CTP:phosphocholine cytidylyltransferase and protein kinase C recognize different physical features of membranes: differential responses to an oxidized phosphatidylcholine

Phospholipases and Membrane Curvature: What Is Happening at the Surface?

In this revision work, we emphasize the close relationship between the action of phospholipases and the modulation of membrane curvature and curvature stress resulting from this activity. The alteration of the tridimensional structure of membranes upon the action of phospholipases is analyzed based on studies on model lipid membranes. The transient unbalance of both compositional and physical membrane properties between the hemilayers upon phospholipase activity lead to curvature tension and the catalysis of several membrane-related processes. Several proteins' membrane-bound and soluble forms are susceptible to regulation by the curvature stress induced by phospholipase action, which has important consequences in cell signaling. Additionally, the modulation of membrane fusion by phospholipase products regulates membrane dynamics in several cellular scenarios. We commented on vesicle fusion in the Golgi-endoplasmic system, synaptic vesicle fusion to the plasma membrane, viral membrane fusion to host cell plasma membrane and gametes membrane fusion upon acrosomal reaction. Furthermore, we explored the modulation of membrane fusion by the asymmetric adsorption of amphiphilic drugs. A deep understanding of the relevance of lipid membrane structure, particularly membrane curvature and curvature stress, on different cellular events leads to the challenge of its regulation, which may become a powerful tool for pharmacological therapy.

Insights into the behavior of unsaturated diacylglycerols in mixed lipid bilayers in relation to protein kinase C activation-A molecular dynamics simulation study

The lipid second messenger diacylglycerol (DAG) is known for its involvement in many types of cellular signaling, especially as an endogenous agonist for protein kinase C (PKC). Evidence has emerged that the degree of saturation of the DAG molecules can affect PKC activation. DAG molecules with different acyl chain saturation have not only been observed to induce varying extents of PKC activation, but also to express selectivity towards different PKC isozymes. Both qualities are important for precise therapeutic activation of PKC; understanding DAG behavior at the molecular level in different environments has much potential in the development of drugs to target PKC. We used molecular dynamics simulations to study the behavior of two different unsaturated DAG species in lipid environments with varying degrees of unsaturation. We focus on phosphatidylethanolamine (PE) instead of phosphatidylcholine (PC) to more accurately model the relevant biomembranes. The effect of cholesterol (CHOL) on these systems was also explored. We found that both high level of unsaturation in the acyl chains of the DAG species and presence of CHOL in the surrounding membrane increase DAG molecule availability at the lipid-water interface. This can partially explain the previously observed differences in PKC activation strength and specificity, the complete mechanism is, however, likely to be more complex. Our simulations coupled with the current understanding of lipids highlight the need for more simulations of biologically accurate lipid environments in order to determine the correct correlations between molecular mechanisms and biological behavior when studying PKC activation.

Multifaceted Analyses of Isolated Mitochondria Establish the Anticancer Drug 2-Hydroxyoleic Acid as an Inhibitor of Substrate Oxidation and an Activator of Complex IV-Dependent State 3 Respiration

The synthetic fatty acid 2-hydroxyoleic acid (2OHOA) has been extensively investigated as a cancer therapy mainly based on its regulation of membrane lipid composition and structure, activating various cell fate pathways. We discovered, additionally, that 2OHOA can uncouple oxidative phosphorylation, but this has never been demonstrated mechanistically. Here, we explored the effect of 2OHOA on mitochondria isolated by ultracentrifugation from U118MG glioblastoma cells. Mitochondria were analyzed by shotgun lipidomics, molecular dynamic simulations, spectrophotometric assays for determining respiratory complex activity, mass spectrometry for assessing beta oxidation and Seahorse technology for bioenergetic profiling. We showed that the main impact of 2OHOA on mitochondrial lipids is their hydroxylation, demonstrated by simulations to decrease co-enzyme Q diffusion in the liquid disordered membranes embedding respiratory complexes. This decreased co-enzyme Q diffusion can explain the inhibition of disjointly measured complexes I–III activity. However, it doesn’t explain how 2OHOA increases complex IV and state 3 respiration in intact mitochondria. This increased respiration probably allows mitochondrial oxidative phosphorylation to maintain ATP production against the 2OHOA-mediated inhibition of glycolytic ATP production. This work correlates 2OHOA function with its modulation of mitochondrial lipid composition, reflecting both 2OHOA anticancer activity and adaptation to it by enhancement of state 3 respiration.

