Electrostatics and reduction of dimensionality produce apparent cooperativity when basic peptides bind to acidic lipids in membranes

FERM domains recruit ample PI(4,5)P2s to form extensive protein-membrane attachments

The four-point-one ezrin-radixin-moesin homology (FERM) protein domain is a multifunctional protein-lipid binding site, constituting an integral part of numerous membrane-associated proteins. Its interaction with the lipid phosphatidylinositol-4,5-bisphosphate (PIP2), located at the inner leaflet of eukaryotic plasma membranes, is important for localization, anchorage, and activation of FERM-containing proteins. FERM-PIP2 complexes structurally determined so far exclusively feature a 1:1 binding stoichiometry of protein and lipid, with a few basic FERM residues neutralizing the -4 charge of the bound PIP2. Whether this picture from static crystal structures also applies to the dynamic interaction of FERM domains on PIP2 membranes is unknown. We here quantified the stoichiometry of FERM-PIP2 binding in a lipid bilayer using atomistic molecular dynamics simulations and experiments on solid supported membranes for the FERM domains of focal adhesion kinase and ezrin. In contrast to the structural data, we find much higher average stoichiometries of FERM-PIP2 binding, amounting to 1:3 or 1:4 ratios, respectively. In simulations, the full set of basic residues at the membrane interface, 7 and 15 residues for focal adhesion kinase and ezrin, respectively, engages in PIP2 interactions. In addition, Na ions enter the FERM-membrane binding interface, compensating negative PIP2 charges in case of high charge surpluses from bound PIP2. We propose the multivalent binding of FERM domains to PIP2 in lipid bilayers to significantly enhance the stability of FERM-membrane binding and to render the FERM-membrane linkage highly adjustable.

How Tim proteins differentially exploit membrane features to attain robust target sensitivity

Immune surveillance cells such as T cells and phagocytes utilize integral plasma membrane receptors to recognize surface signatures on triggered and activated cells such as those in apoptosis. One such family of plasma membrane sensors, the transmembrane immunoglobulin and mucin domain (Tim) proteins, specifically recognize phosphatidylserine (PS) but elicit distinct immunological responses. The molecular basis for the recognition of lipid signals on target cell surfaces is not well understood. Previous results suggest that basic side chains present at the membrane interface on the Tim proteins might facilitate association with additional anionic lipids including but not necessarily limited to PS. We, therefore, performed a comparative quantitative analysis of the binding of the murine Tim1, Tim3, and Tim4, to synthetic anionic phospholipid membranes under physiologically relevant conditions. X-ray reflectivity and vesicle binding studies were used to compare the water-soluble domain of Tim3 with results previously obtained for Tim1 and Tim4. Although a calcium link was essential for all three proteins, the three homologs differed in how they balance the hydrophobic and electrostatic interactions driving membrane association. The proteins also varied in their sensing of phospholipid chain unsaturation and showed different degrees of cooperativity in their dependence on bilayer PS concentration. Surprisingly, trace amounts of anionic phosphatidic acid greatly strengthened the bilayer association of Tim3 and Tim4, but not Tim1. A novel mathematical model provided values for the binding parameters and illuminated the complex interplay among ligands. In conclusion, our results provide a quantitative description of the contrasting selectivity used by three Tim proteins in the recognition of phospholipids presented on target cell surfaces. This paradigm is generally applicable to the analysis of the binding of peripheral proteins to target membranes through the heterotropic cooperative interactions of multiple ligands.

Membrane partitioning and lipid selectivity of the N-terminal amphipathic H0 helices of endophilin isoforms

Endophilin is an N-BAR protein, which is characterized by a crescent-shaped BAR domain and an amphipathic helix that contributes to the membrane binding of these proteins. The exact function of that H0 helix has been a topic of debate. In mammals, there are five different endophilin isoforms, grouped into A (three members) and B (two members) subclasses, which have been described to differ in their subcellular localization and function. We asked to what extent molecular properties of the H0 helices of these members affect their membrane targeting behavior. We found that all H0 helices of the endophilin isoforms display a two-state equilibrium between disordered and α-helical states in which the helical secondary structure can be stabilized through trifluoroethanol. The helicities in high TFE were strikingly different among the H0 peptides. We investigated H0-membrane partitioning by the monitoring of secondary structure changes via CD spectroscopy. We found that the presence of anionic phospholipids is critical for all H0 helices partitioning into membranes. Membrane partitioning is found to be sensitive to variations in membrane complexity. Overall, the H0 B subfamily displays stronger membrane partitioning than the H0 A subfamily. The H0 A peptide-membrane binding occurs predominantly through electrostatic interactions. Variation among the H0 A subfamily may be attributed to slight alterations in the amino acid sequence. Meanwhile, the H0 B subfamily displays greater specificity for certain membrane compositions, and this may link H0 B peptide binding to endophilin B's cellular function.

Tunable Electromechanical Nanopore Trap Reveals Populations of Peripheral Membrane Protein Binding Conformations

We demonstrate that a naturally occurring nanopore, the voltage-dependent anion channel (VDAC) of the mitochondrion, can be used to electromechanically trap and interrogate proteins bound to a lipid surface at the single-molecule level. Electromechanically probing α-synuclein (αSyn), an intrinsically disordered neuronal protein intimately associated with Parkinson's pathology, reveals wide variation in the time required for individual proteins to unbind from the same membrane surface. The observed distributions of unbinding times span up to 3 orders of magnitude and depend strongly on the lipid composition of the membrane; surprisingly, lipid membranes to which αSyn binds weakly are most likely to contain subpopulations in which electromechanically driven unbinding is very slow. We conclude that unbinding of αSyn from the membrane surface depends not only on membrane binding affinity but also on the conformation adopted by an individual αSyn molecule on the membrane surface.

Electrostatics, Hydrogen Bonding, and Molecular Structure at Polycation and Peptide:Lipid Membrane Interfaces

Polycation and peptide-modified surfaces represent opportunities for developing potentially novel biocidal materials in a growing effort to combat bacterial resistance to traditional bactericides. It is well-known that the positive charge of these compounds is crucial to their function in biofouling prevention and as antimicrobials; however, methods for quantifying the number of positive charges on surface-bound polycations and peptides are necessary to predict, control, and optimize the design and therefore the utility of these compounds. This Spotlight on Applications reports on such an approach that combines second harmonic generation (SHG) spectroscopy, quartz crystal microbalance with dissipation monitoring (QCM-D), and atomistic simulations to obtain mechanistic insight into polycation-membrane interactions using supported lipid bilayers (SLBs) as our model system. We find that at high surface coverage, the large polycations we surveyed feature a considerably smaller percentage of ionization when compared to the smaller polycations and peptides. At these high charge densities, we suspect a pK_a shift of the charged groups to lower charge-charge repulsion as well as the formation of a looplike conformation such that less monomeric units form contact-ion pairs with the bilayer. Our sum frequency generation (SFG) spectroscopy results complement our understanding of the polycation-membrane interaction. At a high density of the polycation poly(allylamine hydrochloride) (PAH), second-order spectral line shapes are consistent with the expulsion of interfacial water molecules possibly due to contact-ion pair formation between PAH and the lipid bilayer. This finding could be essential for understanding the underlying first steps of cell lysis and penetration by polycations and should be explored further.

