Chlorpromazine associates with phosphatidylserines to cause an increase in the lipid's own interfacial molecular area--role of the fatty acyl composition

Partition coefficients of the drug chlorpromazine were determined for five different molecular species of diacylglycerophosphatidylserine in a monolayer kept at constant surface pressure (20 mN/m). Two models of adsorption of chlorpromazine in phosphatidylserine monolayers were compared. The first model correlated the amount of inserted drug molecules with the induced increase in area. The second model introduced the effect of drug adsorption on the lipid's own area by comparing the effect of increasing temperature on the lipid's own interfacial area. From the second model, the extrapolated work of insertion of one drug molecule per lipid molecule in a monolayer kept at 20 mN/m was correlated to the partition of the drug in liposomes. The work of insertion of chlorpromazine was insignificant in the unsaturated dioleoylphosphatidylserine and was maximum in the saturated distearoylphosphatidylserine monolayers. The presence of one double bond in the acyl chains dramatically reduces the work of insertion of chlorpromazine between lipid molecules and also reduces the effect chlorpromazine induces on the lipids own interfacial area in monolayers.     

The mitochondrial precursor protein apocytochrome c strongly influences the order of the headgroup and acyl chains of phosphatidylserine dispersions. A 2H and 31P NMR study

Deuterium and phosphorus nuclear magnetic resonance techniques were used to study the interaction of the mitochondrial precursor protein apocytochrome c with headgroup-deuterated (dioleoylphosphatidyl-L-[2-2H1]serine) and acyl chain deuterated (1,2-[11,11-2H2]dioleoylphosphatidylserine) dispersions. Binding of the protein to dioleoylphosphatidylserine liposomes results in phosphorus nuclear magnetic resonance spectra typical of phospholipids undergoing fast axial rotation in extended liquid-crystalline bilayers with a reduced residual chemical shift anisotropy and an increased line width. 2H NMR spectra on headgroup-deuterated dioleoylphosphatidylserine dispersions showed a decrease in quadrupolar splitting and a broadening of the signal on interaction with apocytochrome c. Addition of increasing amounts of apocytochrome c to the acyl chain deuterated dioleoylphosphatidylserine dispersions results in the gradual appearance of a second component in the spectra with a 44% reduced quadrupolar splitting. Such large reduction of the quadrupolar splitting has never been observed for any protein studied yet. The lipid structures corresponding to these two components could be separated by sucrose gradient centrifugation, demonstrating the existence of two macroscopic phases. In mixtures of phosphatidylserine and phosphatidylcholine similar effects are observed. The induction of a new spectral component with a well-defined reduced quadrupolar splitting seems to be confined to the N-terminus since addition of a small hydrophilic amino-terminal peptide (residues 1-38) also induces a second component with a strongly reduced quadrupolar splitting. A chemically synthesized peptide corresponding to amino acid residues 2-17 of the presequence of the mitochondrial protein cytochrome oxidase subunit IV also has a large perturbing effect on the order of the acyl chains, indicating that the observed effects may be a property shared by many mitochondrial precursor proteins. In contrast, binding of the mature protein, cytochrome c, to acyl chain deuterated phosphatidylserine dispersions has no effect on the deuterium and phosphorus nuclear magnetic resonance spectra, thereby demonstrating precursor-specific perturbation of the phospholipid order. The inability of holocytochrome c to perturb the phospholipid order is due to folding of this protein, since unfolding of cytochrome c by heat or urea treatment results in similar effects on dioleoylphosphatidylserine bilayers, as observed for the unfolded precursor. Implications of these data for the import of apocytochrome c into mitochondria will be discussed.     