The Implications for Cells of the Lipid Switches Driven by Protein-Membrane Interactions and the Development of Membrane Lipid Therapy

The cell membrane contains a variety of receptors that interact with signaling molecules. However, agonist-receptor interactions not always activate a signaling cascade. Amphitropic membrane proteins are required for signal propagation upon ligand-induced receptor activation. These proteins localize to the plasma membrane or internal compartments; however, they are only activated by ligand-receptor complexes when both come into physical contact in membranes. These interactions enable signal propagation. Thus, signals may not propagate into the cell if peripheral proteins do not co-localize with receptors even in the presence of messengers. As the translocation of an amphitropic protein greatly depends on the membrane's lipid composition, regulation of the lipid bilayer emerges as a novel therapeutic strategy. Some of the signals controlled by proteins non-permanently bound to membranes produce dramatic changes in the cell's physiology. Indeed, changes in membrane lipids induce translocation of dozens of peripheral signaling proteins from or to the plasma membrane, which controls how cells behave. We called these changes "lipid switches", as they alter the cell's status (e.g., proliferation, differentiation, death, etc.) in response to the modulation of membrane lipids. Indeed, this discovery enables therapeutic interventions that modify the bilayer's lipids, an approach known as membrane-lipid therapy (MLT) or melitherapy.

Lipid Replacement Therapy: a natural medicine approach to replacing damaged lipids in cellular membranes and organelles and restoring function

Lipid Replacement Therapy, the use of functional oral supplements containing cell membrane phospholipids and antioxidants, has been used to replace damaged, usually oxidized, membrane glycerophospholipids that accumulate during aging and in various clinical conditions in order to restore cellular function. This approach differs from other dietary and intravenous phospholipid interventions in the composition of phospholipids and their defense against oxidation during storage, ingestion, digestion and uptake as well as the use of protective molecules that noncovalently complex with phospholipid micelles and prevent their enzymatic and bile disruption. Once the phospholipids have been taken in by transport processes, they are protected by several natural mechanisms involving lipid receptors, transport and carrier molecules and circulating cells and lipoproteins until their delivery to tissues and cells where they can again be transferred to intracellular membranes by specific and nonspecific transport systems. Once delivered to membrane sites, they naturally replace and stimulate removal of damaged membrane lipids. Various chronic clinical conditions are characterized by membrane damage, mainly oxidative but also enzymatic, resulting in loss of cellular function. This is readily apparent in mitochondrial inner membranes where oxidative damage to phospholipids like cardiolipin and other molecules results in loss of trans-membrane potential, electron transport function and generation of high-energy molecules. Recent clinical trials have shown the benefits of Lipid Replacement Therapy in restoring mitochondrial function and reducing fatigue in aged subjects and patients with a variety of clinical diagnoses that are characterized by loss of mitochondrial function and include fatigue as a major symptom. This Article is Part of a Special Issue Entitled: Membrane Structure and Function: Relevance in the Cell's Physiology, Pathology and Therapy.