Structure of voltage-dependent anion channel-tethered bilayer lipid membranes determined using neutron reflectivity

Neutron reflectivity (NR) has emerged as a powerful technique to study the structure and behavior of membrane proteins at planar lipid interfaces. Integral membrane proteins (IMPs) remain a significant challenge for NR owing to the difficulty of forming complete bilayers with sufficient protein density for scattering techniques. One strategy to achieve high protein density on a solid substrate is the capture of detergent-stabilized, affinity-tagged IMPs on a nitrilotriacetic acid (NTA)-functionalized self-assembled monolayer (SAM), followed by reconstitution into the lipids of interest. Such protein-tethered bilayer lipid membranes (ptBLMs) have the notable advantage of a uniform IMP orientation on the substrate. Here, NR is used to provide a structural characterization of the ptBLM process from formation of the SAM to capture of the detergent-stabilized IMP and lipid reconstitution. The mitochondrial outer-membrane voltage-dependent anion channel (VDAC), which controls the exchange of bioenergetic metabolites between mitochondria and the cytosol, was used as a model β-barrel IMP. Molecular dynamics simulations were used for comparison with the experimental results and to inform the parameters of the physical models describing the NR data. The detailed structure of the SAM is shown to depend on the density of the NTA chelating groups. The relative content of detergent and protein in surface-immobilized, detergent-stabilized VDAC is measured, while the reconstituted lipid bilayer is shown to be complete to within a few percent, using the known atomic structure of VDAC. Finally, excess lipid above the reconstituted bilayer, which is of consequence for more indirect structural and functional studies, is shown to be present.

A TEM-traceable physiologically functional gold nanoprobe that permeates non-endocytic cells

Background

Nanoparticles’ intracellular fate requires proper internalization. Most cells make use of a battery of internalization pathways, but some are practically sealed, as they lack the biochemical machinery for cellular intake. Non-endocytic cells, such as mammals’ spermatozoa, challenge standard drug-delivery strategies.

Purpose

In this article, we present a gold nanoprobe that permeates the external and internal membranes of human sperm.

Methods

Our design makes use of a gold nanoparticle functionalized with a membrane-permeable cysteine-rich recombinant protein. The chimeric protein contains two units of physiologically active metallothioneins (MT) that also provide binding motifs to gold and a cell-penetrating-peptide sequence (CPP) that confers cell permeability to the nanoparticle.

Results

Transmission electron microscopy, indirect immunofluorescence, and functional assays show that the nanoprobe is readily internalized in sperm, without compromising cell integrity, while preserving MT’s physiological activity. Our findings highlight the potential of CPP-functionalized nanogold for investigating the physiology of otherwise impermeable non-endocytic cells.

Interfacial electrostatics of poly(vinylamine hydrochloride), poly(diallyldimethylammonium chloride), poly-l-lysine, and poly-l-arginine interacting with lipid bilayers

Charge densities of cationic polymers adsorbed to lipid bilayers are estimated from SHG spectroscopy and QCM-D measurements.

Structural features and lipid binding domain of tubulin on biomimetic mitochondrial membranes

Dimeric tubulin, an abundant water-soluble cytosolic protein known primarily for its role in the cytoskeleton, is routinely found to be associated with mitochondrial outer membranes, although the structure and physiological role of mitochondria-bound tubulin are still unknown. There is also no consensus on whether tubulin is a peripheral membrane protein or is integrated into the outer mitochondrial membrane. Here the results of five independent techniques-surface plasmon resonance, electrochemical impedance spectroscopy, bilayer overtone analysis, neutron reflectometry, and molecular dynamics simulations-suggest that α-tubulin's amphipathic helix H10 is responsible for peripheral binding of dimeric tubulin to biomimetic "mitochondrial" membranes in a manner that differentiates between the two primary lipid headgroups found in mitochondrial membranes, phosphatidylethanolamine and phosphatidylcholine. The identification of the tubulin dimer orientation and membrane-binding domain represents an essential step toward our understanding of the complex mechanisms by which tubulin interacts with integral proteins of the mitochondrial outer membrane and is important for the structure-inspired design of tubulin-targeting agents.

Polyarginine Interacts More Strongly and Cooperatively than Polylysine with Phospholipid Bilayers

The interactions of two highly positively charged short peptide sequences with negatively charged lipid bilayers were explored by fluorescence binding assays and all-atom molecular dynamics simulations. The bilayers consisted of mixtures of phosphatidylglycerol (PG) and phosphatidylcholine (PC) lipids as well as a fluorescence probe that was sensitive to the interfacial potential. The first peptide contained nine arginine repeats (Arg9), and the second one had nine lysine repeats (Lys9). The experimentally determined apparent dissociation constants and Hill cooperativity coefficients demonstrated that the Arg9 peptides exhibited weakly anticooperative binding behavior at the bilayer interface at lower PG concentrations, but this anticooperative effect vanished once the bilayers contained at least 20 mol % PG. By contrast, Lys9 peptides showed strongly anticooperative binding behavior at all PG concentrations, and the dissociation constants with Lys9 were approximately 2 orders of magnitude higher than with Arg9. Moreover, only arginine-rich peptides could bind to the phospholipid bilayers containing just PC lipids. These results along with the corresponding molecular dynamics simulations suggested two important distinctions between the behavior of Arg9 and Lys9 that led to these striking differences in binding and cooperativity. First, the interactions of the guanidinium moieties on the Arg side chains with the phospholipid head groups were stronger than for the amino group. This helped facilitate stronger Arg9 binding at all PG concentrations that were tested. However, at PG concentrations of 20 mol % or greater, the Arg9 peptides came into sufficiently close proximity with each other so that favorable like-charge pairing between the guanidinium moieties could just offset the long-range electrostatic repulsions. This led to Arg9 aggregation at the bilayer surface. By contrast, Lys9 molecules experienced electrostatic repulsion from each other at all PG concentrations. These insights may help explain the propensity for cell penetrating peptides containing arginine to more effectively cross cell membranes in comparison with lysine-rich peptides.