Studies on the lipid dependency and mechanism of the translocation of the mitochondrial precursor protein apocytochrome c across model membranes

The lipid dependency of apocytochrome c binding to model membranes and of the translocation of the precursor protein across these membranes was studied by using large unilamellar, trypsin-containing vesicles. These vesicles were improved with respect to those used in a previous article (Rietveld, A., and de Kruijff, B. (1984) J. Biol. Chem. 259, 6704-6706), in the sense that a lower amount of trypsin was enclosed. In mixed egg phosphatidylcholine/bovine brain phosphatidylserine vesicles, both the Kd of apocytochrome c binding (about 20 microM) and the number of phosphatidylserine molecules interacting with the protein was found to be constant. When the phosphatidylserine fraction in the vesicles is more than 15-30% apocytochrome c addition results in the exposure of (a part of) the protein to the internal, trypsin-containing vesicle medium, which process we conceive as a translocation event. Also the interaction of apocytochrome c with vesicles composed of phosphatidylcholine and another acidic phospholipid in a 1:1 ratio, leads to the translocation of the protein across the model membrane. The affinity of this binding was found to be in the order cardiolipin greater than phosphatidylglycerol greater than phosphatidylinositol greater than phosphatidylserine. By varying the lipid composition of the vesicles, it could be demonstrated that the translocation requires a fluid bilayer. In addition, protein specificity was shown for the translocation process. Although apocytochrome c-lipid interaction causes vesicle aggregation, fusion by lipid mixing could not be detected. Due to the apocytochrome c-lipid interaction also, protein aggregates and oligomers have been formed. These results will be discussed in the light of a model for translocation of a precursor protein across a model membrane. The relevance for the mitochondrial system will also be discussed.     

Structural investigation of the covalent and electrostatic binding of yeast cytochrome c to the surface of various ultrathin lipid multilayers using x-ray diffraction.

X-Ray diffraction was used to characterize the profile structures of ultrathin lipid multilayers having a bound surface layer of cytochrome c. The lipid multilayers were formed on an alkylated glass surface, using the Langmuir-Blodgett method. The ultrathin lipid multilayers of this study were: five monolayers of arachidic acid, four monolayers of arachidic acid with a surface monolayer of dimyristoyl phosphatidylserine, and four monolayers of arachidic acid acid with a surface monolayer of thioethyl stearate. Both the phosphatidylserine and the thioethyl stearate surfaces were found previously to covalently bind yeast cytochrome c, while the arachidic acid surface electrostatically binds yeast cytochrome c. Meridional x-ray diffraction data were collected from these lipid multilayer films with and without a bound yeast cytochrome c surface layer. A box refinement technique, previously shown to be effective in deriving the profile structures of ultrathin multilayer lipid films with and without electrostatically bound cytochrome c, was used to determine the multilayer electron density profiles. The surface monolayer of bound cytochrome c was readily apparent upon comparison of the multilayer electron density profiles for the various pairs of ultrathin multilayer films plus/minus cytochrome c for all cases. In addition, cytochrome c binding to the multilayer surface significantly perturbs the underlying lipid monolayers.     

Investigations on the insertion of the mitochondrial precursor protein apocytochrome c into model membranes

Different aspects of the interaction of apocytochrome c and model membranes composed of negatively charged lipids, were studied in order to get insight into the nature of this interaction. The effect of the protein on the lipid packing properties are revealed by DSC, ESR and monolayer techniques. These experiments clearly demonstrate that upon electrostatic interaction with the negatively charged phospholipids, apocytochrome c is able to penetrate into the hydrophobic region of the model membrane. In the case of 1,2-dimyristoyl-sn-glycero-3-phosphoglycerol, this results in a perturbation of 160 lipid molecules per apocytochrome c molecule. Most likely, apocytochrome c disrupts the formation of the gel phase and restricts the lipid chain motion above the gel to liquid-crystalline phase transition. Tryptophan fluorescence measurements confirm that at least a part of the protein penetrates into the bilayer, and suggest that after this penetration, the tryptophan (residue no. 59) is located in the glycerol backbone region of the phospholipids. Although the secondary structure of apocytochrome c is predicted to contain about 35% of alpha-helical structure, the CD pattern of an aqueous solution of the protein is featureless. However, negatively charged lipids are able to express this alpha-helical potency in the apocytochrome c, which might be important for the insertion of the protein into lipid membranes.     