A 22-mer segment in the structurally pliable regulatory domain of metazoan CTP: phosphocholine cytidylyltransferase facilitates both silencing and activating functions

CTP:phosphocholine cytidylyltransferase (CCT), an amphitropic enzyme that regulates phosphatidylcholine synthesis, is composed of a catalytic head domain and a regulatory tail. The tail region has dual functions as a regulator of membrane binding/enzyme activation and as an inhibitor of catalysis in the unbound form of the enzyme, suggesting conformational plasticity. These functions are well conserved in CCTs across diverse phyla, although the sequences of the tail regions are not. CCT regulatory tails of diverse origins are composed of a long membrane lipid-inducible amphipathic helix (m-AH) followed by a highly disordered segment, reminiscent of the Parkinson disease-linked protein, α-synuclein, which we show shares a novel sequence motif with vertebrate CCTs. To unravel features required for silencing, we created chimeric enzymes by fusing the catalytic domain of rat CCTα to the regulatory tail of CCTs from Drosophila, Caenorhabditis elegans, or Saccharomyces cerevisiae or to α-synuclein. Only the tail domains of the two invertebrate CCTs were competent for both suppression of catalytic activity and for activation by lipid vesicles. Thus, both silencing and activating functions of the m-AH can tolerate significant changes in length and sequence. We identified a highly amphipathic 22-residue segment in the m-AH with features conserved among animal CCTs but not yeast CCT or α-synuclein. Deletion of this segment from rat CCT increased the lipid-independent V(max) by 10-fold, equivalent to the effect of deleting the entire tail, and severely weakened membrane binding affinity. However, membrane binding was required for additional increases in catalytic efficiency. Thus, full activation of CCT may require not only loss of a silencing conformation in the m-AH but a gain of an activating conformation, promoted by membrane binding.

The cold but not hard fats in ectotherms: consequences of lipid restructuring on susceptibility of biological membranes to peroxidation, a review

The production of reactive oxygen species is a regular feature of life in the presence of oxygen. Some reactive oxygen species possess sufficient energy to initiate lipid peroxidation in biological membranes, self-propagating reactions with the potential to damage membranes by altering their physical properties and ultimately their function. Two of the most prominent patterns of lipid restructuring in membranes of ectotherms involve contents of polyunsaturated fatty acids and ratios of the abundant phospholipids, phosphatidylcholine and phosphatidylethanolamine. Since polyunsaturated fatty acids and phosphatidylethanolamine are particularly vulnerable to oxidation, it is likely that higher contents of these lipids at low body temperature elevate the inherent susceptibility of membranes to lipid peroxidation. Although membranes from animals living at low body temperatures may be more prone to oxidation, the generation of reactive oxygen species and lipid peroxidation are sensitive to temperature. These scenarios raise the possibility that membrane susceptibility to lipid peroxidation is conserved at physiological temperatures. Reduced levels of polyunsaturated fatty acids and phosphatidylethanolamine may protect membranes at warm temperatures from deleterious oxidations when rates of reactive oxygen species production and lipid peroxidation are relatively high. At low temperatures, enhanced susceptibility may ensure sufficient lipid peroxidation for cellular processes that require lipid oxidation products.

Tocopherols and tocotrienols in membranes: a critical review

The familiar role of tocols (tocopherols and tocotrienols) as lipid-soluble chain-terminating inhibitors of lipid peroxidation is currently in the midst of a reinterpretation. New biological activities have been described for tocols that apparently are not dependent on their well-established antioxidant behaviour. These activities could well be real, but there remain large gaps in our understanding of the behaviour of tocols in membranes, especially when it comes to the alpha-, beta-, gamma-, delta-chroman methylation patterns and the seemingly special nature of tocotrienols. It is inappropriate to make conclusions and develop models based on in vivo (or cell culture) results with reference to in vitro measurements of antioxidant activity. When present in biological membranes, tocols will experience a large variation in the local composition of phospholipids and the presence of neutral lipids such as cholesterol, both of which would be expected to change the efficiency of antioxidant action. It is likely that tocols are not homogeneously dispersed in a membrane, but it is still not known whether any specific combination of lipid head group and acyl chains are conferred special protection from peroxidation, nor do we currently appreciate the structural role that tocols play in membranes. Tocols may enhance curvature stress or counteract similar stresses generated by other lipids such as lysolipids. This review will outline what is known about the location and behaviour of tocols in phospholipid bilayers. We will draw mainly from the biophysical literature, but will attempt to extend the discussion to biologically relevant phenomena when appropriate. We hope that it will assist researchers when designing new experiments and when critically assessing the results, in turn providing a more thorough understanding of the biochemistry of tocols.

