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Biological membranes contain a substantial amount of "nonbilayer lipids", which have a tendency to form nonlamellar phases. In this study the hypothesis was tested that the presence of nonbilayer lipids in a membrane, due to their overall small headgroup, results in a lower packing density in the headgroup region, which might facilitate the interfacial insertion of proteins. Using the catalytic domain of leader peptidase (delta2-75) from Escherichia coli as a model protein, we studied the lipid class dependence of its insertion and binding. In both lipid monolayers and vesicles, the membrane binding of (catalytically active) delta2-75 was much higher for the nonbilayer lipid DOPE compared to the bilayer lipid DOPC. For the nonbilayer lipids DOG and MGDG a similar effect was observed as for DOPE, strongly suggesting that no specific interactions are involved but that the small headgroups create hydrophobic interfacial insertion sites. On the basis of the results of the monolayer experiments, calculations were performed to estimate the space between the lipid headgroups accessible to the protein. We estimate a maximal size of the insertion sites of 15 +/- 7 A2/lipid molecule for DOPE, relative to DOPC. The size of the insertion sites decreases with an increase in headgroup size. These results show that nonbilayer lipids stimulate the membrane insertion of delta2-75 and support the idea that such lipids create insertion sites by reducing the packing density at the membrane-water interface. It is suggested that PE in the bacterial membrane facilitates membrane insertion of the catalytic domain of leader peptidase, allowing the protein to reach the cleavage site in preproteins.

We have examined the influence of surface pressure and phospholipid composition on hydrolysis of phosphatidylinositol (4,5)-bisphosphate (PIP2) by phospholipase C beta 1 (PLC beta 1) and PLC beta 2 in mixed composition phospholipid monolayers. Increasing the monolayer surface pressure from 15 to 36 mN/m reduced the rate at which PIP2 was hydrolyzed by PLC beta 1 and PLC beta 2 by 4-6-fold, although PLC beta 1 was more active than PLC beta 2, even at high surface pressure. Reduced enzyme activity was accompanied by an increase in reaction induction times, suggesting that increasing surface pressure reduced the penetration rate of the enzymes into the monolayer. Quantitation of interfacial enzyme concentration using 35S-labeled PLC beta 1 confirmed that less enzyme was associated with the monolayer at higher pressures. The relationship between PLC activity and substrate concentration was examined at a single surface pressure of 30 mN/m. This relationship was not hyperbolic, and increases in the mole percentage (mol %) of PIP2 in the monolayer resulted in an upwardly-curving increase in PLC activity. Thus, PLC beta 1 activity increased 7-fold and PLC beta 2 activity increased 4-fold when the mol % of PIP2 in the monolayer increased from 17.9% to 29%, increasing further thereafter. Paradoxically, increasing the mol % of PIP2 from 0 to 60% was accompanied by a 3-fold decrease in interfacial enzyme concentrations. Taken together, these data show that the catalytic activity of PLC beta involves some element of penetration of lipid interfaces, and suggest that the organization of the substrate facilitates PLC activity, giving credence to the substrate theory of interfacial activation of phospholipases. We present a hypothesis suggesting that PIP2 molecules coalesce into enriched lateral domains which favor PLC beta activity.

Pulmonary surfactant protein C (SP-C) is a small hydrophobic peptide that is palmitoylated on 2 adjacent cysteine residues. SP-C enhances the adsorption of phospholipids into a monolayer. The function of the acylation is not clear yet. The experiments described in this article were carried out in order to investigate the function of SP-C acylation in (protein-catalyzed) lipid monolayer formation, and in bilayer interactions. Palmitoylated and nonpalmitoylated human recombinant SP-C were used. In addition, a nonacylated SP-C with a Cys--Ser mutations was included in these studies. In Wilhelmy plate experiments using negatively charged, protein-containing phospholipid monolayers and negatively charged vesicles, CaCl2 was required to obtain a maximal insertion rate of lipids into the monolayer. If the negatively charged phospholipids in the monolayer were replaced by neutral phospholipids, CaCl2 was only required to show a maximal SP-C-catalyzed insertion rate (if the molecule is palmitoylated, but not if nonpalmitoylated proteins were added). In pressure area measurements, the palmitoylated protein showed a different change in pressure as a function of the surface area, as compared with the nonpalmitoylated proteins. Circular dichroism experiments showed that all three proteins had a high content of alpha-helix. All three proteins showed a preferential orientation at the air-water interface, but the palmitoylated protein has an orientation which is more parallel to the monolayer than that of the nonpalmitoylated proteins. It is concluded that acylation of SP-C alters structural and physical properties of this protein.

