

Sterol carrier protein-2 (SCP-2) involvement in cholesterol hydroperoxide cytotoxicity as revealed by SCP-2 inhibitor effects

Sterol carrier protein-2 (SCP-2) plays an important role in cholesterol trafficking and metabolism in mammalian cells. The purpose of this study was to determine whether SCP-2, under oxidative stress conditions, might also traffic hydroperoxides of cholesterol, thereby disseminating their cytotoxic effects. Two inhibitors, SCPI-1 and SCPI-3, known to block cholesterol binding by an insect SCP-2, were used to investigate this. A mouse fibroblast transfectant clone (SC2F) overexpressing SCP-2 was found to be substantially more sensitive to apoptotic killing induced by liposomal 7α-hydroperoxycholesterol (7α-OOH) than a wild-type control. 7α-OOH uptake by SC2F cells and resulting apoptosis were both inhibited by SCPI-1 or SCPI-3 at a subtoxic concentration. Preceding cell death, reactive oxidant accumulation and loss of mitochondrial membrane potential were also strongly inhibited. Similar SCPI protection against 7α-OOH was observed with two other types of SCP-2-expressing mammalian cells. In striking contrast, neither inhibitor had any effect on H(2)O(2)-induced cell killing. To learn whether 7α-OOH cytotoxicity is due to uptake/transport by SCP-2, we used a fluorescence-based competitive binding assay involving recombinant SCP-2, NBD-cholesterol, and SCPI-1/SCPI-3 or 7α-OOH. The results clearly showed that 7α-OOH binds to SCP-2 in SCPI-inhibitable fashion. Our findings suggest that cellular SCP-2 not only binds and translocates cholesterol but also cholesterol hydroperoxides, thus expanding their redox toxicity and signaling ranges under oxidative stress conditions.

PC-TP/StARD2: Of membranes and metabolism

Phosphatidylcholine transfer protein (PC-TP, synonym StARD2) binds phosphatidylcholines, and catalyzes their intermembrane transfer and exchange in vitro. The structure of PC-TP comprises a hydrophobic pocket and a well-defined head group binding site, and its gene expression is regulated by peroxisome proliferator activated receptor-alpha. Recent studies have revealed key regulatory roles for PC-TP in lipid and glucose metabolism. Notably, Pctp(-/-) mice are sensitized to the action of insulin, and exhibit more efficient brown fat-mediated thermogenesis. PC-TP appears to limit access of fatty acids to mitochondria by stimulating the activity of thioesterase superfamily member 2, a newly characterized long-chain fatty acyl-coenzyme A thioesterase. Because PC-TP discriminates between phosphatidylcholines within lipid bilayers, it might function as a sensor that links metabolic regulation to membrane composition.

High-throughput computational structure-based characterization of protein families: START domains and implications for structural genomics

SkyLine, a high-throughput homology modeling pipeline tool, detects and models true sequence homologs to a given protein structure. Structures and models are stored in SkyBase with links to computational function annotation, as calculated by MarkUs. The SkyLine/SkyBase/MarkUs technology represents a novel structure-based approach that is more objective and versatile than other protein classification resources. This structure-centric strategy provides a multi-dimensional organization and coverage of protein space at the levels of family, function, and genome. The concept of "modelability", the ability to model sequences on related structures, provides a reliable criterion for membership in a protein family ("leverage") and underlies the unique success of this approach. The overall procedure is illustrated by its application to START domains, which comprise a Biomedical Theme for the Northeast Structural Genomics Consortium as part of the Protein Structure Initiative. START domains are typically involved in the non-vesicular transport of lipids. While 19 experimentally determined structures are available, the family, whose evolutionary hierarchy is not well determined, is highly sequence diverse, and the ligand-binding potential of many family members is unknown. The SkyLine/SkyBase/MarkUs approach provides significant insights and predicts: (1) many more family members (approximately 4,000) than any other resource; (2) the function for a large number of unannotated proteins; (3) instances of START domains in genomes from which they were thought to be absent; and (4) the existence of two types of novel proteins, those containing dual START domain and those containing N-terminal START domains.