Molecular associations of vitamin E

To understand how vitamin E fulfills its functions in membranes and lipoproteins, it is necessary to know how it associates with the lipid components of these structures and the effects its presence has on their structure and stability. Studies of model membrane systems containing vitamin E have proved to be an informative approach to address these questions. A review of the way vitamin E interacts with phospholipid bilayers, how it distributes within the structure, its motional diffusion characteristics, and orientation has been undertaken. The effect of vitamin E on membrane stability and permeability has been described. The tendency of vitamin E to form complexes with certain phospholipids is examined as is the way modulation of protein functions takes place. Finally, recent evidence relevant to the putative role of vitamin E in protecting membranes from free radical attack and the consequences of lipid oxidation in lipoproteins and membranes is examined.

Diacylglycerols, multivalent membrane modulators

Diacylglycerols are second messengers confined to biomembranes and, although relatively simple molecules from the structural point of view, they are able of triggering a surprisingly wide range of biological responses. Diacylglycerols are recognized by a well conserved protein motif, such as the C1 domain. This domain was observed for the first time in protein kinases C but is now known to be present in many other proteins. The effect of diacylglycerols is not limited to binding to C1 domains and they are able to alter the biophysical properties of biomembranes and hence modulate the activity of membrane associated proteins and also facilitate some processes like membrane fusion.

Membrane curvature stress controls the maximal conversion of violaxanthin to zeaxanthin in the violaxanthin cycle--influence of alpha-tocopherol, cetylethers, linolenic acid, and temperature

Zeaxanthin, an important component in protection against overexcitation in higher plants, is formed from violaxanthin by the enzyme violaxanthin de-epoxidase. We have investigated factors that may control the maximal degree of conversion in the violaxanthin cycle. The conversion of violaxanthin to zeaxanthin in isolated spinach thylakoids was followed at different temperatures and in the presence of lipid packing modifiers. The maximum degree of conversion was found to be 35%, 70% and 80% at 4 degrees C, 25 degrees C and 37 degrees C respectively. In the presence of membrane modifying agents, known to promote non-lamellar structures (H(II)), such as linolenic acid the conversion increased, and the maximal level of violaxanthin de-epoxidation obtained was close to 100%. In contrast, substances promoting lamellar phases (L(alpha)), such as alpha-tocopherol and 8-cetylether (C(16)EO(8)), only 55% and 35% of the violaxanthin was converted at 25 degrees C, respectively. The results are interpreted in light of the lipid composition of the thylakoid membrane, and we propose a model where a negative curvature elastic stress in the thylakoid lipid bilayer is required for violaxanthin de-epoxidase activity. In this model zeaxanthin with its longer hydrophobic stretch is proposed to promote lamellar arrangements of the membrane. As a result, zeaxanthin relieves the curvature elastic stress, which in turn leads to inactivation of violaxanthin de-epoxidase.

High Cationic Charge and Bilayer Interface-Binding Helices in a Regulatory Lipid Glycosyltransferase,

In the prokaryote Acholeplasma laidlawii, membrane bilayer properties are sensed and regulated by two interface glycosyltransferases (GTs), synthesizing major nonbilayer- (alMGS GT) and bilayer-prone glucolipids. These enzymes are of similar structure, as many soluble GTs, but are sensitive to lipid charge and curvature stress properties. Multivariate and bioinformatic sequence analyses show that such interface enzymes, in relation to soluble ones of similar fold, are characterized by high cationic charge, certain distances between small and cationic amino acids, and by amphipathic helices. Varying surface contents of Lys/Arg pairs and Trp indicate different membrane-binding subclasses. A predicted potential (cationic) binding helix from alMGS was structurally verified by solution NMR and CD. The helix conformation was induced by a zwitterionic as well as anionic lipid environment, and the peptide was confined to the bilayer interface. Bilayer affinity of the peptide, analyzed by surface plasmon resonance, was higher than that for soluble membrane-seeking proteins/peptides and rose with anionic lipid content. Interface intercalation was supported by phase equilibria in membrane lipid mixtures, analyzed by 31P NMR and DSC. An analogous, potentially binding helix has a similar location in the structurally determined Escherichia coli cell wall precursor GT MurG. These two helices have little sequence conservation in alMGS and MurG homologues but maintain their amphipathic character. The evolutionary modification of the alMGS binding helix and its location close to the acceptor substrate site imply a functional importance in enzyme catalysis, potentially providing a mechanism by which glycolipid synthesis will be sensitive to membrane surface charge and intrinsic curvature strain.