Molecular mechanism for differential recognition of membrane phosphatidylserine by the immune regulatory receptor Tim4

Recognition of phosphatidylserine (PS) lipids exposed on the extracellular leaflet of plasma membranes is implicated in both apoptotic cell removal and immune regulation. The PS receptor T cell immunoglobulin and mucin-domain-containing molecule 4 (Tim4) regulates T-cell immunity via phagocytosis of both apoptotic (high PS exposure) and nonapoptotic (intermediate PS exposure) activated T cells. The latter population must be removed at lower efficiency to sensitively control immune tolerance and memory cell population size, but the molecular basis for how Tim4 achieves this sensitivity is unknown. Using a combination of interfacial X-ray scattering, molecular dynamics simulations, and membrane binding assays, we demonstrate how Tim4 recognizes PS in the context of a lipid bilayer. Our data reveal that in addition to the known Ca(2+)-coordinated, single-PS binding pocket, Tim4 has four weaker sites of potential ionic interactions with PS lipids. This organization makes Tim4 sensitive to PS surface concentration in a manner capable of supporting differential recognition on the basis of PS exposure level. The structurally homologous, but functionally distinct, Tim1 and Tim3 are significantly less sensitive to PS surface density, likely reflecting the differences in immunological function between the Tim proteins. These results establish the potential for lipid membrane parameters, such as PS surface density, to play a critical role in facilitating selective recognition of PS-exposing cells. Furthermore, our multidisciplinary approach overcomes the difficulties associated with characterizing dynamic protein/membrane systems to reveal the molecular mechanisms underlying Tim4's recognition properties, and thereby provides an approach capable of providing atomic-level detail to uncover the nuances of protein/membrane interactions.

Photoinduced membrane damage of E. coli and S. aureus by the photosensitizer-antimicrobial peptide conjugate eosin-(KLAKLAK)2

Background/Objectives

Upon irradiation with visible light, the photosensitizer-peptide conjugate eosin-(KLAKLAK)₂ kills a broad spectrum of bacteria without damaging human cells. Eosin-(KLAKLAK)₂ therefore represents an interesting lead compound for the treatment of local infection by photodynamic bacterial inactivation. The mechanisms of cellular killing by eosin-(KLAKLAK)₂, however, remain unclear and this lack of knowledge hampers the development of optimized therapeutic agents. Herein, we investigate the localization of eosin-(KLAKLAK)₂ in bacteria prior to light treatment and examine the molecular basis for the photodynamic activity of this conjugate.

Methodology/Principal Findings

By employing photooxidation of 3,3-diaminobenzidine (DAB), (scanning) transmission electron microscopy ((S)TEM), and energy dispersive X-ray spectroscopy (EDS) methodologies, eosin-(KLAKLAK)₂ is visualized at the surface of E. coli and S. aureus prior to photodynamic irradiation. Subsequent irradiation leads to severe membrane damage. Consistent with these observations, eosin-(KLAKLAK)₂ binds to liposomes of bacterial lipid composition and causes liposomal leakage upon irradiation. The eosin moiety of the conjugate mediates bacterial killing and lipid bilayer leakage by generating the reactive oxygen species singlet oxygen and superoxide. In contrast, the (KLAKLAK)₂ moiety targets the photosensitizer to bacterial lipid bilayers. In addition, while (KLAKLAK)₂ does not disrupt intact liposomes, the peptide accelerates the leakage of photo-oxidized liposomes.

Conclusions/Significance

Together, our results suggest that (KLAKLAK)₂ promotes the binding of eosin Y to bacteria cell walls and lipid bilayers. Subsequent light irradiation results in membrane damage from the production of both Type I & II photodynamic products. Membrane damage by oxidation is then further aggravated by the (KLAKLAK)₂ moiety and membrane lysis is accelerated by the peptide. These results therefore establish how photosensitizer and peptide act in synergy to achieve bacterial photo-inactivation. Learning how to exploit and optimize this synergy should lead to the development of future bacterial photoinactivation agents that are effective at low concentrations and at low light doses.

Electrostatically-controlled protein adsorption onto lipid bilayer: modeling adsorbate aggregation behavior

Using adsorption models based on scaled particle (SPT) and double layer theories the electrostatically-controlled protein adsorption onto membrane surface has been simulated for non-associating and self-associating ligands. The binding isotherms of monomeric and oligomeric protein species have been calculated over a range of variable parameters including lipid and protein concentrations, protein and membrane charges, pH and ionic strength. Adsorption behavior of monomers appeared to be the most sensitive to the changes in the protein aggregation state. The hallmarks of the protein oligomerization are identified. The practical guides for optimal design of binding experiments focused on obtaining proofs of protein self-association are suggested.

Real-time detection reveals that effectors couple dynamin's GTP-dependent conformational changes to the membrane

The GTPase dynamin is a mechanochemical enzyme involved in membrane fission, but the molecular nature of its membrane interactions and their regulation by guanine nucleotides and protein effectors remain poorly characterized. Using site-directed fluorescence labeling and several independent fluorescence spectroscopic techniques, we have developed robust assays for the detection and real-time monitoring of dynamin-membrane and dynamin-dynamin interactions. We show that dynamin interacts preferentially with highly curved, PIP2-dense membranes and inserts partially into the lipid bilayer. Our kinetic measurements further reveal that cycles of GTP binding and hydrolysis elicit major conformational rearrangements in self-assembled dynamin that favor dynamin-membrane association and dissociation, respectively. Sorting nexin 9, an abundant dynamin partner, transiently stabilizes dynamin on the membrane at the onset of stimulated GTP hydrolysis and may function to couple dynamin's mechanochemical conformational changes to membrane destabilization. Amphiphysin I has the opposite effect. Thus, dynamin's mechanochemical properties on a membrane surface are dynamically regulated by its GTPase cycle and major binding partners.

Cytoplasmic domain of zebrafish myelin protein zero: adhesive role depends on beta-conformation

Solution spectroscopy studies on the cytoplasmic domain of human myelin protein zero (P0) (hP0-cyt) suggest that H-bonding between beta-strands from apposed molecules is likely responsible for the tight cytoplasmic apposition in compact myelin. As a follow-up to these findings, in the current study we used circular dichroism and x-ray diffraction to analyze the same type of model membranes previously used for hP0-cyt to investigate the molecular mechanism underlying the zebrafish cytoplasmic apposition. This space is significantly narrower in teleosts compared with that in higher vertebrates, and can be accounted for in part by the much shorter cytoplasmic domain in the zebrafish protein (zP0-cyt). Circular dichroism measurements on zP0-cyt showed similar structural characteristics to those of hP0-cyt, i.e., the protein underwent a beta-->alpha structural transition at lipid/protein (L/P) molar ratios >50, and adopted a beta-conformation at lower L/P molar ratios. X-ray diffraction was carried out on lipid vesicle solutions with zP0-cyt before and after dehydration to study the effect of protein on membrane lipid packing. Solution diffraction revealed the electron-density profile of a single membrane bilayer. Diffraction patterns of dried samples suggested a multilamellar structure with the beta-folded P0-cyt located at the intermembrane space. Our findings support the idea that the adhesive role of P0 at the cytoplasmic apposition in compact myelin depends on the cytoplasmic domain of P0 being in the beta-conformation.

