Calcium and prothrombin-induced lateral phase separation in membranes

Langmuir monolayer approaches to protein recognition through molecular imprinting

Surface plasmon resonance (SPR) spectroscopy and atomic force microscopy (AFM) have been employed to investigate ferritin adsorption to binary surfactant monolayers of cationic dioctadecyldimethylammonium bromide (DOMA) and non-ionic methyl stearate (SME). Surfactant molar ratios, miscibility, and lateral mobility were controlled to define the number, size, and distribution of "binding sites" for ferritin, which under the low ionic strength conditions investigated, adsorbed to the monolayers predominantly through electrostatic interactions. Successive adsorption/desorption cycles revealed that fluid monolayers, capable of laterally restructuring during the initial protein adsorption event, bound up to 60% more ferritin (dependent on SME:DOMA ratios) as compared to monolayers that were immobilized on a hydrophobic support during this first adsorption step. The enhanced binding of ferritin to fluid monolayers was accentuated in films having non-ionic SME as the principal component. These findings support the premise that the surfactants reorganize to form favorable interactions with an adsorbing protein, leading to protein specific charge patterns, or templates, in the films. Template assessment, however, was complicated by the presence of an irreversibly bound protein fraction, which AFM revealed to be locally ordered protein clusters.

H-aggregation of azobenzene-substituted amphiphiles in vesicular membranes

Photochemical switching has been studied of double-tailed phosphate amphiphiles containing azobenzene units in both tails in aqueous vesicular dispersions and in mixed vesicular systems with 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC). Since the ease of switching depends on the strength of the bilayer packing, particular emphasis has been placed on the occurrence of H-aggregation in the hydrophobic core of the vesicles. UV-vis spectrometry was employed to monitor H-aggregation and showed how this process depends on the ionic strength and on the mode of preparation of the vesicles. Two types of H-aggregates were observed in mixed DOPC vesicles with 5 mol % of azobenzene phosphate: one with lambda(max) at around 300 nm and one with lambda(max) at 305-320 nm. Those with lambda(max) at 300 nm could not be trans-cis photoisomerized, whereas those with lambda(max) at 305-320 nm are more loosely packed and can be photochemically switched. The permeability of the vesicular bilayers, as probed with leakage experiments using calcein as a fluorescent probe, was examined as another measure for the strength of bilayer packing. Leakage occurred only for DOPC vesicles containing more than 20 mol % of azobenzenephosphate, irradiated with UV light to induce trans-cis photoisomerization. We contend that detailed information on bilayer packing will be of crucial importance for fine-tuning the lateral pressure in vesicular membranes with the ultimate aim to steer the opening and closing of mechanosensitive protein channels of large conductance.

An index of lipid phase diagrams

There is a growing awareness of the utility of lipid phase behavior data in studies of membrane-related phenomena. Such miscibility information is commonly reported in the form of temperature-composition (T-C) phase diagrams. The current index is a conduit to the relevant literature. It lists lipid phase diagrams, their components and conditions of measurement, and complete bibliographic information. The main focus of the index is on lipids of membrane origin where water is the dispersing medium. However, it also includes records on acylglycerols, fatty acids, cationic lipids, and detergent-containing systems. The miscibility of synthetic and natural lipids with other lipids, with water, and with biomolecules (proteins, nucleic acids, carbohydrates, etc.) and non-biological materials (drugs, anesthetics, organic solvents, etc.) is within the purview of the index. There are 2188 phase diagram records in the index, the bulk (81%) of which refers to binary (two-component) T-C phase diagrams. The remainder is made up of more complex (ternary, quaternary) systems, pressure-T phase diagrams, and other more exotic miscibility studies. The index covers the period from 1965 through to July, 2001.

Selective protein interactions with phosphatidylserine containing liposomes alter the steric stabilization properties of poly(ethylene glycol)

Incorporation of 5 mol% poly(ethylene glycol)-conjugated lipids (PEG-lipids) has been shown to extend the circulation longevity of neutral liposomes due to steric repulsion of PEG at the membrane surface. The effects of PEG-lipids on protein interactions with biologically reactive membranes were examined using phosphatidylserine (PS) containing liposomes as the model. Incorporating 15 mol% 1,2-distearoyl-sn-glycero-3-phosphoethanolamine (DSPE)-PEG 2000 into PS liposomes resulted in circulation lifetimes comparable to that obtained with neutral liposomes containing 5 mol% DSPE-PEG 2000. These results suggested that 15 mol% DSPE-PEG 2000 may be effective in protecting PS liposomes from the high affinity, PS-mediated binding of plasma proteins. This was determined by monitoring the effects of PEG-lipids on calcium-mediated blood coagulation protein interactions with PS liposomes. Prothrombin binding and procoagulant activity of PS liposomes could be inhibited >80% when 15 mol% DSPE-PEG 2000 was used. These results are consistent with PS on membrane surfaces forming transient nucleation sites for protein binding that may result in lateral exclusion of PEG-lipids incorporated at <10 mol%. These nucleation sites may be inaccessible when PEG-lipids are present at elevated levels where they adopt a highly compressed brush conformation. This suggests that liposomes with reactive groups and PEG-lipids may be appropriately designed to impart selectivity to protein interactions with membrane surfaces.