Membrane lipid polymorphism: relationship to bilayer properties and protein function

Bilayers are the most familiar arrangement of phospholipids. However, even as bilayers, phospholipids can arrange themselves in a variety of morphologies from essentially flat structures found in large liposomes or when adhered to a flat solid support, to the curved structures found in small liposomes or as bicontinuous cubic phases. Phospholipids can also arrange themselves as curved monolayers, such as in the hexagonal phase, and they can even form spherical or ellipsoid-shaped micelles. A number of factors will determine the final morphology of a lipid aggregate including the structure of the lipid, the nature of the lipid headgroup and its degree of hydration, and the temperature. In addition to being interesting in its own right, the property of lipid polymorphism can be applied to understand how fundamental intrinsic curvature properties of a membrane alter the physical properties of a membrane bilayer. This, in turn, will affect the functional characteristics of membrane proteins, with several possible mechanisms explaining the coupling of membrane properties with protein function.

Biophysical properties of lipids and dynamic membranes

The lipid bilayer is a 3D assembly with a rich variety of physical features that modulate cell signaling and protein function. Lateral and transverse forces within the membrane are significant and change rapidly as the membrane is bent or stretched and as new constituents are added, removed or chemically modified. Recent studies have revealed how differences in structure between the two leaflets of the bilayer and between different areas of the bilayer can interact together with membrane deformation to alter the activities of transmembrane channels and peripheral membrane binding proteins. Here, we highlight some recent reports that the physical properties of the membrane can help control the function of transmembrane proteins and the motor-dependent elongation of internal organelles, such as the endoplasmic reticulum.

Membrane-lipid therapy: a new approach in molecular medicine

Although most drugs bind to proteins and regulate their activity, some drugs act through a new therapeutic approach called membrane-lipid therapy and bind to lipids, thus modulating the structure of membranes. Most cellular functions are highly dependent on the lipid environment because they are controlled by proteins in or around membranes. The wide variety of cell and organelle membranes and the existence of special lipid regions (e.g. microvilli) and domains (e.g. lipid rafts) support the possibility of designing specific lipid therapies. Indeed, recent evidence suggests that lipid therapy might have potential for the treatment of cancer, cardiovascular pathologies, neurodegenerative processes, obesity, metabolic disorders, inflammation, and infectious and autoimmune diseases.

Stearoyl-CoA desaturase 1 deficiency increases CTP:choline cytidylyltransferase translocation into the membrane and enhances phosphatidylcholine synthesis in liver