Differential interactions of apo- and holocytochrome c with acidic membrane lipids in model systems and the implications for their import into mitochondria

Monomolecular layers of lipid extracts of microsomal, mitochondrial outer and inner membranes, and pure lipid species have been used to measure their interaction with apo- and holocytochrome c. Large differences were observed both with respect to the nature and the lipid specificity of the interaction. The initial electrostatic interaction of the hemefree precursor apocytochrome c with anionic phospholipids is followed by penetration of the protein in between the acyl chains. Apocytochrome c shows similar interactions for all anionic lipids tested. In strong contrast the holoprotein discriminates enormously between cardiolipin for which it has a high affinity and phosphatidylserine and phosphatidylinositol for which it has a much lower affinity. For these latter lipids the interaction with cytochrome c is primarily electrostatic. The cytochrome c-cardiolipin interaction shows several unique features which suggest the formation of a specific complex between the two molecules. These properties account for the preference in interaction of the apoprotein with the lipid extract of the outer mitochondrial membrane over that of the endoplasmic reticulum and the large preference of cytochrome c for the inner over that of the outer mitochondrial membrane lipid extract. Only apocytochrome c was able to induce close contacts between monolayers of the mitochondrial outer membrane lipids and vesicles of mitochondrial inner membrane lipids. Experiments with fragments of both protein and unfolding experiments with cytochrome c revealed that the differences in interaction between the two proteins are mainly due to differences in their tertiary structure and not the presence of the heme group itself. The initial unfolded structure of apocytochrome c is responsible for the high penetrative power of the protein and its ability to induce close membrane contact, whereas the folded structure of cytochrome c is responsible for the specific interaction with cardiolipin. The results are discussed in the light of the apocytochrome c import process in mitochondria and suggest that lipid-protein interactions contribute to targeting the precursor toward mitochondria and are important for its translocation across the outer mitochondrial membrane and the final localization of cytochrome c toward the outside of the inner mitochondrial membrane.     

Apocytochrome c binding to negatively charged lipid dispersions studied by spin-label electron spin resonance

The interaction of apocytochrome c with aqueous dispersions of phosphatidylserine from bovine spinal cord and with other negatively charged phospholipids has been studied as a function of pH and salt concentration by using spin-label electron spin resonance (ESR) spectroscopy and chemical binding assays. The ESR spectra of phospholipids spin-labeled at different positions on the sn-2 chain indicate a generalized decrease in mobility of the lipids, while the characteristic flexibility gradient toward the terminal methyl end of the chain is maintained, on binding of apocytochrome c to phosphatidylserine dispersions. This perturbation of the bulk lipid mobility or ordering is considerably greater than that observed on binding of cytochrome c. In addition, a second, more motionally restricted, lipid component is observed with lipids labeled close to the terminal methyl ends of the chains. This second component is not observed on binding of cytochrome c and can be taken as direct evidence for penetration of apocytochrome c into the lipid bilayer. It is less strongly motionally restricted than similar spectral components observed with integral membrane proteins and displays a steep flexibility gradient. The proportion of this second component increases with increasing protein-to-lipid ratio, but the stoichiometry per protein bound decreases from 4.5 lipids per 12 000-dalton protein at low protein contents to 2 lipids per protein at saturating amounts of protein. Apocytochrome c binding to phosphatidylserine dispersions decreases with increasing salt concentration from a saturation value corresponding to approximately 5 lipids per protein in the absence of salt to practically zero at 0.4 M NaCl.(ABSTRACT TRUNCATED AT 250 WORDS)     

Interactions of cytochrome c and [14C]-carboxymethylated cytochrome c with monolayers of phosphatidylcholine, phosphatidic acid and cardiolipin