Stabilization of O-pyromellitylgramicidin channels in bilayer lipid membranes through electrostatic interaction with polylysines of different chain lengths

Functioning of membrane proteins, in particular ionic channels, can be modulated by alteration of their arrangement in membranes. We addressed this issue by studying the effect of different chain length polylysines on the kinetics of ionic channels formed in a bilayer lipid membrane (BLM) by O-pyromellitylgramicidin carrying three negative charges at the C-terminus. The method of sensitized photoinactivation was applied to the analysis of the channel association-dissociation kinetics (characterized by the exponential factor of the curve describing the time course of the flash-induced decrease in the transmembrane current, tau). Addition of polylysine to the bathing solutions of BLM led to the deceleration of the photoinactivation kinetics, i.e. to the increase in tau. It was shown here that for a series of polylysines differing in their chain lengths, the value of tau grew as their concentration increased above a threshold level until at a certain concentration of each polylysine tau reached maximum. At higher polylysine concentrations tau began to decrease and finally became close to the control level observed in the absence of polylysine. With lengthening of the polylysine chain the maximum value of tau increased, the concentration dependence became steeper, and the threshold concentration decreased. The increase in the ionic strength of the medium shifted the concentration dependence of tau to higher polylysine concentrations and decreased the maximum value of tau. It was concluded that the increase in tau was caused by the formation of domains of O-pyromellitylgramicidin molecules induced by binding of polylysines. This can be related to functional aspects of polycation-induced sequestering of negatively charged transmembrane peptides in neutral membranes.

Roles of factor Va heavy and light chains in protein and lipid rearrangements associated with the formation of a bovine factor Va-membrane complex

Factor Va is an essential protein cofactor of the enzyme factor Xa, which activates prothrombin to thrombin during blood coagulation. Peptides with an apparent Mr of approximately 94,000 (heavy chain; HC) and approximately 74,000 or 72,000 (light chain; LC) interact in the presence of Ca2+ to form active Va. The two forms of Va-LC differ in their carboxyl-terminal C2 domain. Using Va reconstituted with either LC form, we examined the effects of the two LC species on membrane binding and on the activity of membrane-bound Va. We found that 1) Va composed of the 72,000 LC bound only slightly more tightly to membranes composed of a mixture of neutral and acidic lipids, the Kd being reduced by a factor of approximately 3 at 5 mM and by a factor of 6 at 2 mM Ca2+. 2) The two forms of Va seemed to undergo different conformational changes when bound to a membrane. 3) The activity of bovine Va varied somewhat with LC species, the difference being greatest at limiting Xa concentration. We have also addressed the role of the two Va peptides in membrane lipid rearrangements and binding: 1) Va binding increased lateral packing density in mixed neutral/acidic lipid membranes. In the solid phase, Va-HC had no effect, whereas Va-LC and whole Va had similar but small effects. In the fluid phase, Va-HC and whole Va both altered membrane packing, with Va-HC having the largest effect. 2) Va-HC bound reversibly and in a Ca2+-independent fashion to membranes composed of neutral phospholipid (Kd, approximately 0.3 microM; stoichiometry approximately 91). High ionic strength had little effect on binding. 3) The substantial effect of Va on packing within neutral phospholipid membranes was mimicked by Va-HC. 4) Based on measurements of membrane phase behavior, binding of Va or its peptide components did not induce thermodynamically discernible lateral membrane domains. These results suggest that the membrane association of factor Va is a complex process involving both chains of Va, changes in lipid packing, and changes in protein structure.

Evidence for phospholipid microdomain formation in liquid crystalline liposomes reconstituted with Escherichia coli lactose permease