The interaction of the precursor of the chloroplast protein ferredoxin with membrane lipids was studied in monolayer experiments in order to investigate the possible involvement of membrane lipids in the protein translocation process. The precursor efficiently and specifically inserts into a total lipid extract of its biological target the outer envelope membrane of chloroplasts. This interaction is mediated by the transit sequence as it can also be observed for the chemically prepared transit peptide of ferredoxin but neither for the ferredoxin apoprotein nor holoprotein. Interactions with the individual chloroplast lipids, monogalactosyl-diacylglycerol, sulfoquinovosyl-diacylglycerol, and phosphatidylglycerol are predominantly involved which corresponds to the results obtained for transit peptide fragments of the small subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase (van't Hof, R., Demel, R. A., Keegstra, K., and De Kruijff, B. (1991) FEBS Lett. 291, 350-354). No efficient interaction was obtained with digalactosyl-diacylglycerol and phosphatidylcholine, suggesting that a loose lipid headgroup packing due to small lipid headgroups and/or electrostatic repulsions facilitates efficient insertion. The observed preferences for interaction of the precursor and transit peptide of ferredoxin for the chloroplast outer envelope membrane lipid extract and the presequence of cytochrome c oxidase subunit IV for the mitochondrial outer membrane lipid extract indicate that targeting sequence-lipid interactions contribute to organelle-specific protein targeting.

2H and 31P NMR techniques were used to study the effects on acyl chain order and lipid organization of the well-characterized pore-forming domain of colicin A (20-kDa thermolytic fragment of colicin A) upon insertion in model membrane systems derived from the Escherichia coli fatty acid auxotrophic strain K 1059, which was grown in the presence of [11,11-2H2]-labeled oleic acid. Addition of the protein to dispersions of the E. coli total lipid extract, in a 1/70 molar ratio of peptide to lipids, resulted in a large pH-dependent decrease in quadrupolar splitting of the 2H NMR spectra. The decrease of the quadrupolar splitting obtained at the various pH values was correlated with the pH dependence of the insertion of the protein in monolayer films using the same E. coli lipid extracts. The pK governing the perturbing effects on the order of the fatty acyl chains was around 5, in agreement with the values of the pH-dependent conformational changes of the pore-forming domain of colicin A required for membrane insertion as reported by van der Goot et al. [(1991) Nature 354, 408-410]. 31P NMR measurements show that the bilayer organization remains intact upon addition of the protein to dispersions of lipid extract. Surprisingly, 31P NMR measurements as a function of temperature indicate that the pore-forming domain of colicin A even stabilizes bilayer lipid structure at pH 4. Both the large effect of the protein on acyl chain order and its bilayer-stabilizing activity are indicative of a surface localization of the protein.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of cholesteryl oleate on the lipoprotein lipase-catalyzed hydrolysis of trioleoylglycerol was determined in monolayers of egg phosphatidylcholine at a constant surface pressure of 24 mN m-1. The phospholipid monolayers contained 1.0 to 7.5 mol % trioleoylglycerol and various amounts (0 to 20 mol %) of cholesteryl oleate. The initial rates of trioleoylglycerol hydrolysis were determined with lipoprotein lipase purified from bovine milk. In phospholipid monolayers containing 5.0 or 7.5 mol % trioleoylglycerol, the further addition of cholesteryl oleate caused a decrease in lipoprotein lipase activity. In contrast, addition of cholesteryl oleate to phospholipid monolayers containing 1.0 or 2.5 mol % trioleoylglycerol enhanced enzyme activity; a 3-fold enhancement was observed with 5.0-7.5 mol % cholesteryl oleate. Based on force-area measurements, the cholesteryl ester-mediated decrease in lipoprotein lipase activity observed at high substrate concentrations may be explained by displacement of trioleoylglycerol from the interface, thereby reducing the interfacial trioleoylglycerol concentration available for enzyme catalysis. One explanation for the cholesteryl oleate-mediated enhancement of lipoprotein lipase activity at low trioleoylglycerol concentrations is that the additional spreading of cholesteryl oleate disrupts microemulsions of trioleoylglycerol, thereby increasing the effective monomer substrate concentration available for enzyme catalysis. Based on these monolayer studies with model systems, we suggest that the relative amount of cholesteryl esters in plasma triacylglycerol-rich lipoproteins plays a regulatory role in determining the rate at which triacylglycerols are cleared from the circulation.