The role of phospholipid transfer protein in lipoprotein-mediated neutralization of the procoagulant effect of anionic liposomes

BACKGROUND

Serum has the ability to neutralize the procoagulant properties of anionic liposomes, with transfer of phospholipids (PLs) to both high-density lipoprotein (HDL) and low-density lipoprotein (LDL) particles. Phospholipid transfer protein (PLTP) mediates transfer of PLs between HDL and other lipoproteins and conversion of HDL into larger and smaller particles.

OBJECTIVES

To examine the role of PLTP in the neutralization of procoagulant liposomes.

METHODS

Procoagulant liposomes were incubated with different lipoproteins in the presence or absence of PLTP, and then tested for their ability to stimulate thrombin formation.

RESULTS AND CONCLUSIONS

In the absence of added PLTP, the lipoprotein-enriched fraction, total HDL, HDL(3) and very high-density lipoprotein (VHDL) were all able to neutralize the procoagulant properties of the liposomes. In these samples, endogenous PLTP was present, as judged by Western blotting. In contrast, no PLTP was present in LDL, HDL(2) and lipoprotein-deficient serum, all of which displayed no ability to neutralize the procoagulant liposomes. The phospholipid (PL) transfer activity was dependent on both enzyme (PLTP) and PL acceptor (lipoproteins). After treatment of the VHDL fraction with antiserum against PLTP, the neutralization of procoagulant activity was reduced, but could be regained by the addition of active PLTP. The neutralizing activity was dependent on a catalytically active form of PLTP, and addition of a low activity form of PLTP had no effect. In conclusion, PLTP was found to mediate transfer of anionic PLs to HDL and LDL, thereby neutralizing the effect of procoagulant liposomes, resulting in a reduction of procoagulant activity.

Phosphatidylinositol transfer proteins: sequence motifs in structural and evolutionary analyses

Phosphatidylinositol transfer proteins (PITP) are a family of monomeric proteins that bind and transfer phosphatidylinositol and phosphatidylcholine between membrane compartments. They are required for production of inositol and diacylglycerol second messengers, and are found in most metazoan organisms. While PITPs are known to carry out crucial cell-signaling roles in many organisms, the structure, function and evolution of the majority of family members remains unexplored; primarily because the ubiquity and diversity of the family thwarts traditional methods of global alignment. To surmount this obstacle, we instead took a novel approach, using MEME and a parsimony-based analysis to create a cladogram of conserved sequence motifs in 56 PITP family proteins from 26 species. In keeping with previous functional annotations, three clades were supported within our evolutionary analysis; two classes of soluble proteins and a class of membrane-associated proteins. By, focusing on conserved regions, the analysis allowed for in depth queries regarding possible functional roles of PITP proteins in both intra- and extra- cellular signaling.

Substrate efflux propensity plays a key role in the specificity of secretory A-type phospholipases

To better understand the principles underlying the substrate specificity of A-type phospholipases (PLAs), a high throughput mass spectrometric assay was employed to study the effect of acyl chain length and unsaturation of phospholipids on their rate of hydrolysis by three different secretory PLAs in micelles and vesicle bilayers. With micelles, each enzyme responded differently to substrate acyl chain unsaturation and double bond position, probably reflecting differences in the accommodative properties of their substrate binding sites. Experiments with saturated acyl positional isomers indicated that the length of the sn2 chain was more critical than that of the sn1 chain, suggesting tighter association of the former with the enzyme. Only the first 9-10 carbons of the sn2 acyl chain seem to interact intimately with the active site. Strikingly, no discrimination between positional isomers was observed with vesicles, and the rate of hydrolysis decreased far more with increasing chain length than with micelles, suggesting that translocation of the phospholipid substrate to the active site is rate-limiting with bilayers. Supporting this conclusion, acyl chain structure affected hydrolysis and spontaneous intervesicle transfer, which correlates with lipid efflux propensity, analogously. We conclude that substrate efflux propensity plays a more important role in the specificity of secretory PLA(2)s than commonly thought and could also be a key attribute in phospholipid homeostasis in which (unknown) PLA(2)s are key players.