1. The interactions between cytochrome c (native and [(14)C]carboxymethylated) and monolayers of phosphatidylcholine, phosphatidic acid and cardiolipin at the air/water interface was investigated by measurements of surface radioactivity, pressure and potential. 2. On a subphase of 10mm-or m-sodium chloride, penetration of cytochrome c into egg phosphatidylcholine monolayers, as measured by an increase of surface pressure, and the number of molecules penetrating, as judged by surface radioactivity, were inversely proportional to the initial pressure of the monolayer and became zero at 20dynes/cm. The constant of proportionality was increased when the cytochrome c was carboxymethylated or decreased when the phospholipid was hydrogenated, but the cut-off point remained at 20dynes/cm. 3. Penetrated cytochrome c could be removed almost entirely by compression of the phosphatidylcholine monolayer above 20dynes/cm. 4. With phosphatidic acid and cardiolipin monolayers on 10mm-sodium chloride the binding of cytochrome c was much stronger and cytochrome c penetrated into films nearing the collapse pressure (>40dynes/cm.). The penetration was partly electrostatically facilitated, since it was decreased by carrying out the reaction on a subphase of m-sodium chloride, and the relationship between the surface pressure increment and the initial film pressure moved nearer to that observed with phosphatidylcholine. 5. Surface radioactivity determinations showed that [(14)C]carboxymethylated cytochrome c was still adsorbed on phosphatidic acid and cardiolipin monolayers after the cessation of penetration. This adsorption was primarily electrostatic in nature because it could be prevented and substantially reversed by adding m-sodium chloride to the subphase and there was no similar adsorption on phosphatidylcholine films. 6. The penetration into and adsorption on the three phospholipid monolayers was examined as a function of the pH of the subphase and compared with the state of ionization of both the phospholipid and the protein, and the area occupied by the latter at an air/water interface. 7. It is concluded that the binding of cytochrome c to phospholipids can only be partially understood by a consideration of the ionic interaction between the components and that subtle conformational changes in the protein must affect the magnitude and stability of the complex. 8. If cytochrome c is associated with a phospholipid in mitochondria then cardiolipin would fulfil the characteristics of the binding most adequately.     

Insertion behavior of the Bacillus thuringiensis Cry4Ba insecticidal protein into lipid monolayers

Toxicity mechanisms of Bacillus thuringiensis Cry insecticidal proteins involve membrane insertion and lytic pore formation in lipid bilayers of the target larval midgut cell membranes. The B. thuringiensis Cry4Ba mosquito-larvicidal protein has been shown to be capable of permeabilizing liposome vesicles and of forming ion channels in planar lipid bilayers. Here, the membrane interaction of the 65-kDa activated Cry4Ba protein with the lipid monolayers, comprising dipalmitoyl phosphatidylcholine, dioleoyl phosphatidylethanolamine, and cholesterol (Chol), was studied using Langmuir-Blodgett technique. The interactions of the Cry4Ba protein with the lipid monolayers were measured from the surface pressure versus area isotherms of the protein-lipid monolayers. The increase in the mean molecular area was demonstrated as an incorporation of the protein into lipid monolayers. The insertion of the Cry4Ba protein was monitored by measuring as an increase of the surface pressure at constant molecular area. For a given monolayer, the membrane insertion of the Cry4Ba reduced as the initial surface pressure increased. The Cry4Ba protein showed a strong preference of an insertion towards a Chol monolayer. In addition, the mixed monolayers of Chol showed an enhanced effect on the insertion kinetics of Cry4Ba into lipid films, suggesting its involvement in the modulation of the protein insertion. These findings provide the first evidence that the Cry4Ba protein is capable of inserting itself into lipid monolayers, depending on the packing density of the monolayers. Our results also indicate that only a limited part of the protein is likely to be involved in the insertion.     

Effect of helper lipids on the interaction of DNA with cationic lipid monolayers studied by specular neutron reflection

The interaction of DNA with monolayers of the cationic lipid dimethyldioctadecylammonium bromide, with/without 50 mol % of a neutral "helper" lipid, either dioleoylphosphatidylethanolamine or cholesterol, has been studied using specular neutron reflection, surface pressure-area isotherms, and Brewster angle microscopy. The amount of DNA bound to the lipid head groups has been comprehensively quantified in the range of 8-39 vol% of DNA with respect to the monolayer composition (monolayers composed of dimethyldioctadecylammonium bromide binding the most DNA and monolayers containing dioleoylphosphatidylethanolamine binding the least) and surface pressure (DNA binding being greatest at highest surface pressures). Surprisingly, regardless of these variables, the thickness of the DNA-containing layer remained approximately constant between 18 and 25 ?. This systematic study is the first direct quantification of the binding of DNA with two different helper-lipid-containing multicomponent monolayers, an important step toward understanding interaction parameters in more realistic models of gene delivery systems.     