The well-characterized integral membrane protein lactose (lac) permease from Escherichia coli was reconstituted together with trace amounts (molar fraction X = 0.005 of the total phospholipid) of different pyrene-labeled phospholipid analogs into 1-palmitoyl-2-oleoyl-sn-glycero-3-sn-glycero-3-phospho-rac'-glycerol (POPG) liposomes. Effects of lac permease on bilayer lipid dynamics were investigated by measuring the excimer-to-monomer fluorescence intensity ratio IE/IM. Compared to control vesicles, the presence of lac permease (at a protein:phospholipid stoichiometry P/L of 1:4.000) increased the rate of excimer formation by 1-palmitoyl-2[6-(pyren-1-yl)]decanoyl-sn-glycero-3-phosphocholine (PPDPC) by approximately fivefold. Decreasing P/L from approximately 1:4.000 to 1:7.600 decreased the IE/IM for PPDPC from 0.16 to 0.05, respectively. An increase in bilayer fluidity due to permease is unlikely, thus implying that the augmented IE/IM should arise from partial lateral segregation of PPDPC in the vesicles. This notion is supported by the further 38% increase in IE/IM observed for the pyrene-labeled Cys-148 lac permease reconstituted into POPG vesicles at P/L 1:4000. The importance of the length of the lipid-protein boundary is implicated by the reduction in IE/IM resulting from the aggregation of the lac permease in vesicles by a monoclonal antibody. Interestingly, excimer formation by 1-palmitoyl-2[6-(pyren-1-yl)hexanoyl-sn-glycero-3-phosphocholine (PPHPC) was enhanced only fourfold in the presence of lac permease. Results obtained with the corresponding pyrenyl phosphatidylglycerols and -methanols were qualitatively similar to those above, thus indicating that lipid headgroup-protein interactions are not involved. Inclusion of 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamino-N-(5-fluoresce inthio- carbamoyl) (DPPF, X = 0.005) into reconstituted lactose permease vesicles containing PPDPC caused a nearly 90% decrease in excimer fluorescence, whereas in control vesicles lacking the reconstituted protein only 40% quenching was evident. The addition of 1,2-dipalmitoyl-sn-glycero-3-phospho-rac'-glycerol (DPPG) decreased IE/IM for PPDPC, revealing the driving force for the lateral segregation of this probe to become attenuated. More specifically for protein-free bilayers at XDPPG = 0.10 the rate of lateral diffusion of PPDPC in POPG is diminished, as evidenced by the 24% decrement in IE/IM, under these conditions the increase in IE/IM due to lac permease was strongly reduced, by approximately 84%. The present data are interpreted in terms of the hydrophobic mismatch theory, which predicts that integral membrane proteins will draw lipids of similar hydrophobic thickness into their vicinity. In brief, the approximate lengths of most of the predicted 12 hydrophobic, membrane-spanning alpha-helical segments of lactose permease range between 28.5 and 37.5 A and thus exceed the hydrophobic thickness of POPG of approximately 25.8 A. Therefore, to reduce the free energy of the assembly, longer lipids such as PPDPC and DPPF are accumulated in the immediate vicinity of lactose permease in fluid, liquid crystalline POPG bilayers.

Dynamic features of prothrombin interaction with phospholipid vesicles of different size and composition: implications for protein--membrane contact

The dynamics of prothrombin interaction with membrane vesicles of different size and composition was investigated to ascertain the impact of membrane surface characteristics and particle size on this interaction. Dissociation rates were highly sensitive to membrane composition and varied from about 20/s for membranes of 10% PS to 0.1/s for membranes of 50% PS. Overall affinity also varied by more than two orders of magnitude. Very small differences between prothrombin binding to SUV versus LUV were found. Association with large unilamellar vesicles (LUV of 115 nm diameter) was about 4-fold slower, when expressed on the basis of binding sites, than association with small unilamellar vesicles (SUV, 30 nm diameter) of the same composition. Both reactions proceeded at less than 25% of the collisional limit so that the differences were largely due to intrinsic binding properties. Vesicles of 325 nm diameter showed even slower association velocities. Dissociation rates from LUV were about 2-fold slower than from SUV. Again, these differences arose primarily from intrinsic binding properties. Dissociation conformed to a single first order reaction over a wide range of protein occupancy on the membrane. At very high packing density, the dissociation rate increased by about 2-fold. At equilibrium, prothrombin preferred binding to SUV over LUV by about 2-fold. This very small difference, despite substantial differences in phospholipid headgroup packing and hydrocarbon exposure, appeared inconsistent with an important role for protein insertion into the hydrocarbon region of the membrane. However, prothrombin-membrane interaction may arise from a series of interaction forces that have compensating features at equilibrium. The small differences in prothrombin binding to SUV versus LUV, together with differences in the number of protein binding sites per vesicle, were important to identify mechanisms of substrate delivery to the active site of the prothrombinase enzyme [Lu, Y., & Nelsestuen, G. L. (1996) Biochemistry 35, 8201-8209].

Insights into the complex association of bovine factor Va with acidic-lipid-containing synthetic membranes

The mechanism of binding of blood coagulation cofactor factor Va to acidic-lipid-containing membranes has been addressed. Binding isotherms were generated at room temperature using the change in fluorescence anisotropy of pyrene-labeled bovine factor Va to detect binding to sonicated membrane vesicles containing either bovine brain phosphatidylserine (PS) or 1,2-dioleoyl-3-sn-phosphatidylglycerol (DOPG) in combination with 1-palmitoyl-2-oleoyl-3-sn-phosphatidylcholine (POPC). The composition of the membranes was varied from 0 to 40 mol% for PS/POPC and from 0 to 65 mol % for DOPG/POPC membranes. Fitting the data to a classical Langmuir adsorption model yielded estimates of the dissociation constant (Kd) and the stoichiometry of binding. The values of Kd defined in this way displayed a maximum at low acidic lipid content but were nearly constant at intermediate to high fractions of acidic lipid. Fitting the binding isotherms to a two-process binding model (nonspecific adsorption in addition to binding of acidic lipids to sites on the protein) suggested a significant acidic-lipid-independent binding affinity in addition to occupancy of three protein sites that bind PS in preference to DOPG. Both analyses indicated that interaction of factor Va with an acidic-lipid-containing membrane is much more complex than those of factor Xa or prothrombin. Furthermore, a change in the conformation of bound pyrene-labeled factor Va with surface concentration of acidic lipid was implied by variation of both the saturating fluorescence anisotropy and the binding parameters with the acidic lipid content of the membrane. Finally, the results cannot support the contention that binding occurs through nonspecific adsorption to a patch or domain of acidic lipids in the membrane. Factor Va is suggested to associate with membranes by a complex process that includes both acidic-lipid-specific and acidic-lipid-independent sites and a protein structure change induced by occupancy of acidic-lipid-specific sites on the factor Va molecule.