A gate-latch-lock mechanism for hormone signalling by abscisic acid receptors

Abscisic acid (ABA) is a ubiquitous hormone that regulates plant growth, development, and responses to environmental stresses. Its action is mediated by the PYR/PYL/RCAR family of START proteins, but it remains unclear how these receptors bind ABA and in turn, how hormone binding leads to inhibition of the downstream type 2C protein phosphatase (PP2C) effectors. Here we report crystal structures of apo and ABA-bound receptors as well as a ternary PYL2-ABA-PP2C complex. The apo receptors contain an open ligand-binding pocket flanked by a gate that closes in response to ABA via conformational changes in two highly conserved β-loops that serve as a gate and latch. Moreover, ABA-induced closure of the gate creates a surface that enables the receptor to dock into and competitively inhibit the PP2C active site. A conserved tryptophan in the PP2C inserts directly between the gate and latch, which functions to further lock the receptor in a closed conformation. Together, our results identify a conserved gate-latch-lock mechanism underlying ABA signaling.

Stomatin-like protein-1 interacts with stomatin and is targeted to late endosomes

The human stomatin-like protein-1 (SLP-1) is a membrane protein with a characteristic bipartite structure containing a stomatin domain and a sterol carrier protein-2 (SCP-2) domain. This structure suggests a role for SLP-1 in sterol/lipid transfer and transport. Because SLP-1 has not been investigated, we first studied the molecular and cell biological characteristics of the expressed protein. We show here that SLP-1 localizes to the late endosomal compartment, like stomatin. Unlike stomatin, SLP-1 does not localize to the plasma membrane. Overexpression of SLP-1 leads to the redistribution of stomatin from the plasma membrane to late endosomes suggesting a complex formation between these proteins. We found that the targeting of SLP-1 to late endosomes is caused by a GYXXPhi (Phi being a bulky, hydrophobic amino acid) sorting signal at the N terminus. Mutation of this signal results in plasma membrane localization. SLP-1 and stomatin co-localize in the late endosomal compartment, they co-immunoprecipitate, thus showing a direct interaction, and they associate with detergent-resistant membranes. In accordance with the proposed lipid transfer function, we show that, under conditions of blocked cholesterol efflux from late endosomes, SLP-1 induces the formation of enlarged, cholesterol-filled, weakly LAMP-2-positive, acidic vesicles in the perinuclear region. This massive cholesterol accumulation clearly depends on the SCP-2 domain of SLP-1, suggesting a role for this domain in cholesterol transfer to late endosomes.

Genetic and biochemical analysis of non-vesicular lipid traffic

Robust lipid traffic within and among membranes is essential for cell growth and membrane biogenesis. Many of these transport reactions occur by nonvesicular pathways, and the genetic and biochemical details of these processes are now beginning to emerge. Intramembrane lipid transport reactions utilize P-type ATPases, ABC transporters, scramblases, and Niemann-Pick type C (NPC) family proteins. The intramembrane processes regulate the establishment and elimination of membrane lipid asymmetry, the cellular influx and efflux of sterols and phospholipids, and the egress of lysosomally deposited lipids. The intermembrane lipid transport processes play important roles in membrane biogenesis, sterol sequestration, and steroid hormone formation. The roles of soluble lipid carriers and membrane-bound lipid-transporting complexes, as well as the mechanisms for regulation of their targeting and assembly, are now becoming apparent. Elucidation of the details of these systems is providing new perspectives on the regulation of lipid traffic within cells.