Adsorption of bovine prothrombin to spread phospholipid monolayers

The interaction of bovine prothrombin with phospholipids was measured, using as the lipid source monolayers spread at the air-buffer interface. Fluorescence spectroscopy was implemented to determine the equilibrium concentration of free prothrombin in the aqueous subphase of the protein-monolayer suspensions, in a continuous assay system. The increase in surface pressure (pi) from the protein-monolayer adsorption was also measured and, with values of the adsorbed protein concentration (c[s]), was used to calculate dc(s)/d(pi). At a particular phosphatidylserine (PS) content of liquid-expanded (LE) phosphatidylcholine (PC)/PS monolayers, dc(s)/d(pi) was independent of the initial surface pressure (pi[i]), when this latter value exceeded 30 mN/m. However, dc(s)/d(pi) varied significantly with the relative PS content of the monolayer. Values of the equilibrium dissociation constants calculated from the concentration dependence of delta(pi) indicated that the affinity of prothrombin for LE monolayers was higher at higher PS contents and lower packing densities. The affinity of prothrombin for liquid-condensed (LC) PC/PS monolayers was found to be much weaker relative to LE monolayers of similar phospholipid composition. This approach, employing spread monolayers to study prothrombin-phospholipid binding, coupled with a simple and accurate method to determine the free protein concentration in protein-monolayer suspensions, offers significant advantages for the investigation of protein-membrane interaction. The equilibrium characteristics that describe the interaction of prothrombin with the different phospholipid monolayers under various conditions also provide support for previous results which indicated that hydrophobic interactions are involved in the adsorption of vitamin K-dependent coagulation and anticoagulation proteins to model membrane systems.     

The interaction of cytochrome c with monolayers of phosphatidylethanolamine

1. The interaction between [(14)C]carboxymethylated cytochrome c and monolayers of egg phosphatidylethanolamine at the air/water interface has been investigated by measurements of surface radioactivity, pressure and potential. 2. On adding (14)C-labelled cytochrome c to the subphase under monolayers with a surface pressure below 24dynes/cm. there was an initial surface pressure increment as the protein penetrated, followed by an adsorption that could be detected only by a continued increase in the surface radioactivity. 3. Above film pressures of 24dynes/cm. only adsorption was observed, i.e. an increment in surface radioactivity with none in surface pressure. 4. The changes in surface parameters with penetration of cytochrome c added to the subphase were indirectly proportional to the initial pressure of the monolayer. With hydrogenated phosphatidylethanolamine the constant of proportionality was increased but penetration again ceased at 24dynes/cm. 5. On compressing a phosphatidylethanolamine film containing penetrated cytochrome c to 40dynes/cm. only a proportion of the protein was ejected on a subphase of 10mm-sodium chloride, whereas on a subphase of m-sodium chloride nearly all the protein was lost. 6. With both penetration and adsorption only a small proportion of the added cytochrome c interacted with the phospholipid films, and initially the amount bound was proportional to the added protein concentration. There was no evidence of a stoicheiometric relationship between the protein and phospholipid or the build-up of multilayers. The bonded protein was not released by removing cytochrome c from the subphase. 7. The addition of m-sodium chloride to the subphase delays the rate of protein penetration into low-pressure films, but the final surface-pressure increment is not appreciably decreased. In contrast, m-sodium chloride almost completely stops adsorption on to films at all pressures. 8. When sodium chloride is added to the subphase below cytochrome c adsorbed to monolayers at high pressures, so that the final concentration is 1m, only a proportion of the protein is desorbed and this decreases as the time of the interaction increases. This indicates that adsorption is initially electrostatic, followed by the formation of non-ionic bonds. 9. Alteration of the subphase pH under a high-pressure film leads to a steady increase in adsorption from pH3 to 8.5 followed by a rapid fall to zero adsorption at pH11. 10. The penetration into phospholipid monolayers at 10dynes/cm. shows a rate that is consistent with the relative electrostatic status of the two components of the interaction as the subphase pH is varied between 3 and 10.5. The final equilibrium penetration shows a pronounced peak in the increments of surface pressure at pH9.0 although a similar peak is not observed in the surface radioactivity. This indicates that more residues of the protein are penetrating into the film at about this pH. 11. Determinations were made of the electrophoretic mobilities of phosphatidylethanolamine particles both alone and after interaction with cytochrome c. 12. The electrophoretic mobilities of cytochrome c adsorbed on lipid particles showed an isoelectric point below that of cytochrome c. This and the observations on the monolayers suggest that, with cytochrome c, protein-protein interactions are weak compared with other proteins.     