Binding of the dystrophin second repeat to membrane di-oleyl phospholipids is dependent upon lipid packing

Dystrophin is the genetically deficient protein in Duchenne Muscular Dystrophy. Its C- and N-terminal ends interact with cytoskeletal and membrane proteins, establishing a link between the cytoskeleton and the extracellular matrix. In a previous study, we showed that there is an interaction between the second repeat of the rod domain and membrane phospholipids, which places tryptophan residues in close contact with the membrane. Here, we examine the binding of the dystrophin repeat-2 to small unilamellar vesicles with varying composition. We find that the protein binds predominantly to di-oleyl-phosphatidylserine. The binding as a function of increasing mol% of DOPS appears to be cooperative due to reduction of dimensionality, greatly enhanced in the absence of salts, and partly modulated by pH. Substituting small by large unilamellar vesicles induces a 30-fold lower affinity of the protein for the membrane phospholipids. However, modifying the packing of the acyl chains by introducing lipids such as phosphatidylethanolamine and cholesterol to the vesicle leads to an approximately 7-fold increase in affinity. Taken together, these results show that the binding involves electrostatic forces in addition to hydrophobic ones.

To B or not to B: PIP2 answers the question

In a recent issue of Molecular Cell, Papayannopoulos et al. (2005) show that N-WASP activation of actin-related protein2/3 (Arp2/3) is ultrasensitive to PI(4,5)P(2) concentration. We discuss how interactions between basic regions in proteins and negatively charged membranes can produce sharp on-off switches that may regulate biological activity.

The ABCs of artificial antigen presentation

Artificial antigen presentation aims to accelerate the establishment of therapeutic cellular immunity. Artificial antigen-presenting cells (AAPCs) and their cell-free substitutes are designed to stimulate the expansion and acquisition of optimal therapeutic features of T cells before therapeutic infusion, without the need for autologous antigen-presenting cells. Compelling recent advances include fibroblast AAPCs that process antigens, magnetic beads that are antigen specific, novel T-cell costimulatory combinations, the augmentation of therapeutic potency of adoptively transferred T lymphocytes by interleukin-15, and the safe use of dendritic cell-derived exosomes pulsed with tumor antigen. Whereas the safety and potency of the various systems warrant further preclinical and clinical studies, these emerging technologies are poised to have a major impact on adoptive T-cell therapy and the investigation of T cell-mediated immunity.

The effect of lipid demixing on the electrostatic interaction of planar membranes across a salt solution

We study the effect of lipid demixing on the electrostatic interaction of two oppositely-charged membranes in solution, modeled here as an incompressible two-dimensional fluid mixture of neutral and charged mobile lipids. We calculate, within linear and nonlinear Poisson-Boltzmann theory, the membrane separation at which the net electrostatic force between the membranes vanishes, for a variety of different system parameters. According to Parsegian and Gingell, contact between oppositely-charged surfaces in an electrolyte is possible only if the two surfaces have exactly the same charge density (sigma(1) = -sigma(2)). If this condition is not fulfilled, the surfaces can repel each other, even though they are oppositely charged. In our model of a membrane, the lipidic charge distribution on the membrane surface is not homogeneous and frozen, but the lipids are allowed to freely move within the plane of the membrane. We show that lipid demixing allows contact between membranes even if there is a certain charge mismatch, /sigma(1)/ not equal /sigma(2)/, and that in certain limiting cases, contact is always possible, regardless of the value of sigma(1)/sigma(2) (if sigma(1)/sigma(2) < 0). We furthermore find that of the two interacting membranes, only one membrane shows a major rearrangement of lipids, whereas the other remains in exactly the same state it has in isolation and that, at zero-disjoining pressure, the electrostatic mean-field potential between the membranes follows a Gouy-Chapman potential from the more strongly charged membrane up to the point of the other, more weakly charged membrane.

Cytochrome c location in phosphatidylcholine/cardiolipin model membranes: resonance energy transfer study

Resonance energy transfer between lipid-bound fluorescent probe 3-methoxybenzanthrone as a donor and heme group of cytochrome c as an acceptor has been examined to ascertain the protein disposition relative to the surface of model membranes composed of phosphatidylcholine and cardiolipin (10, 50 and 80 mol%). The model of energy transfer in membrane systems has been extended to the case of donors distributed between the two-bilayer leaflets and acceptors located at the outer monolayer taking into account the donor and acceptor orientational behavior. Assuming specific protein orientation relative to the membrane surface and varying lateral distance of the donor-acceptor closest approach in the range from 0 to 3.5 nm the limits for possible heme distances from the bilayer midplane have been found to be 0.8-3 nm (10 mol% CL), 0-2.6 nm (50 mol% CL), and 1.4-3.3 nm (80 mol% CL).

Phosphatidylinositol polyphosphate binding to the mammalian septin H5 is modulated by GTP

BACKGROUND

Septins are members of a conserved family of GTPases found in organisms as diverse as budding yeast and mammals. In budding yeast, septins form hetero-oligomeric filaments that lie adjacent to the membrane at the mother-bud neck, whereas in mammals, they concentrate at the cleavage furrow of mitotic cells; in both cases, septins provide a required function for cytokinesis. What directs the location and determines the stability of septin filaments, however, remains unknown.

RESULTS

Here we show that the mammalian septin H5 is associated with the plasma membrane and specifically binds the phospholipids phosphatidylinositol 4, 5-bisphosphate (PtdIns(4,5)P(2)) and phosphatidylinositol 3,4, 5-trisphosphate (PtdIns(3,4,5)P(3)). Deletion analysis revealed that this binding occurs at a site rich in basic residues that is conserved in most septins and is located adjacent to the GTP-binding motif. Phosphoinositide binding was inhibited by mutations within this motif and was also blocked by agents known to associate with PtdInsP(2) or by a peptide corresponding to the predicted PtdInsP(2)-binding sequence of H5. GTP binding and hydrolysis by H5 significantly reduced its PtdInsP(2)-binding capability. Treatment of cells with agents that occluded, dephosphorylated or degraded PtdInsP(2) altered the appearance and localization of H5.

CONCLUSIONS

These results indicate that the interaction of septins with PtdInsP(2) might be an important cellular mechanism for the spatial and temporal control of septin accumulation.

The electrostatics of lipid surfaces

Charged lipids constitute a substantial fraction of all membrane lipids. Their charges vary in quantity and distribution within their headgroup regions. In long range interactions, their charges' value and electrostatic potential in the vicinity of the membrane surface can be approximated by the Guy-Chapman theory. This theory treats the interface as a charged structureless plain surrounded by uniform environments. However, if one considers intermolecular interactions, such assumptions need to be revised. The interface is in reality a thick region containing the residual charges of lipid headgroups. Their arrangement depends on the type of lipid present in the membrane. The variety of lipids and their biological functions suggests that charge distribution determines the extent and type of interaction with surface associated molecules. Numerous examples show that protein behavior at the lipid bilayer surface is determined by the type of lipid present, indicating protein specificity towards certain surface locations and local properties (determined by lipid composition) of a particular type. Such specificity is achieved by a combination of electrostatic, hydrophobic and enthropic effects. Comparing lipid biological activity, it can be stated that residual charge distribution is one of the factors of intermolecular recognition leading to the specific interaction of lipid molecules and selected proteins in various processes, particularly those involved with signal transduction pathways. Such specificity enables a variety of processes occurring simultaneously on the same membrane surface to function without cross-reaction interference.