Alkoxyalkyl prodrugs of acyclic nucleoside phosphonates enhance oral antiviral activity and reduce toxicity: current state of the art

Although the acyclic nucleoside phosphonates cidofovir, adefovir and tenofovir are approved for treating human cytomegalovirus, hepatitis B and HIV infections, respectively, their utility is limited by low oral bioavailability, renal toxicity and poor cell penetration. Research over the past decade has shown that these undesirable features can be eliminated by esterifying the compounds with an alkoxyalkyl group, in effect disguising them as lysophospholipids. In this modified form, the drugs are readily taken up in the gastrointestinal tract and have a prolonged circulation time in plasma. The active metabolite also has a long half life within cells, permitting infrequent dosing. Because these modified drugs are not recognized by the transport mechanisms that cause the accumulation of acyclic nucleoside phosphonates in renal tubular cells, they lack nephrotoxicity. Alkoxyalkyl esterification also markedly increases the in vitro antiviral activity of acyclic nucleoside phosphonates by improving their delivery into cells. For example, an alkoxyalkyl ester of cyclic-cidofovir, a less soluble compound, retains anti-CMV activity for 3 months following a single intravitreal injection. Two of these novel compounds, hexadecyloxypropyl-cidofovir (CMX001) and hexadecyloxypropyl-tenofovir (CMX157) are now in clinical development. This article focuses on the hexadecyloxypropyl and octadecyloxyethyl esters of cidofovir and (S)-HPMPA, describing their synthesis and the evaluation of their in vitro and in vivo activity against a range of orthopoxviruses, herpesviruses, adenoviruses and other double-stranded DNA viruses. The extension to other nucleoside phosphonate antivirals is highlighted, demonstrating that this novel approach can markedly improve the medicinal properties of these drugs.

Arabidopsis ACBP6 is an acyl-CoA-binding protein associated with phospholipid metabolism

In our recent paper in Plant Physiology, we showed that the Arabidopsis thaliana 10-kD acyl-CoA-binding protein, ACBP6, is subcellularly localized to the cytosol and that the overexpression of ACBP6 in transgenic Arabidopsis enhanced freezing tolerance. ACBP6-conferred freezing tolerance was independent of induced cold-regulated (COLD-RESPONSIVE) gene expression, but was correlated to an enhanced expression of phospholipase Ddelta (PLDdelta). Lipid analyses on cold-acclimated freezing-treated ACBP6-overexpressors revealed a decline in phosphatidylcholine (PC) and an elevation of phosphatidic acid (PA) in comparison to wild type. Furthermore, the His-tagged ACBP6 recombinant protein was observed using in vitro filter-binding assays to bind PC, but not PA or lysophosphatidylcholine. Taken together, our results implicate roles for ACBP6 in phospholipid metabolism that is related to gene regulation and PC-binding/transfer. This represents the first report demonstrating the in vitro binding of an ACBP to a phospholipid. The effect of ACBP6 on PLDdelta expression is reminiscent of yeast 10-kD ACBP function in the regulation of genes associated with stress responses, fatty acid synthesis and phospholipid synthesis. However, the yeast ACBP regulates the expression of genes involved in phospholipid synthesis by donation of acyl-CoA esters and its binding to phospholipids remains to be demonstrated.

Non-vesicular sterol transport in cells

Sterols such as cholesterol are important components of cellular membranes. They are not uniformly distributed among organelles and maintaining the proper distribution of sterols is critical for many cellular functions. Both vesicular and non-vesicular pathways move sterols between membranes and into and out of cells. There is growing evidence that a number of non-vesicular transport pathways operate in cells and, in the past few years, a number of proteins have been proposed to facilitate this transfer. Some are soluble sterol transfer proteins that may move sterol between membranes. Others are integral membranes proteins that mediate sterol efflux, uptake from cells, and perhaps intracellular sterol transfer as well. In most cases, the mechanisms and regulation of these proteins remains poorly understood. This review summarizes our current knowledge of these proteins and how they could contribute to intracellular sterol trafficking and distribution.