Specificity of the interaction of amino- and carboxy-terminal fragments of the mitochondrial precursor protein apocytochrome c with negatively charged phospholipids. A spin-label electron spin resonance study

The contribution of the various regions of the mitochondrial precursor protein apocytochrome c to the interaction of the protein with phosphatidylserine dispersions has been studied with chemically and enzymatically prepared fragments of horse heart apocytochrome c and phospholipids spin-labeled at different positions of the sn-2 chain. Three amino-terminal heme-less peptides, two heme-containing amino-terminal fragments, one central fragment, and three carboxy-terminal fragments were studied. The electron spin resonance spectra of phospholipids spin-labeled at the C5 position of the fatty acid chain indicate that both amino-terminal and carboxy-terminal fragments of the apocytochrome c molecule cause a restriction of motion of the lipids, whereas the heme-containing peptides and protein have less effect. In addition, a second motionally more restricted lipid component, which is observed for apocytochrome c interacting with phosphatidylserine dispersions containing lipids spin-labeled at the C12 or C14 position [G?rrissen, H., Marsh, D., Rietveld, A., & de Kruijff, B. (1986) Biochemistry 25, 2904-2910], was observed both on binding the carboxy-terminal fragments and on binding of the amino-terminal fragments of the precursor protein. Interestingly, even a small water-soluble peptide consisting of the 24 carboxy-terminal residues gave rise to a two-component spectrum, with an outer hyperfine splitting of the restricted lipid component of 59 G, indicating a considerable restriction of the chain motion. This suggests that both the carboxy- and amino-terminal parts of the protein penetrate into the center of the bilayer and cause a strong perturbation of the fatty acyl chain motion. The implications of these findings for the mechanism of apocytochrome c translocation across membranes are discussed.     

Interaction of a nonspecific wheat lipid transfer protein with phospholipid monolayers imaged by fluorescence microscopy and studied by infrared spectroscopy.

The interaction of a nonspecific wheat lipid transfer protein (LTP) with phospholipids has been studied using the monolayer technique as a simplified model of biological membranes. The molecular organization of the LTP-phospholipid monolayer has been determined by using polarized attenuated total internal reflectance infrared spectroscopy, and detailed information on the microstructure of the mixed films has been investigated by using epifluorescence microscopy. The results show that the incorporation of wheat LTP within the lipid monolayers is surface-pressure dependent. When LTP is injected into the subphase under a dipalmytoylphosphatidylglycerol monolayer at low surface pressure (< 20 mN/m), insertion of the protein within the lipid monolayer leads to an expansion of dipalmytoylphosphatidylglycerol surface area. This incorporation leads to a decrease in the conformational order of the lipid acyl chains and results in an increase in the size of the solid lipid domains, suggesting that LTP penetrates both expanded and solid domains. By contrast, when the protein is injected under the lipid at high surface pressure (> or = 20 mN/m) the presence of LTP leads neither to an increase of molecular area nor to a change of the lipid order, even though some protein molecules are bound to the surface of the monolayer, which leads to an increase of the exposure of the lipid ester groups to the aqueous environment. On the other hand, the conformation of LTP, as well as the orientation of alpha-helices, is surface-pressure dependent. At low surface pressure, the alpha-helices inserted into the monolayers are rather parallel to the monolayer plane. In contrast, at high surface pressure, the alpha-helices bound to the surface of the monolayers are neither parallel nor perpendicular to the interface but in an oblique orientation.     