Evidence for the formation of microdomains in liquid crystalline large unilamellar vesicles caused by hydrophobic mismatch of the constituent phospholipids

The excimer-to-monomer fluorescence emission intensity ratio (IE/IM) of the fluorescent probe 1-palmitoyl-2-[(pyren-1-yl)]decanoyl-sn-glycero-3-phosphocholine (PPDPC, 1 mol%) was measured at 30 degrees C as a function of the thickness of fluid liposomal membranes composed of phosphatidylcholines (PCs) with homologous monounsaturated acyl chains of varying lengths N (= number of carbon atoms). Upon decreasing N from di-24:1 PC to di-14:1 PC, the rate of excimer formation was sigmoidally augmented from 0.02 to 0.06. This increase in IE/IM can arise either from enhanced lateral mobility or from the lateral enrichment of PPDPC into domains, or both. Direct evidence for partial lateral segregation of PPDPC being involved is provided by experiments where 1.6 mol% of 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamino-N- (5-fluoresceinthiocarbamoyl) (DPPF) was included together with PPDPC into the bilayers. Notably, because of spectral overlap DPPF can function as a resonance energy transfer acceptor for pyrene excimer. Fluorescence intensity ratio (F/Fo) measured at 480 nm for PPDPC/DPPF (yielding F) and PPDPC (yielding Fo) containing membranes as a function of N reveals a sharp maximum for di-20:1 PC, i.e., the quenching of pyrene excimer fluorescence by DPPF is least efficient in this lipid and is enhanced upon either decrease or increase in N. This is compatible with colocalization of DPPF in PPDPC enriched domains when N not equal to 20, whereas in di-20:1 PC these probes appear to be effectively dispersed. The driving force for the enrichment of PPDPC in thin (N < 20) and thick (N > 20) PC matrices is likely to be hydrophobic mismatch of the effective lengths of the matrix phospholipids and the fluorescent probes. We also measured fluorescence polarization (P) for 1,6-diphenyl-1,3,5-hexatriene (DPH) as well as the IE/IM for the intramolecular excimer forming probe 1,2-bis[(pyren-1-yl)]decanoyl-sn-glycero-3-phosphocholine (bisPDPC) as a function of N. In brief, neither the fluorescence polarization data and nor the measurements of intramolecular chain dynamics using bisPDPC concur with enhanced lateral diffusion as the sole cause for the increase in the IE/IM for PPDPC in thin membranes. Our findings suggest hydrophobic mismatch as the cause of microdomain formation of lipids in fluid, liquid crystalline bilayers, while simultaneously allowing for a high rates of lateral diffusion. Such hydrophobic mismatch-induced compositional fluctuations would also offer one plausible explanation for the chain length diversity observed for biological membranes.

Poly(ethylene glycol)-induced and temperature-dependent phase separation in fluid binary phospholipid membranes

Exclusion of the strongly hygroscopic polymer, poly(ethylene glycol) (PEG), from the surface of phosphatidylcholine liposomes results in an osmotic imbalance between the hydration layer of the liposome surface and the bulk polymer solution, thus causing a partial dehydration of the phospholipid polar headgroups. PEG (average molecular weight of 6000 and in concentrations ranging from 5 to 20%, w/w) was added to the outside of large unilamellar liposomes (LUVs). This leads to, in addition to the dehydration of the outer monolayer, an osmotically driven water outflow and shrinkage of liposomes. Under these conditions phase separation of the fluorescent lipid 1-palmitoyl-2[6-(pyren-1-yl)]decanoyl-sn-glycero-3-phosphocholine (PPDPC) embedded in various phosphatidylcholine matrices was observed, evident as an increase in the excimer-to-monomer fluorescence intensity ratio (IE/IM). Enhanced segregation of the fluorescent lipid was seen upon increasing and equal concentrations of PEG both inside and outside of the LUVs, revealing that osmotic gradient across the membrane is not required, and phase separation results from the dehydration of the lipid. Importantly, phase separation of PPDPC could be induced by PEG also in binary mixtures with 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), 1-stearoyl-2-oleoyl-sn-glycero-3-phosphocholine (SOPC), and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), for which temperature-induced phase segregation of the fluorescent lipid below Tm was otherwise not achieved. In the different lipid matrices the segregation of PPDPC caused by PEG was abolished above characteristic temperatures T0 well above their respective main phase transition temperatures Tm. For 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), DMPC, SOPC, and POPC, T0 was observed at approximately 50, 32, 24, and 20 degrees C, respectively. Notably, the observed phase separation of PPDPC cannot be accounted for the 1 degree C increase in Tm for DMPC or for the increase by 0.5 degrees C for DPPC observed in the presence of 20% (w/w) PEG. At a given PEG concentration maximal increase in IE/IM (correlating to the extent of segregation of PPDPC in the different lipid matrices) decreased in the sequence 1,2-dihexadecyl-sn-glycero-3-phosphocholine (DHPC) > DPPC > DMPC > SOPC > POPC, whereas no evidence for phase separation in 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) LUV was observed (Lehtonen and Kinnunen, 1994, Biophys. J. 66: 1981-1990). Our results indicate that PEG-induced dehydration of liposomal membranes provides the driving force for the segregation of the pyrene lipid. In brief, phase separation of PPDPC from the matrix lipid could be attributed to the diminishing effective size of the phosphatidylcholine polar headgroup resulting from its partial dehydration by PEG. This in turn would allow for enhanced van der Waals interactions between the acyl chains of the matrix lipid, which then caused the exclusion of PPDPC due to the perturbing bulky pyrene moiety. Phase separation in DMPC/PPDPC liposomes was abolished by the inclusion of 25 mol % cholesterol and to a lesser extent by epicholesterol.