Amphitropic proteins: regulation by reversible membrane interactions (review)

What do Src kinase, Ras-guanine nucleotide exchange factor, cytidylyltransferase, protein kinase C, phospholipase C, vinculin, and DnaA protein have in common? These proteins are amphitropic, that is, they bind weakly (reversibly) to membrane lipids, and this process regulates their function. Proteins functioning in transduction of signals generated in cell membranes are commonly regulated by amphitropism. In this review, the strategies utilized by amphitropic proteins to bind to membranes and to regulate their membrane affinity are described. The recently solved structures of binding pockets for specific lipids are described, as well as the amphipathic alpha-helix motif. Regulatory switches that control membrane affinity include modulation of the membrane lipid composition, and modification of the protein itself by ligand binding, phosphorylation, or acylation. How does membrane binding modulate the protein's function? Two mechanisms are discussed: (1) localization with the substrate, activator, or downstream target, and (2) activation of the protein by a conformational switch. This paper also addresses the issue of specificity in the cell membrane targetted for binding.

Binding of peripheral proteins to mixed lipid membranes: effect of lipid demixing upon binding

Binding isotherms have been determined for the association of horse heart cytochrome c with dioleoyl phosphatidylglycerol (DOPG)/dioleoyl phosphatidylcholine (DOPC) bilayer membranes over a range of lipid compositions and ionic strengths. In the absence of protein, the DOPG and DOPC lipids mix nearly ideally. The binding isotherms have been analyzed using double layer theory to account for the electrostatics, either the Van der Waals or scaled particle theory equation of state to describe the protein surface distribution, and a statistical thermodynamic formulation consistent with the mass-action law to describe the lipid distribution. Basic parameters governing the electrostatics and intrinsic binding are established from the binding to membranes composed of anionic lipid (DOPG) alone. Both the Van der Waals and scaled particle equations of state can describe the effects of protein distribution on the DOPG binding isotherms equally well, but with different values of the maximum binding stoichiometry (13 lipids/protein for Van der Waals and 8 lipids/protein for scaled particle theory). With these parameters set, it is then possible to derive the association constant, Kr, of DOPG relative to DOPC for surface association with bound cytochrome c by using the binding isotherms obtained with the mixed lipid membranes. A value of Kr (DOPG:DOPC) = 3.3-4.8, depending on the lipid stoichiometry, is determined that consistently describes the binding at different lipid compositions and different ionic strengths. Using the value of Kr obtained it is possible to derive the average in-plane lipid distribution and the enhancement in protein binding induced by lipid redistribution using the statistical thermodynamic theory.

Polylysine decelerates kinetics of negatively charged gramicidin channels as shown by sensitized photoinactivation

Effect of a cationic polymer, poly(L-lysine), on the kinetic properties of ionic channels formed by neutral gramicidin A (gA) and its negatively charged analogue O-pyromellitylgramicidin (OPg) in a bilayer lipid membrane is studied using a method of sensitized photoinactivation. This newly developed method is based on the analysis of transmembrane current transients induced by a flash in the presence of a photosensitizer. It has been shown previously that the time course of the flash-induced current decrease in most cases follows a single exponential decay with an exponential factor (tau, the characteristic time of photoinactivation) that correlates well with the single-channel lifetime. Addition of polylysine does not affect tau for gA channels, but causes a substantial increase in tau for OPg channels. This effect is reversed by addition of polyacrylic acid. The deceleration of the photoinactivation kinetics is ascribed to electrostatic interaction of polylysine with OPg probably resulting in OPg clustering. The latter can stabilize the channel state by reducing the rotational and lateral mobility of OPg monomers and dimers, and thus increase the single channel lifetime.

Cationic amphiphile interactions with polyadenylic acid as probed via 2H-NMR

2H-NMR spectroscopy was used to investigate the effects of polyadenylic acid (PolyA) on three aminomethyl-deuterated cationic amphiphiles: specifically, N-[1-(2,3-dioleoyloxy)propyl]-N',N',N'-trimethylammonium chloride (DOTAP-gamma-d3), 3beta-[N-(N',N',N'-trimethylaminoethane)carbamoyl] cholesterol (TC-CHOL-gamma-d3), and cetyltrimethylammonium bromide (CTAB-gamma-d9). When mixed with 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and incorporated into lipid bilayer membranes, each of the cationic amphiphiles yielded 2H-NMR spectra consisting of a motionally averaged Pake powder pattern. The 2H-NMR quadrupolar splitting generally increased with increasing mole fraction of cationic amphiphile in the lipid bilayer with the exception of CTAB-gamma-d9. Adding PolyA caused the quadrupolar splitting to increase progressively in every case, until a 1:1 cation:anion charge ratio was achieved, after which the quadrupolar splitting changed no further. Deuterium NMR relaxation time measurements showed a parallel increase in T(qe)2 with increasing PolyA. The size of these changes produced by PolyA increased in the order: TC-CHOL < DOTAP < CTAB. NaCl addition reversed much, but not all, of the PolyA-related changes in 2H-NMR quadrupolar splittings and T(qe)2 relaxation times. A UV-based PolyA-membrane binding assay showed that salt addition caused PolyA desorption, and that the salt concentration required to do so increased in the order: TC-CHOL < DOTAP < CTAB. The results are consistent with an electrostatic binding of PolyA to the cationic lipid bilayer surface, accompanied by formation of a stoichiometric charge complex between PolyA and the cationic amphiphile, in which the cationic amphiphile retains considerable motional freedom. The strength of the complex increases in the order: TC-CHOL < DOTAP < CTAB.