Parameters modulating the maximum insertion pressure of proteins and peptides in lipid monolayers

The lipid monolayer model membrane is useful for studying the parameters responsible for protein and peptide membrane binding. Different approaches have been used to determine the extent of protein and peptide binding to lipid monolayers. This review focuses on the use of the “maximum insertion pressure” (MIP) to estimate the extent of protein and peptide penetration in lipid monolayers. The MIP data obtained with different proteins and peptides have been reviewed and discussed which allowed to draw conclusions on the parameters modulating the monolayer binding of proteins and peptides. In particular, secondary structure components such as amphipathic α-helices of proteins and peptides as well as electrostatic interactions play important roles in monolayer binding. The MIPs have been compared to the estimated lateral pressure of biomembranes which allowed to evaluate the possible association between proteins or peptides with natural membranes. For example, the MIP of a membrane-anchored protein with a glycosylphosphatidylinositol (GPI) was found to be far below the estimated lateral pressure of biomembranes. This allowed us to conclude that this protein is probably unable to penetrate the membrane and should thus be hanged at the membrane surface by use of its GPI lipid anchor. Moreover, the values of MIP obtained with myristoylated and non-myristoylated forms of calcineurin suggest that the myristoyl group does not contribute to monolayer binding. However, the acylation of a peptide resulted in a large increase of MIP. Finally, the physical state of lipid monolayers can have a strong effect on the values of MIP such that it is preferable to perform measurements with lipids showing a single physical state. Altogether the data show that the measurement of the maximum insertion pressure provides very useful information on the membrane binding properties of proteins and peptides although uncertainties must be provided to make sure the observed differences are significant.     

Interaction of α-thrombin and prethrombin 2, with phosphatidylserine-containing monolayers

Prothrombin activation complex is located at a phospholipid surface on activated platelets. To see whether the thrombin domain of the molecule plays a role in the interaction with lipids, we investigated the direct interaction of human α-thrombin and its precursor prethrombin 2 with phospholipid monolayers of varous compositions (PS/PC). Adsorption of the labeled proteins was determined by surface radioactive measurements. Penetrations of the proteins in the lipid layer was inferred from capacitance variation of the monolayer measured by a palarography. Disulfide bridges reduced at the electrode were determined by cycle voltametry.In all the cases studied, although in different manners thrombin was found both to adsorb and penetrate the lipid layer, whereas prethrombin 2 did not penetrate pure phosphatidylcholine (PC). In the case of thrombin but not of prethrombin 2, penetration is accompanied by S-S reduction which is maximum at 10 per cent of phosphatidylserine (PS). This indicate a different orientation for prethrombin 2 and thrombin in the lipid layer. This observation might be of importance for the comprehension of the architecture of the prothrombin might be of for the regulation of thrombin formation within the complex.     

Cholesterol affects spectrin-phospholipid interactions in a manner different from changes resulting from alterations in membrane fluidity due to fatty acyl chain composition