Interacting proteins dictate function of the minimal START domain phosphatidylcholine transfer protein/StarD2

The Star (steroidogenic acute regulatory protein)-related transfer (START) domain superfamily is characterized by a distinctive lipid-binding motif. START domains typically reside in multidomain proteins, suggesting their function as lipid sensors that trigger biological activities. Phosphatidylcholine transfer protein (PC-TP, also known as StarD2) is an example of a START domain minimal protein that consists only of the lipid-binding motif. PC-TP, which binds phosphatidylcholine exclusively, is expressed during embryonic development and in several tissues of the adult mouse, including liver. Although it catalyzes the intermembrane exchange of phosphatidylcholines in vitro, this activity does not appear to explain the various metabolic alterations observed in mice lacking PC-TP. Here we demonstrate that PC-TP function may be mediated via interacting proteins. Yeast two-hybrid screening using libraries prepared from mouse liver and embryo identified Them2 (thioesterase superfamily member 2) and the homeodomain transcription factor Pax3 (paired box gene 3), respectively, as PC-TP-interacting proteins. These were notable because the START domain superfamily contains multidomain proteins in which the START domain coexists with thioesterase domains in mammals and with homeodomain transcription factors in plants. Interactions were verified in pulldown assays, and colocalization with PC-TP was confirmed within tissues and intracellularly. The acyl-CoA thioesterase activity of purified recombinant Them2 was markedly enhanced by recombinant PC-TP. In tissue culture, PC-TP coactivated the transcriptional activity of Pax3. These findings suggest that PC-TP functions as a phosphatidylcholine-sensing molecule that engages in diverse regulatory activities that depend upon the cellular expression of distinct interacting proteins.

Characterization and tissue-specific expression of phosphatidylcholine transfer protein gene from amphioxus Branchiostoma belcheri

An amphioxus cDNA, encoding phosphatidylcholine transfer protein (AmphiPCTP), was identified for the first time from the gut cDNA library of Branchiostoma belcheri. It contains a 660-bp open reading frame corresponding to a deduced protein of 219 amino acids. Phylogenetic tree analysis showed that AmphiPCTP clustered with PCTP subgroup of PCTP subfamily containing steroidogenic acute regulatory protein (StAR)-related lipid transfer (START) domains. AmphiPCTP had an exon-intron organization similar to that of human and rat PCTP genes in terms of both exon number and sequence homology of each exon, suggesting that PCTP has probably maintained a similar function in both amphioxus and mammalian species. Both in situ hybridization histochemistry and whole-mount in situ hybridization revealed a tissue-specific expression pattern of AmphiPCTP with the high levels in the hepatic caecum and primitive gut, including the region where the hepatic caecum will form later during development. This apparently agrees with the hypothesis that amphioxus hepatic caecum is equivalent to vertebrate liver. These results suggest a conserved role of PCTPs in amphioxus as well as mammalian species.

Phosphorylation of StarD10 on Serine 284 by Casein Kinase II Modulates Its Lipid Transfer Activity

StarD10 is a dual specificity lipid transfer protein capable of shuttling phosphatidylcholine and phosphatidylethanolamine between membranes in vitro. We now provide evidence that, in vivo, StarD10 is phosphorylated on serine 284. This novel phosphorylation site was identified by tandem mass spectrometry of immunoaffinity-purified StarD10 from lysates of HEK293T cells transiently expressing the protein. In vitro kinase assays revealed that casein kinase II was capable of phosphorylating wild-type StarD10 but not a S284A mutant protein. Interestingly, hypotonic extracts prepared from HEK293T cells expressing the serine to alanine mutant exhibited increased lipid transfer activity compared with those from wild-type StarD10-expressing cells, suggesting that, in a cellular context, phosphorylation on serine 284 negatively regulates StarD10 activity. Because casein kinase II phosphorylation also inhibited lipid transfer activity of the purified recombinant StarD10 protein, inhibition is not dependent on any cellular cofactors. Instead, our data show that C-terminal StarD10 phosphorylation on serine 284 regulates its association with cellular membranes.