Binding and dissociation of cytochrome c to and from membranes containing acidic phospholipids

Membrane association and detachment of cytochrome c (cyt c) in millisecond to second time domain were investigated by stopped-flow fluorescence spectroscopy monitoring the efficiency of energy transfer from a pyrene-fatty acid containing phospholipid derivative, 1-palmitoyl-2-[10-(pyren-1-yl)-decanoyl]-sn-glycero-3-phosphoglyce rol (PPDPG, mole fraction X = 0.01) to the heme of the cyt c. Large unilamellar liposomes composed of egg phosphatidylcholine (eggPC) with varying content of the acidic phospholipid phosphatidylglycerol (eggPG) were employed. Unexpectedly, the rate of binding of cyt c to membranes was attenuated upon increasing the mole fraction of the acidic phospholipid (XPG). For example, at 50 microM phospholipid and 5 microM cyt c, when XPG was increased from 0.20 to 0.40 the half-time for the single-exponential decay in fluorescence increased from 4.7 to 8.6 ms. A similar observation was made for the membrane binding of another cationic protein, histone H1. We suggest that the formation of cooperative hydrogen-bonded networks by deprotonated and protonated PG in the vesicle surface retards the binding of cyt c to the liposome surface. However, once formed, the complex of cyt c with acidic phospholipids is stabilized by increasing XPG. Accordingly, significantly prolonged half-times of dissociation of cyt c from liposomes by NaCl, ATP, and different cationic proteins are measured upon increasing XPG. Differences between the latter cationic membrane binding ligands most likely reflect the varying relative contributions of hydrophobicity and Coulombic forces to their attachment to liposomes. Our data on the release and binding of cyt c to liposomes as a function of XPG and in the presence of ATP also provide the first direct experimental evidence for multiple lipid binding sites in cyt c.

Studies of kinetics and equilibrium membrane binding of class A and class L model amphipathic peptides

We studied the kinetics and equilibrium membrane binding of two amphipathic alpha-helical peptides: the 18L peptide, which belongs to the class L (lytic peptides), and the Ac-18A-NH2 peptide of the class A (apolipoprotein), according to classification of Segrest et al. ((1990) Proteins, 8, 103-117). Both for cationic 18L and zwitterionic Ac-18A-NH2, the presence of acidic lipids increased the membrane binding constants by two orders of magnitude. The free energy of peptide-membrane association was in the range of 8.5-12.8 kcal/mol. Binding isotherms corresponded to monomer partitioning with saturation at high peptide/lipid ratios. This was also supported by stopped flow studies of the kinetics of peptide-membrane association as measured by peptide tryptophan fluorescence or by energy transfer from the peptide to the lipid-anchored anthrylvinyl fluorophor. The apparent time required for peptide-membrane equilibration was in the millisecond range. At low peptide/lipid ratios it depended on lipid concentration and was independent of the peptide concentration. The rate of peptide-membrane association was found to be relatively close to the diffusion limit. Thus peptide-membrane affinity was mostly determined by the peptide dissociation rate, i.e. higher membrane affinity correlated with a decrease in dissociation rate and with a slower peptide exchange. We have shown that the dynamic character of the peptide membrane equilibrium can be used for selective peptide targeting and disruption of membranes with a specific lipid composition.

Adjuvant lipopeptide interaction with model membranes

The cationic lipohexapeptide Pam3Cys-Ser-(Lys)4 is a synthetic model for the triacylated N-terminal part of bacterial lipoproteins, and it is used as an adjuvant and macrophage activator. The amphiphilic lipopeptide was injected below a phosphatidylserine monolayer at the air-water interface. It interacted with the interface, as seen by a decrease in the surface potential (deltaV), and it was inserted in the monolayer, until surface charge neutralization was reached, as seen by the parallel increases of deltaV and of the surface pressure. No insertion occurred above 29 mN/m. The interaction kinetics was sensitive to ionic strength and to the nature of acidic phospholipids and of their acyl chains, but the final equilibrium was independent of these factors. Addition of the lipopeptide to large unilamellar vesicles (LUVs) induced their aggregation, and an exchange of lipids between fluorophor-labelled and non-labelled LUVs. However, no fusion was observed, just as reported for polylysine. The lipopeptide strongly inhibited calcium-induced fusion of PS LUVs, in contrast to the published effect of polylysine. This was probably due to inhibition of calcium fixation on liposomes, since it was observed that the lipopeptide efficiently displaced 45Ca2+ from a PS monolayer. In addition, a phospholipid segregation was observed in SUVs for a few ten micromolar of the lipopeptide.

Conformational equilibria of terminally blocked single amino acids at the water-hexane interface. A molecular dynamics study

The conformational equilibria of the acetyl and methyl amide terminally blocked L-alanine, L-leucine and L-glutamine amino acids are examined in vacuum, in bulk water, and at the water-hexane interface, using multi-nanosecond molecular dynamics simulations. The two-dimensional probability distribution functions of finding the peptides at different dihedral angles of the backbone, phi and psi, are calculated, and free energy differences between different conformational states are determined. All three peptides are interfacially active, i.e. tend to accumulate at the interface even though they are not amphiphilic. Conformational states stable in both gas phase and water are also stable in the interfacial environment. Their populations, however, cannot be simply predicted from the knowledge of conformational equilibria in the bulk phases, indicating that the interface exerts a unique effect on the peptides. Conformational preferences in the interfacial environment arise from the interplay between electrostatic and hydrophobic effects. As in an aqueous solution, electrostatic solute-solvent interactions lead to the stabilization of more polar peptide conformations. The hydrophobic effect is manifested at the interface by a tendency to segregate polar and nonpolar moieties of the solute into the aqueous and the hexane phases, respectively. For the terminally blocked glutamine, this favors conformations for which such a segregation is compatible with the formation of strong, backbone-side chain intramolecular hydrogen bonds on the hexane side of the interface. The influence of the hydrophobic effect can be also noted in the orientational preferences of the peptides at the interface. The terminally blocked leucine is oriented such that its nonpolar side chain is buried in hexane, whereas the polar side chain of glutamine is immersed in water. The free energies of rotating the peptides along the axis parallel to the interface by more than 90 degrees are substantial. This indicates that peptide folding at interfaces is strong by driven by the tendency to adopt amphiphilic structures.

Spin-label electron spin resonance studies on the interactions of lysine peptides with phospholipid membranes

The interactions of lysine oligopeptides with dimyristoyl phosphatidylglycerol (DMPG) bilayer membranes were studied using spin-labeled lipids and electron spin resonance spectroscopy. Tetralysine and pentalysine were chosen as models for the basic amino acid clusters found in a variety of cytoplasmic membrane-associating proteins, and polylysine was chosen as representative of highly basic peripherally bound proteins. A greater motional restriction of the lipid chains was found with increasing length of the peptide, while the saturation ratio of lipids per peptide was lower for the shorter peptides. In DMPG and dimyristoylphosphatidylserine host membranes, the perturbation of the lipid chain mobility by polylysine was greater for negatively charged spin-labeled lipids than for zwitterionic lipids, but for the shorter lysine peptides these differences were smaller. In mixed bilayers composed of DMPG and dimyristoylphosphatidylcholine, little difference was found in selectivity between spin-labeled phospholipid species on binding pentalysine. Surface binding of the basic lysine peptides strongly reduced the interfacial pK of spin-labeled fatty acid incorporated into the DMPG bilayers, to a greater extent for polylysine than for tetralysine or pentalysine at saturation. The results are consistent with a predominantly electrostatic interaction with the shorter lysine peptides, but with a closer surface association with the longer polylysine peptide.