We previously showed that erythrocyte and brain spectrins bind phospholipid vesicles and monolayers prepared from phosphatidylethanolamine and phosphatidylserine and their mixtures with phosphatidylcholine (Review: A.F. Sikorski, B. Hanus-Lorenz, A. Jezierski, A. R. Dluzewski, Interaction of membrane skeletal proteins with membrane lipid domain, Acta Biochim. Polon. 47 (2000) 565). Here, we show how changes in the fluidity of the phospholipid monolayer affect spectrin-phospholipid interaction. The presence of up to 10%-20% cholesterol in the PE/PC monolayer facilitates the penetration of the monolayer by both types of spectrin. For monolayers constructed from mixtures of PI/PC and cholesterol, the effect of spectrins was characterised by the presence of two maxima (at 5 and 30% cholesterol) of surface pressure for erythroid spectrin, and a single maximum (at 20% cholesterol) for brain spectrin. The binding assay results indicated a small but easily detectable decrease in the affinity of erythrocyte spectrin for FAT-liposomes prepared from a PE/PC mixture containing cholesterol, and a 2- to 5-fold increase in maximal binding capacity (B(max)) depending on the cholesterol content. On the other hand, the results from experiments with a monolayer constructed from homogenous synthetic phospholipids indicated an increase in deltapi change with the increase in the fatty acyl chain length of the phospholipids used to prepare the monolayer. This was confirmed by the results of a pelleting experiment. Adding spectrins into the subphase of raft-like monolayers constructed from DOPC, SM and cholesterol (1/1/1) induced an increase in surface pressure. The deltapi change values were, however, much smaller than those observed in the case of a natural PE/PC (6/4) monolayer. An increased binding capacity for spectrins of liposomes prepared from a "raft-like" mixture of lipids could also be concluded from the pelleting assay. In conclusion, we suggest that the effect of membrane lipid fluidity on spectrin-phospholipid interactions is not simple but depends on how it is regulated, i.e., by cholesterol content or by the chemical structure of the membrane lipids.     

Interactions of adriamycin,cytochrome c,and serum albumin with lipid monolayers containing poly(ethylene glycol)-ceramide

Poly(ethylene glycol)(2000)C(20)ceramide (PEG-Cer) containing monolayers at an air/water interface were characterized by measuring their surface pressure versus area/molecule (pi-A) and surface potential versus area/molecule (Delta V-A) isotherms. The behavior of pi-A as well as Delta V versus lipid density (Delta V-n) and Delta V-pi isotherms for PEG-Cer are in keeping with two transitions of the lipopolymer, starting at pi approximately equal to 9 and 21 mN/m. We also investigated the effects of PEG-Cer on the binding of adriamycin, cytochrome c and bovine serum albumin to monolayers containing varying mole fractions X of PEG-Cer. PEG-Cer impedes the penetration of these ligands into lipid monolayers with similar effects at both X = 0.04 and 0.08. This effect of PEG-Cer depends on the conformation of the lipopolymer and the interactions between the lipid surface and the surface-interacting molecule as well as the size of the latter.     

Activation of protein kinase C in lipid monolayers

The potential of lipid monolayers spread at an air-water interface was investigated as a well defined membrane model able to support protein kinase C (PKC) association and activation. PKC association to a mixed phospholipid film (phosphatidylcholine, phosphatidylserine) could be detected by an increase of the monolayer surface pressure. This association was strikingly dependent upon the presence of submicromolar concentrations of Ca2+. The effect of Ca2+ resulted in an increase of the PKC penetration into the lipid core at a given permissive surface pressure as well as in a marked increase of the critical surface pressure (29-38 dynes/cm) above which the enzyme was excluded from the membrane. Inclusion of diacylglycerol or tetradecanoate phorbol acetate (TPA) did not modify the PKC-monolayer association in a detectable manner. PKC associated to the lipid layer exhibited the expected catalytic property and was fully activated when diacylglycerol or TPA was included in the membrane. PKC activity was highly dependent upon the surface pressure of the lipid monolayer, being optimal between 30 and 35 dynes/cm. Study of the compression isotherm of various diacylglycerol structures revealed that all potent PKC agonists exhibited an expanded liquid phase behavior with collapse pressure below 40 dynes/cm, in contrast to weak activators which showed condensed isotherms with high collapse pressure (approximately equal to 60 dynes/cm). These observations showed that the lipid monolayer system is well adapted to the study of the molecular mechanisms involved in the regulation of PKC activity at a model membrane interface. They are in line with the suggestion of a major role of Ca2+ in the association (translocation) of PKC to membrane in living cell and suggest that diacylglycerol (and TPA) might activate membrane-associated PKC through local change in the surrounding lipid phase organization.