Stearoyl-CoA desaturase (SCD) is the rate-limiting enzyme in monounsaturated fatty acid synthesis. Previously, we showed that Scd1 deficiency reduces liver triglyceride accumulation and considerably decreases synthesis of very low density lipoprotein and its secretion in both lean and obese mice. In the present study, we found that Scd1 deficiency significantly modulates hepatic glycerophospholipid profile. The content of phosphatidylcholine (PC) was increased by 40% and the activities of CTP:choline cytidylyltransferase (CCT), the rate-limiting enzyme in de novo PC synthesis, and choline phosphotransferase were increased by 64 and 53%, respectively, in liver of Scd1-/- mice. In contrast, the protein level of phosphatidylethanolamine N-methyltransferase, an enzyme involved in PC synthesis via methylation of phosphatidylethanolamine, was decreased by 80% in the liver of Scd1-/- mice. Membrane translocation of CCT is required for its activation. Immunoblot analyses demonstrated that twice as much CCTalpha was associated with plasma membrane in livers of Scd1-/- compared with wild type mice, suggesting that Scd1 mutation leads to an increase in CCT membrane affinity. The incorporation of [(3)H]glycerol into PC was increased by 2.5-fold in Scd1-/- primary hepatocytes compared with those of wild type mice. Furthermore, mitochondrial glycerol-3-phosphate acyltransferase activity was reduced by 42% in liver of Scd1-/- mice; however, the activities of microsomal glycerol-3-phosphate acyltransferase, diacylglycerol acyltransferase, and ethanolamine phosphotransferase were not affected by Scd1 mutation. Our study revealed that SCD1 deficiency specifically increases CCT activity by promoting its translocation into membrane and enhances PC biosynthesis in liver.

Diversity and versatility of lipid-protein interactions revealed by molecular genetic approaches

The diversity in structures and physical properties of lipids provides a wide variety of possible interactions with proteins that affect their assembly, organization, and function either at the surface of or within membranes. Because lipids have no catalytic activity, it has been challenging to define many of their precise functions in vivo in molecular terms. Those processes responsive to lipids are attuned to the native lipid environment for optimal function, but evidence that lipids with similar properties or even detergents can sometimes partially replace the natural lipid environment has led to uncertainty as to the requirement for specific lipids. The development of strains of microorganisms in which membrane lipid composition can be genetically manipulated in viable cells has provided a set of reagents to probe lipid functions. These mutants have uncovered previously unrecognized roles for lipids and provided in vivo verification for putative functions described in vitro. In this review, we summarize how these reagent strains have provided new insight into the function of lipids. The role of specific lipids in membrane protein folding and topological organization is reviewed. The evidence is summarized for the involvement of anionic lipid-enriched domains in the organization of amphitropic proteins on the membrane surface into molecular machines involved in DNA replication and cell division.

Phospholipid biosynthesis in mammalian cells

Identification of the genes and gene products involved in the biosynthesis of phosphatidylcholine, phosphatidylethanolamine, and phosphatidylserine has lagged behind that in many other fields because of difficulties encountered in purifying the respective proteins. Nevertheless, most of these genes have now been identified. In this review article, we have highlighted important new findings on the individual enzymes and the corresponding genes of phosphatidylcholine synthesis via its two major biosynthetic pathways: the CDP-choline pathway and the methylation pathway. We also review recent studies on phosphatidylethanolamine biosynthesis by two pathways: the CDP-ethanolamine pathway, which is active in the endoplasmic reticulum, and the phosphatidylserine decarboxylase pathway, which operates in mitochondria. Finally, the two base-exchange enzymes, phosphatidylserine synthase-1 and phosphatidylserine synthase-2, that synthesize phosphatidylserine in mammalian cells are also discussed.

Characterization of the lipid-binding domain of the Plasmodium falciparum CTP:phosphocholine cytidylyltransferase through synthetic-peptide studies

Phospholipid biosynthesis plays a key role in malarial infection and is regulated by CCT (CTP:phosphocholine cytidylyltransferase). This enzyme belongs to the group of amphitropic proteins which are regulated by reversible membrane interaction. To assess the role of the putative membrane-binding domain of Plasmodium falciparum CCT (PfCCT), we synthesized three peptides, K21, V20 and K54 corresponding to residues 274-294, 308-327 and 274-327 of PfCCT respectively. Conformational behaviour of the peptides, their ability to bind to liposomes and to destabilize lipid bilayers, and their insertion properties were investigated by different biophysical techniques. The intercalation mechanisms of the peptides were refined further by using surface-pressure measurements on various monolayers at the air/water interface. In the present study, we show that the three studied peptides are able to bind to anionic and neutral phospholipids, and that they present an alpha-helical conformation upon lipid binding. Peptides V20 and the full-length K54 intercalate their hydrophobic parts into an anionic bilayer and, to a lesser extent, a neutral one for V20. Peptide K21 interacts only superficially with both types of phospholipid vesicles. Adsorption experiments performed at the air/water interface revealed that peptide K54 is strongly surface-active in the absence of lipid. Peptide V20 presents an atypical behaviour in the presence of phosphatidylserine. Whatever the initial surface pressure of a phosphatidylserine film, peptide V20 and phosphatidylserine entities seem linked together in a special organization involving electrostatic and hydrophobic interactions. We showed that PfCCT presents different lipid-dependence properties from other studied CCTs. Although the lipid-binding domain seems to be located in the C-terminal region of the enzyme, as with the mammalian counterpart, the membrane anchorage, which plays a key role in the enzyme regulation, is driven by two alpha-helices, which behave differently from one another.