Myristoylated alanine-rich C kinase substrate (MARCKS) produces reversible inhibition of phospholipase C by sequestering phosphatidylinositol 4,5-bisphosphate in lateral domains

The myristoylated alanine-rich protein kinase C substrate (MARCKS) is a major protein kinase C (PKC) substrate in many different cell types. MARCKS is bound to the plasma membrane, and several recent studies suggest that this binding requires both hydrophobic insertion of its myristate chain into the bilayer and electrostatic interaction of its cluster of basic residues with acidic lipids. Phosphorylation of MARCKS by PKC introduces negative charges into the basic cluster, reducing its electrostatic interaction with acidic lipids and producing translocation of MARCKS from membrane to cytoplasm. The present study shows that physiological concentrations of MARCKS (<10 microM) inhibit phospholipase C (PLC)-catalyzed hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP2) in phospholipid vesicles. A peptide corresponding to the basic cluster, MARCKS(151-175), produces a similar inhibition, which was observed with both PLC-delta1 and -beta1. Direct fluorescence microscopy observations demonstrate that the MARCKS peptide forms lateral domains enriched in the acidic lipids phosphatidylserine and PIP2 but not PLC, which accounts for the observed inhibition of PIP2 hydrolysis. Phosphorylation of MARCKS(151-175) by PKC releases the inhibition and allows PLC to produce a burst of inositol 1,4, 5-trisphosphate and diacylglycerol.

Binding of small basic peptides to membranes containing acidic lipids: theoretical models and experimental results

We measured directly the binding of Lys3, Lys5, and Lys7 to vesicles containing acidic phospholipids. When the vesicles contain 33% acidic lipids and the aqueous solution contains 100 mM monovalent salt, the standard Gibbs free energy for the binding of these peptides is 3, 5, and 7 kcal/mol, respectively. The binding energies decrease as the mol% of acidic lipids in the membrane decreases and/or as the salt concentration increases. Several lines of evidence suggest that these hydrophilic peptides do not penetrate the polar headgroup region of the membrane and that the binding is mainly due to electrostatic interactions. To calculate the binding energies from classical electrostatics, we applied the nonlinear Poisson-Boltzmann equation to atomic models of the phospholipid bilayers and the basic peptides in aqueous solution. The electrostatic free energy of interaction, which arises from both a long-range coulombic attraction between the positively charged peptide and the negatively charged lipid bilayer, and a short-range Born or image charge repulsion, is a minimum when approximately 2.5 A (i.e., one layer of water) exists between the van der Waals surfaces of the peptide and the lipid bilayer. The calculated molar association constants, K, agree well with the measured values: K is typically about 10-fold smaller than the experimental value (i.e., a difference of about 1.5 kcal/mol in the free energy of binding). The predicted dependence of K (or the binding free energies) on the ionic strength of the solution, the mol% of acidic lipids in the membrane, and the number of basic residues in the peptide agree very well with the experimental measurements. These calculations are relevant to the membrane binding of a number of important proteins that contain clusters of basic residues.

Raf-1 kinase possesses distinct binding domains for phosphatidylserine and phosphatidic acid. Phosphatidic acid regulates the translocation of Raf-1 in 12-O-tetradecanoylphorbol-13-acetate-stimulated Madin-Darby canine kidney cells

Previous studies demonstrated that the cysteine-rich amino-terminal domain of Raf-1 kinase interacts selectively with phosphatidylserine (Ghosh, S., Xie, W. Q., Quest, A. F. G., Mabrouk, G. M., Strum, J. C., and Bell, R. M. (1994) J. Biol. Chem. 269, 10000-10007). Further analysis showed that full-length Raf-1 bound to both phosphatidylserine and phosphatidic acid (PA). Specifically, a carboxyl-terminal domain of Raf-1 kinase (RafC; residues 295 648 of human Raf-1) interacted strongly with phosphatidic acid. The binding of RafC to PA displayed positive cooperativity with Hill numbers between 3.3 and 6.2; the apparent Kd ranged from 4.9 +/- 0.6 to 7.8 +/- 0.9 mol % PA. The interaction of RafC with PA displayed a pH dependence distinct from the interaction between the cysteine-rich domain of Raf-1 and PA. Also, the RafC-PA interaction was unaffected at high ionic strength. Of all the lipids tested, only PA and cardiolipin exhibited high affinity binding; other acidic lipids were either ineffective or weakly effective. By deletion mutagenesis, the PA binding site within RafC was narrowed down to a 35-amino acid segment between residues 389 and 423. RafC did not bind phosphatidyl alcohols; also, inhibition of PA formation in Madin-Darby canine kidney cells by treatment with 1% ethanol significantly reduced the translocation of Raf-1 from the cytosol to the membrane following stimulation with 12-O-tetradecanoylphorbol-13-acetate. These results suggest a potential role of the lipid second messenger, PA, in the regulation of translocation and subsequent activation of Raf-1 in vivo.

Calculations of the electrostatic potential adjacent to model phospholipid bilayers

We used the nonlinear Poisson-Boltzmann equation to calculate electrostatic potentials in the aqueous phase adjacent to model phospholipid bilayers containing mixtures of zwitterionic lipids (phosphatidylcholine) and acidic lipids (phosphatidylserine or phosphatidylglycerol). The aqueous phase (relative permittivity, epsilon r = 80) contains 0.1 M monovalent salt. When the bilayers contain < 11% acidic lipid, the -25 mV equipotential surfaces are discrete domes centered over the negatively charged lipids and are approximately twice the value calculated using Debye-Hückel theory. When the bilayers contain > 25% acidic lipid, the -25 mV equipotential profiles are essentially flat and agree well with the values calculated using Gouy-Chapman theory. When the bilayers contain 100% acidic lipid, all of the equipotential surfaces are flat and agree with Gouy-Chapman predictions (including the -100 mV surface, which is located only 1 A from the outermost atoms). Even our model bilayers are not simple systems: the charge on each lipid is distributed over several atoms, these partial charges are non-coplanar, there is a 2 A ion-exclusion region (epsilon r = 80) adjacent to the polar headgroups, and the molecular surface is rough. We investigated the effect of these four factors using smooth (or bumpy) epsilon r = 2 slabs with embedded point charges: these factors had only minor effects on the potential in the aqueous phase.

Does the binding of clusters of basic residues to acidic lipids induce domain formation in membranes?