Are acidic lipid domains induced by extrinsic protein binding to membranes?

A widely accepted model for the association of extrinsically bound proteins with acidic lipid-containing membranes has been that approach of the protein to the membrane induces a domain of acidic lipids that serves as the protein binding site. This model has been applied to a variety of membrane proteins including those that participate in the proteolytic complex that converts prothrombin to thrombin during the final stages of the blood coagulation cascade. The 'prothrombinase complex' consists of a serine protease (factor Xa), its protein co-factor (factor Va) and the substrate itself (prothrombin), all bound to phosphatidylserine (PS)-containing membranes derived from stimulated platelets. We have used three approaches to test the domain model as it applies to the proteins of this complex. First, phase diagrams describing the mixing of acidic and neutral lipids have failed to provide evidence for extensive acidic lipid domains (on the order of 50 or more lipid molecules) induced by protein biding. Second, pyrene-containing neutral and acidic phospholipids have been used to test for the occurrence of domains of as few as 20-30 lipids associated with binding of the membrane-binding fragment 1 region of prothrombin. Again, no evidence for domains was obtained. Finally, we have shown that binding of these proteins can be described in terms of a generalized model that presumes an acidic-lipid-independent surface adsorption combined with specific binding of acidic lipids to 'm' sites on a protein. Our results suggest that the concept of a protein-induced domain should not be applied indistriminately to explain binding of extrinsic membrane proteins such as the protein kinase C.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Pyrularia thionin binding to and the role of tryptophan-8 in the enhancement of phosphatidylserine domains in erythrocyte membranes

Pyrularia thionin is a small, strongly basic peptide which interacts readily with cellular and synthetic membranes. With cells it induces hemolysis, depolarizes the cellular membrane with an accompanying influx of Ca2+, and activates an endogenous phospholipase A2. Evidence points toward a binding site involving phosphatidylserine (PS). This study shows that addition of the peptide to erythrocyte membranes as well as to vesicles formed from phospholipids isolated from erythrocyte membranes causes an enhancement of phospholipid domains which are made visible by the use of fluorescence digital imaging microscopy with fluorescent derivatives of PS (NBD-PS) and phosphatidylcholine (NBD-PC). Addition of thionin caused a large increase in NBD-PS domains, with an accompanying enrichment of NBD-PC in another separate domain. Double-labeling experiments performed with a Texas Red derivative of thionin show that the peptide binds to the domain enriched in NBD-PS. P thionin inactivated by modification of Trp-8 with N-bromosuccinimide lost the ability to enhance PS domains, although it bound to the membrane with the same affinity as native P thionin. This shows that binding to the membrane is not in itself sufficient to cause the NBD-PS and NBD-PC redistribution into domains.

Effect of bovine prothrombin fragment 1 on the translational diffusion of phospholipids in Langmuir-Blodgett monolayers

Previous work has shown that bovine prothrombin fragment 1 binds to supported planar membranes composed of phosphatidylcholine and phosphatidylserine in a Ca(2+)-specific manner (Tendian et al. (1991) Biochemistry 30, 10991; Pearce et al. (1992) Biochemistry 31, 5983-5995). In the present work, fluorescence pattern photobleaching recovery has been used to examine the effect of membrane-bound fragment 1 on the translational diffusion coefficients of two fluorescent phospholipids in fluid-like phosphatidylserine/phosphatidylcholine Langmuir-Blodgett monolayers. The results show that saturating concentrations of fragment 1, in the presence of Ca2+, reduce the diffusion coefficient of nitrobenzoxadiazolyl-conjugated phosphatidylserine (NBD-PS) and nitrobenzoxadiazolyl-conjugated phosphatidylcholine (NBD-PC) by factors of approximately four and two, respectively. Ca2+ or fragment 1 alone do not have a statistically significant effect on NBD-PS or NBD-PC diffusion. In addition, a nonspecific protein (ovalbumin) does not change the diffusion coefficients of the fluorescent phospholipids either in the absence or presence of Ca2+. The fractions of the fluorescent phospholipids that are laterally mobile are approximately 0.9 for all samples. These results are interpreted with several models for possible mechanisms by which extrinsically bound proteins might retard phospholipid diffusion in membranes.