The effect of vitamin E on the structure of membrane lipid assemblies

The effects of vitamin E on the activity of membrane-dependent enzymes suggest that it acts indirectly by modifying some properties of the lipid host. The effects of alpha-tocopherol (alpha-T) and alpha-tocopherol hemisuccinate (alpha-THS) on phospholipid monolayer structure, curvature, and bending elasticity were examined using X-ray diffraction and the osmotic stress method. These ligands were mixed with the hexagonal phase-forming lipid, dioleoylphosphatidylethanolamine (DOPE). Increasing levels up to 50 mol% alpha-T in DOPE in excess water result in a systematic decrease in the lattice dimension. Analysis of the structural changes imposed by alpha-T shows that it contributes a spontaneous radius of curvature of -13.7 A. This unusually negative value is comparable to diacylglycerols. alpha-T does not affect the bending elasticity of these monolayers. alpha-THS in its charged form decreases membrane curvature, but in its undissociated neutral form has a qualitatively similar but reduced effect on monolayer curvature, as does alpha-T. We discuss these results in terms of the local stresses such ligands would produce in the vicinity of a membrane protein, and how one might expect proteins to respond to such stress.

Effects of phospholipid composition on MinD-membrane interactions in vitro and in vivo

The peripheral membrane ATPase MinD is a component of the Min system responsible for correct placement of the division site in Escherichia coli cells. By rapidly migrating from one cell pole to the other, MinD helps to block unwanted septation events at the poles. MinD is an amphitropic protein that is localized to the membrane in its ATP-bound form. A C-terminal domain essential for membrane localization is predicted to be an amphipathic alpha-helix with hydrophobic residues interacting with lipid acyl chains and cationic residues on the opposite face of the helix interacting with the head groups of anionic phospholipids (Szeto, T. H., Rowland, S. L., Rothfield, L. I., and King, G. F. (2002) Proc. Natl. Acad. Sci. U. S. A. 99, 15693-15698). To investigate whether E. coli MinD displays a preference for anionic phospholipids, we first examined the localization dynamics of a green fluorescent protein-tagged derivative of MinD expressed in a mutant of E. coli that lacks phosphatidylethanolamine. In these cells, which contain only anionic phospholipids (phosphatidylglycerol and cardiolipin), green fluorescent protein-MinD assembled into dynamic focal clusters instead of the broad zones typical of cells with normal phospholipid content. In experiments with liposomes composed of only zwitterionic, only anionic, or a mixture of anionic and zwitterionic phospholipids, purified MinD bound to these liposomes in the presence of ATP with positive cooperativity with respect to the protein concentration and exhibited Hill coefficients of about 2. Oligomerization of MinD on the liposome surface also was detected by fluorescence resonance energy transfer between MinD molecules labeled with different fluorescent probes. The affinity of MinD-ATP for anionic liposomes as well as liposomes composed of both anionic and zwitterionic phospholipids increased 9- and 2-fold, respectively, relative to zwitterionic liposomes. The degree of acyl chain unsaturation contributed positively to binding strength. These results suggest that MinD has a preference for anionic phospholipids and that MinD oscillation behavior, and therefore cell division site selection, may be regulated by membrane phospholipid composition.