Several proteins that are important components of the calcium/phospholipid second messenger system (e.g. phospholipase C, protein kinase C, myristoylated alanine-rich C kinase substrate (MARCKS) and pp60src) contain clusters of basic residues that can interact with acidic lipids on the cytoplasmic surface of plasma membranes. We have studied the membrane binding of MARCKS and pp60src, peptides that mimic the basic regions of these proteins, and simple model peptides. Specifically, we determined how the binding of these model peptides depends on (1) the number of basic residues in the peptide (2) the fraction of acidic lipids in the membrane (3) the ionic strength of the solution (4) the chemical nature of the basic residues (Arg versus Lys) and the acidic phospholipids [phosphatidylglycerol (PG) versus phosphatidylserine (PS)] (5) the pressure and (6) the temperature. The results are consistent with a simple theoretical model: each basic residue in a peptide binds independently to an acidic lipid with an intrinsic microscopic association constant of 1-10 M-1 (binding energy congruent to 1 kcal/mol). The binding is described with a mass action formalism and the non-specific electrostatic accumulation of the peptides in the aqueous diffuse double layer is described with the Gouy-Chapman theory. This Gouy-Chapman/mass action model accounts surprisingly well for the sigmoidal dependence of binding on the percentage of acidic lipids in the membrane (apparent co-operativity or Hill coefficient > 1); the model assumes that the multivalent basic peptides bind > 1 acidic lipids and thus induce or stabilize domain formation.

Effect of the conformation of a peptide from gp41 on binding and domain formation in model membranes

Binding of the peptide fragment 828-848 (P828), amino acid sequence RVIEVVQGACRAIRHIPRRIR, from the carboxy-terminal region of the envelope glycoprotein gp41 of human immunodeficiency virus type 1 (HIV-1) to membranes composed of a mixture of neutral and negatively charged phospholipids results in domain or cluster formation of the charged lipid. The conformation and dynamics of the peptide are investigated in solution and in the presence of sodium dodecyl sulphate (SDS) micelles using high resolution nuclear magnetic resonance (NMR) spectroscopy and circular dichroism (CD) spectropolarimetry. The CD results demonstrate that addition of either SDS, negatively charged phospholipid liposomes, or trifluoroethanol (TFE) induces a conformational transition of the peptide from a random coil or an extended chain in water to a more ordered structure with an estimated helical content of up to 60%. The structure of the peptide in a membrane mimetic SDS solution was investigated in detail using two-dimensional NMR. The measurements demonstrate the existence of a helical component in the peptide conformation in the SDS-bound state. The peptide most likely exists as an ensemble of conformations with exchange times between them which are fast on the chemical shift NMR time scale (10(-3) s). Simple neutralization of the six arginine sidechain charges does not cause the peptide to adopt an ordered structure. Thus, there is an additional requirement for the structural transition such as that resulting from constraint of the peptide on a surface, or localization of the peptide at the lipid-water interface where the polarity is lower.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of apocytochrome C binding to human erythrocytes

The binding of 125I-labeled apocytochrome c to human erythrocytes was determined for free apocytochrome c concentrations at 10(-10)-10(-6) M. At about 2 x 10(-9) M, maximum cell association of apocytochrome c occurs at 50 mM NaCl and at 22 degrees C. Intact erythrocytes at 22 degrees C have three classes of apocytochrome c binding sites: one high-affinity noncooperative site (n1 = 728 per cell, Kd1 = 1.5 x 10(-9) M) and two positively cooperative sites (n2 = 3.7 x 10(4) per cell, Kd2 = 1.2 x 10(-7) M, alpha 2 = 2.0, and n3 = 2.5 x 10(5) per cell, Kd3 = 7.1 x 10(-7) M, alpha 3 = 12). Erythrocytes at 37 degrees C, and erythrocyte ghosts at 22 degrees C, also have three classes of apocytochrome c binding sites, and most sites are positively cooperative.

Lipid dynamics and peripheral interactions of proteins with membrane surfaces

A large body of evidence strongly indicates biomembranes to be organized into compositionally and functionally specialized domains, supramolecular assemblies, existing on different time and length scales. For these domains and intimate coupling between their chemical composition, physical state, organization, and functions has been postulated. One important constituent of biomembranes are peripheral proteins whose activity can be controlled by non-covalent binding to lipids. Importantly, the physical chemistry of the lipid interface allows for a rapid and reversible control of peripheral interactions. In this review examples are provided on how membrane lipid (i) composition (i.e., specific lipid structures), (ii) organization, and (iii) physical state can each regulate peripheral binding of proteins to the lipid surface. In addition, a novel and efficient mechanism for the control of the lipid surface association of peripheral proteins by [Ca2+], lipid composition, and phase state is proposed. The phase state is, in turn, also dependent on factors such as temperature, lateral packing, presence of ions, metabolites and drugs. Confining reactions to interfaces allows for facile and cooperative large scale integration and control of metabolic pathways due to mechanisms which are not possible in bulk systems.

Phosphorylation reverses the membrane association of peptides that correspond to the basic domains of MARCKS and neuromodulin

Several groups have observed that phosphorylation causes the MARCKS (Myristoylated Alanine-Rich C Kinase Substrate) protein to move off cell membranes and phospholipid vesicles. Our working hypothesis is that significant membrane binding of MARCKS requires both hydrophobic insertion of the N-terminal myristate into the bilayer and electrostatic association of the single cluster of basic residues in the protein with acidic lipids and that phosphorylation reverses this electrostatic association. Membrane binding measurements with myristoylated peptides and phospholipid vesicles show this hydrophobic moiety could, at best, barely attach proteins to plasma membranes. We report here membrane binding measurements with basic peptides that correspond to the phosphorylation domains of MARCKS and neuromodulin. Binding of these peptides increases sigmoidally with the percent acidic lipid in the phospholipid vesicle and can be described by a Gouy-Chapman/mass action theory that explains how electrostatics and reduction of dimensionality produce apparent cooperativity. The electrostatic affinity of the MARCKS peptide for membranes containing 10% acidic phospholipids (10(4) M-1 = chi/[P], where chi is the mole ratio of peptide bound to the outer monolayer of the vesicles and [P] is the concentration of peptide in the aqueous phase) is the same as the hydrophobic affinity of the myristate moiety for bilayer membranes. Phosphorylation decreases the affinity of the MARCKS peptide for membranes containing 15% acidic lipid about 1000-fold and produces a rapid (t1/2 < 30 s) dissociation of the peptide from phospholipid vesicles.

Effect of the lipid hydrolysis products on the phospholipase A2 action towards lipid monolayer

The effect of lauric acid (LA) and lysolauroyllecithin (LLL) on the hydrolysis of lipid in monolayer by phospholipase A2 from Bee venom was studied. It was found that LLL inhibits phospholipase action under both high (39 mN/m) and low (25 mN/m) surface pressure. On the other hand, LA inhibits phospholipase action under the low surface pressure (15 mN/m or 25 mN/m), but increases enzyme activity under high surface pressure (39 mN/m). This activating effect can be suppressed by high ionic strength of the aqueous subphase. It is suggested that an increase of the negative surface charge of the lipid monolayer, followed by an increase of the local concentrations of the positively charged enzyme and calcium near the monolayer is a coupling factor between fatty acid accumulation and phospholipase activation. Such an autocatalytic process can only occur when the substrate is organised into monolayer, bilayer or micelles, therefore it can be considered as a reason for the substrate activation and induction time before lipid hydrolysis.