The interaction of cardiolipin with rat liver carbamoyl phosphate synthetase I

A selective interaction of rat liver carbamoyl phosphate synthetase I with cardiolipin, and other anionic phospholipids, has been demonstrated. The enzymatic activity of the synthetase is inhibited by cardiolipin and, to a lesser extent, by phosphatidylglycerol, phosphatidylinositol, and phosphatidylserine. This group of anionic phospholipids also induced a conformational change in the synthetase, yielding a species with increased exposure of the linkages between independently folded domains of the enzyme, as determined by limited proteolysis under nondenaturing conditions. The interaction of cardiolipin with carbamoyl phosphate synthetase I was a fairly slow process, with complex kinetics, and was apparently irreversible. The inclusion of Mg2+ or of MgATP in the incubation mixture prevented the cardiolipin effects. The zwitterionic phospholipids phosphatidylcholine and phosphatidylethanolamine had negligible effects on the structure and activity of the synthetase. This interaction between cardiolipin and carbamoyl phosphate synthetase I potentially constitutes one of the mechanisms by which the synthetase forms its loose association with the inner mitochondrial membrane. Multiple mechanisms, including synthetase conformational changes, cardiolipin phase changes, and ATP/ADP binding site involvement, are possibly involved in the phospholipid/synthetase interaction and the resulting potential regulatory mechanism(s) for urea cycle activity.

Fourier transform infrared spectroscopic study of Ca2+ and membrane-induced secondary structural changes in bovine prothrombin and prothrombin fragment 1

Fourier transform infrared (FTIR) spectroscopy was used to monitor secondary structural changes associated with binding of bovine prothrombin and prothrombin fragment 1 to acidic lipid membranes. Prothrombin and prothrombin fragment 1 were examined under four different conditions: in the presence of (a) Na2EDTA, (b) 5 mM CaCl2, and in the presence of CaCl2 plus membranes containing 1-palmitoyl-2-oleoyl-3-sn-phosphatidylcholine (POPC) in combination with either (c) bovine brain phosphatidyl-serine (bovPS) or (d) 1,2-dioleoyl-phosphatidylglycerol (DOPG). The widely reported Ca(2+)-induced conformational change in bovine prothrombin fragment 1 was properly detected by our procedures, although Ca(2+)-induced changes in whole prothrombin spectra were too small to be reliably interpreted. Binding of prothrombin in the presence of Ca2+ to procoagulant POPC/bovPS small unilamellar vesicles produced an increase in ordered secondary structures (2% and 3% increases in alpha-helix and beta-sheet, respectively) and a decrease of random structure (5%) as revealed by spectral analysis on both the original and Fourier-self-deconvolved data and by difference spectroscopy with the undeconvolved spectra. Binding to POPC/DOPG membranes, which are less active as procoagulant membranes, produced no detectable changes in secondary structure. In addition, no change in prothrombin fragment 1 secondary structure was detectable upon binding to either POPC/bovPS or POPC/DOPG membranes. This indicates that a membrane-induced conformational change occurs in prothrombin in the nonmembrane-binding portion of the molecule, part of which is activated to form thrombin, rather than in the membrane-binding fragment 1 region. The possible significance of this conformational change is discussed in terms of differences between the procoagulant activities of different acidic lipid membranes.

The effect of factor Va on lipid dynamics in mixed phospholipid vesicles as detected by steady-state and time-resolved fluorescence depolarization of diphenylhexatriene