Regulation of secretory transport by protein kinase D-mediated phosphorylation of the ceramide transfer protein

Protein kinase D (PKD) has been identified as a crucial regulator of secretory transport at the trans-Golgi network (TGN). Recruitment and activation of PKD at the TGN is mediated by the lipid diacylglycerol, a pool of which is generated by sphingomyelin synthase from ceramide and phosphatidylcholine. The nonvesicular transfer of ceramide from the endoplasmic reticulum to the Golgi complex is mediated by the lipid transfer protein CERT (ceramide transport). In this study, we identify CERT as a novel in vivo PKD substrate. Phosphorylation on serine 132 by PKD decreases the affinity of CERT toward its lipid target phosphatidylinositol 4-phosphate at Golgi membranes and reduces ceramide transfer activity, identifying PKD as a regulator of lipid homeostasis. We also show that CERT, in turn, is critical for PKD activation and PKD-dependent protein cargo transport to the plasma membrane. Thus, the interdependence of PKD and CERT is key to the maintenance of Golgi membrane integrity and secretory transport.

Structure and function of phosphatidylcholine transfer protein (PC-TP)/StarD2

Phosphatidylcholine transfer protein (PC-TP) is a highly specific soluble lipid binding protein that transfers phosphatidylcholine between membranes in vitro. PC-TP is a member of the steroidogenic acute regulatory protein-related transfer (START) domain superfamily. Although its biochemical properties and structure are well characterized, the functions of PC-TP in vivo remain incompletely understood. Studies of mice with homozygous disruption of the Pctp gene have largely refuted the hypothesis that this protein participates in the hepatocellular selection and transport of biliary phospholipids, in the production of lung surfactant, in leukotriene biosynthesis and in cellular phosphatidylcholine metabolism. Nevertheless, Pctp(-/-) mice exhibit interesting defects in lipid homeostasis, the understanding of which should elucidate the biological functions of PC-TP.

Investigation of the ligand spectrum of human sterol carrier protein 2 using a direct mass spectrometry assay

Sterol carrier protein 2 (SCP2) has been investigated by nearly native electrospray ionisation mass spectrometry in the presence of long chain fatty acyl CoAs (LCFA-CoAs) and carnitine derivatives of equivalent fatty acid chain length (LCFA-carnitines). Four SCP2 constructs were compared to examine the influence of the N-terminal presequence and the C-terminal peroxisomal targeting signal on ligand binding. Removal of N- or C-terminal residues did not influence ligand binding. The observation that LCFA-CoAs are high affinity ligands for SCP2 was confirmed, while LCFA-carnitines were demonstrated for the first time not to interact with SCP2. LCFA-CoAs formed non-covalent complexes with SCP2 of 2:1 and 1:1 stoichiometry, which could be dissociated by elevating the energy of the ions upon entrance to the mass spectrometer. A fluorescence-competition assay using Nile Red butyric acid confirmed the mass spectrometric observations in solution. The physiological significance of the lack of LCFA-carnitine binding by SCP2 is discussed.

A polymorphism in New Zealand inbred mouse strains that inactivates phosphatidylcholine transfer protein

New Zealand obese (NZO/HlLt) male mice develop polygenic diabetes and altered phosphatidylcholine metabolism. The gene encoding phosphatidylcholine transfer protein (PC-TP) is sited within the support interval for Nidd3, a recessive NZO-derived locus on Chromosome 11 identified by prior segregation analysis between NZO/HlLt and NON/Lt. Sequence analysis revealed that the NZO-derived PC-TP contained a non-synonymous point mutation that resulted in an Arg120His substitution, which was shared by the related NZB/BlNJ and NZW/LacJ mouse strains. Consistent with the structure-based predictions, functional studies demonstrated that Arg120His PC-TP was inactive, suggesting that this mutation contributes to the deficiencies in phosphatidylcholine metabolism observed in NZO mice.