We have monitored the thermotropic behavior of mixed dimyristoylglycerophosphoserine (Myr2GroPSer)/dimyristoylglycerophosphocholine (Myr2GroPCho) and Myr2GroPSer/dipalmitoylglycerophosphocholine (Pam2GroPCho) vesicles in the presence of blood-clotting factor Va, using 1,6-diphenyl-1,3, 5-hexatriene as a lipid probe. The Ca2+-independent interaction of factor Va with these vesicles caused a small increase (1-2 degrees C) in the phase transition temperature, regardless of whether Myr2GroPChe was the lower or higher-melting component of the mixed vesicles. The major effect of factor Va was to increase the polarization of diphenylhexatriene when the mixed vesicles were in the liquid crystalline phase. The protein did not change the anisotropy in the bilayer gel state. The increase in the polarization value above the transition temperature closely correlated with the amount of phospholipid-bound factor Va, as verified by a direct binding technique. In addition, we found that the affinity of factor Va for Myr2GroPSer/Myr2GroPCho and Myr2GroPSer/Pam2GroPCho greatly increased at temperatures above the transition temperatures. Time-dependent fluorescence anisotropy measurements of diphenylhexatriene embedded in vesicles in the liquid crystalline state give fluorescence decay curves which can best be fitted by two exponential functions with two rotational correlation times and a constant term. Vesicles composed of Myr2GroPSer exhibit more ordering than Myr2GroPCho vesicles. However, the order parameter of mixed vesicles composed of 40% Myr2GroPSer and 60% Myr2GroPCho (mol/mol) approached that of Myr2GroPCho. Factor Va dramatically increased the longer rotational correlation time of diphenylhexatriene embedded in mixed vesicles in the liquid crystalline state from 3.7 ns to about 17 ns. The second rank-order parameter increased only slightly, but the calculated steady-state anisotropy increased by twofold. These results indicate that the acidic phospholipid-dependent binding of factor Va to mixed vesicles has an ordering effect on the acyl chains of the acidic phospholipids in the outer layer, but leaves the bulk of the phospholipids, mainly phosphatidylcholine, unaltered. None of the factor-Va-induced alterations in the anisotropy parameters point to the occurrence of lateral phase separation.

Topology of the binding site of blood-clotting factors in model membranes. A fluorescence study

Factors II, X and IX are blood-clotting proteins which bind to phospholipid interfaces in the presence of Ca2+ to activate coagulation. The topology of their binding site on the membrane was investigated in two ways. First, the transition temperature changes of equimolar mixtures of dipalmitoylglycerophosphocholine/phosphatidylserine and dimyristoylglycerophosphocholine/dipalmitoylglycerophosphoserin e were examined by the fluorescence polarization of 1,6-diphenylhexatriene. Results show that Ca2+ triggers a shift of about 3-4 degrees C and that blood-clotting factors further increase this shift by about 1.5 degree C. This suggests that in the gel phase, Ca2+ induces some aggregation of the phosphatidylserine molecules which is reinforced by blood proteins. Second, isothermal energy transfer experiments were performed with natural lipids in their fluid phase. The tryptophan residues of the factors were the energy donors, and pyrene covalently bound to a fatty acid chain of either phosphatidylcholine or phosphatidic acid was the energy acceptor. These pyrene-phospholipids probe either the neutral or the acidic component of phospholipid mixtures. It is concluded that the binding sites of the factors are constituted by both types of lipids and that their composition depends on the membrane. Factor II exhibits some specificity for acidic phospholipids and seems to be surrounded by non-interacting zwitterionic lipids. Factor IX appears to be surrounded by statistically the same amount of charged and zwitterionic lipids. We also demonstrate that binding can also occur without Ca2+. This Ca2+-independent binding probably involves electrostatic and hydrophobic forces but its physiological significance remains to be elucidated.

Calcium-dependent and calcium-independent interactions of prothrombin fragment 1 with phosphatidylglycerol/phosphatidylcholine unilamellar vesicles

We have measured the phase behavior of mixed dipentadecanoylphosphatidylglycerol (DC15PG)/dimyristoylphosphatidylcholine (DMPC) small unilamellar vesicles (SUV) in the presence of saturating (greater than 98% occupancy of binding sites) concentrations of bovine prothrombin fragment 1 and 5 mM Ca2+. Binding of fragment 1 in the presence of Ca2+ was verified by an increase in 90 degrees light scattering. Only in the cases of DC15PG/DMPC SUV below their phase transition and of pure DMPC SUV were such light scattering measurements not reversible upon addition of ethylenediaminetetraacetic acid to complex Ca2+. Phase-behavior changes of DC15PG/DMPC SUV as monitored by diphenylhexatriene fluorescence anisotropy occurred in concert with the binding of fragment 1. The major effects of peptide binding on SUV phase behavior were to raise the phase-transition temperature by 2-15 degrees C, depending on vesicle composition, and, in general, to make the phase diagram for these small vesicles closely resemble that of large vesicles. No evidence was obtained for the existence of lateral membrane domains with distinct compositions induced by the binding of prothrombin fragment 1 plus Ca2+. Surprisingly, fragment 1 without Ca2+ also altered the phase behavior of DC15PG/DMPC SUV. Most striking was the effect of fragment 1 (with or without Ca2+) on DMPC SUV phase behavior. Freeze-fracture electron microscopy demonstrated that pure DMPC vesicles were induced to fuse in the presence of fragment 1, while vesicles containing DC15PG remained intact. The rate of DMPC SUV fusion (followed by 90 degrees light scattering) increased with increasing fragment 1 concentration but was not saturable up to 40 microM fragment 1, suggesting a weak, nonspecific interaction between fragment 1 and the neutral phospholipid vesicle.(ABSTRACT TRUNCATED AT 250 WORDS)

Ca2+-induced phase separation in black lipid membranes and its effect on the transport of a hydrophobic ion

Voltage jump-current relaxation studies have been performed with dipicrylamine-doped black membranes of binary lipid mixtures. As in the case of the carrier-mediated ion transport (Schmidt, G., Eibl, H. and Knoll, W. (1982) J. Membrane Biol. 70, 147-155) no evidence was found that the neutral lipid phosphatidylcholine (DPMPC) and the charged phosphatidic acid (DPMPA) are heterogeneously distributed in the membrane over the whole range of composition. However, besides a continuous dilution of the surface charges of DPMPA by the addition of DPMPC molecules, different structural properties of mixed membranes influence the kinetics of the dipicrylamine transport. The addition of Ca2+ to the electrolyte induces a lipid phase separation within the membrane into two fluid phases of distinctly different characteristics of the translocation of hydrophobic ions. Thus, it is possible to determine a preliminary composition phase diagram for the DPMPA/DPMPC mixtures as a function of the Ca2+ concentration.

Prothrombin fragment 1-membrane interactions: a calcium-43 NMR study

The 43Ca NMR spectra are reported for solutions of prothrombin fragment 1 in the presence and absence of phospholipid. The calcium NMR spectrum permits distinction between three thermodynamic classes of calcium-binding interactions. The calcium ion in the lipid-free solutions was labile, with maximum residence times estimated for the average protein site in the range of 0.5-1 ms. The calcium spectrum was sensitive to the protein association and the addition of phospholipid, which appears to sharpen the calcium specificity for the protein sites. The calcium NMR spectra in the presence of phospholipid are similar to those in lipid-free solutions, which suggests that the calcium ion remains labile in the lipid-protein complex.

Interaction of prothrombin with factor Va-phospholipid complexes

The effects of factor Va and the phospholipid-binding fragment of factor Va [factor Va light chain (LC), Mr 80000] on the binding of prothrombin, factor X, and factor Xa to phospholipid vesicles are reported. Equilibrium binding experiments were performed that utilized large-volume vesicles, which can be removed from the bulk solution by centrifugation. Factor Va decreased the dissociation constant of the prothrombin-phospholipid complex 50-fold, from 2.0 X 10(-7) M to 4.0 X 10(-9) M. For the factor X-phospholipid complex the decrease was 60-fold (1.8 X 10(-7) M to 3.0 X 10(-9) M) and for factor Xa, 160-fold (1.6 X 10(-7) M to 1.0 X 10(-9) M). The ratios of moles of protein bound to moles of total added factor Va at saturation of phospholipid-bound factor Va indicate an 1:1 stoichiometric complex of either factor Xa, factor X, or prothrombin and phospholipid-bound factor Va. In the presence of factor Va LC, the dissociation constants of factor Xa- and prothrombin-phospholipid complexes were increased, while the maximal protein-binding capacities of the vesicles were not affected by factor Va LC. The data suggest a competitive interaction between factor Xa and factor Va LC binding as well as between prothrombin and factor Va LC binding at the phospholipid surface. From this, it is concluded that the phospholipid-binding fragment of factor Va alone does not serve as the binding site for interactions of factor Xa and prothrombin with factor Va.

A theoretical approach to cluster formation in biological membranes

A theoretical analysis of cluster formation within the lipid matrix of biological membranes is presented. Various models are analysed: (a) one-dimensional monolayer, (b) two-dimensional monolayer and (c) one dimensional bilayer. Furthermore, lipid-protein interactions are considered. The model is based on differential equations for the probabilities ai and bi which characterize the occupation of the lattice site i by the lipids A and B, respectively. These differential equations are an approximation of the Master-equation. Steady states as well as time-dependent variations are analysed. Depending on the interaction energies of the two lipids, different stationary lipid distributions are obtained, including clusters of lipids A or B and alternating structures. The distributions may be dynamically stable or unstable. It is shown that phase transitions within the lipid matrix may be induced by alteration of the composition of the membrane, by changing the interaction energies of the lipids, by variation of the temperature or by lipid-protein interactions. The transitions between different stationary distributions are studied by use of bifurcation diagrams. The analysis of time-dependent states reveals that unstable structures of the membrane may be important for certain time periods. Consideration of the lipid bilayer leads to a great number of possible distributions, which may be symmetric or asymmetric with respect to the outer and inner leaflets of the membrane.

Lipid domain structures in biological membranes

Phase separation represents a possibility for segregation of lipidic membrane components into structurally distinct domains. Freeze-fracture electronmicroscopy is a useful method for detection of lipid domains. Indications of a possible domain-nature of structures are a regular pattern within a separated area, a regular outline of such an area and a local modulation of curvature (evagination or invagination). Candidates for domain structures in biological membranes are smooth particle-free areas and arrays of regularly arranged particles. The interpretation of the particle-free areas is more reliable than that of the arrays with regularly arranged particles. Phase separation in biological membranes can be induced experimentally by lowering the temperature, but physiologically the isothermically induced domains are more important. Factors in control of isothermic domain formation are divalent cations, proteins, cholesterol etc. Suggestions on the biological relevance of domain formation concern mainly their role in the mechanism of membrane fusion, but domains in form of transient or stable membrane structures seem to occur also otherwise and disturbances in domain formation or artificially induced domains can be suitable for pathological alterations.