1. 1. Exogenous 1-palmitoyl-sn-glycero-3-phosphorylcholine becomes incorporated into the membrane of A. castellanii within 2 min of incubation. 2. 2. Homogenates of A. castellanii are shown to contain phospholipase A activity. 3. 3. The phospholipase A activity is dependent on the population density of the culture. 4. 4. An increased phospholipase A activity in older cultures of A. castellanii, due to an increased population density, induces aggregation of these cells in a way that communicative contacts are formed. 5. 5. At pH 5.5, the pH value at which the cells were grown, the phospholipase A activity is stimulated by cAMP; at pH 8.0 no stimulation occurs.

The monolayer properties and thermal behavior of different phosphatidylserines are presented. At neutral pH and 22 degrees C, saturated phosphatidylserines form condensed monolayers while unsaturated phosphatidylserines form liquid-expanded films. Under similar conditions, dimyristoylphosphatidylserine undergoes a transition from the liquid-expanded to the condensed state. At pH 4 and 22 degrees C, the surface pressure-area isotherms are shifted to smaller areas relative to the monolayers recorded at neutral pH. The condensation observed at pH 4 is close to that produced at pH 7.4 by the addition of 10 mM CaCl2. As regards the molecular packing in monolayers and the thermal behavior, 1,2-dipalmitoyl-sn-glycero-3-phospho-L-serine (DPPS) and its ether analogue are similar, albeit not identical. Below 30 mN/m, monolayers of the ether analogue are even more condensed than those of DPPS. The order-disorder transition of the ether analogue occurs usually at higher temperatures than that of the diacyl compound. Sonicated phosphatidylserine dispersions consisting of small unilamellar vesicles show anomalous thermal properties compared to sonicated phosphatidylcholine dispersions. They exhibit sharp order-disorder transitions at similar or even slightly elevated temperatures compared to unsonicated phosphatidylserine dispersions. This anomaly is explained in terms of a pH gradient across the bilayer membrane of the small unilamellar phosphatidylserine vesicle. The internal surface pH is more acidic than the external pH, leading to some protonation of phosphatidylserine molecules. This in turn leads to a condensation of phosphatidylserine molecules on the inner bilayer surface. Such a gradient is proposed to be responsible for the thermodynamic stability of highly curved negatively charged bilayer vesicles.

A recent investigation of Kogl, Salemink and Schuller established the constitution of muscaridin as the quaternary trimethylammonium salt of 6-amino-2,3-dihydroxyhexane. The present communication gives a preliminary description of a synthesis which started from 2,3-dihydropyran, leading to racemic muscaridin. The synthetic product gives the same RF-value and a fully similar infrared spectrum as the natural base.

The behavior of the chemically synthesized PhoE signal peptide and signal peptide fragments on hydrophilic-hydrophobic interfaces was studied with circular dichroism and monolayer techniques. The experimental results were compared with computer-calculated predictions of peptide structure, orientation, and molecular area. The complete signal sequence was found to aggregate in a beta-sheet structure when introduced in an aqueous environment; on the other hand, in sodium dodecyl sulfate micelles approximately 75% alpha-helical structure was observed. Assuming this to reflect the actual structure in a peptide monolayer and taking into account the orientations predicted for the fragments, the measured molecular areas suggest a looped orientation of the signal sequence with both N and C terminus in the water phase.

The interaction of the cholinergic (nicotinic) receptor protein purified from Torpedo marmorata electric organ with lipidic monomolecular films is studied at constant film area or constant superficial pressure. In both experimental situations, it is incorporated more efficiently into cholesterol than into egg lecithin films without showing any selectivity for a particular phospholipid class.

The importance of the various regions of the apocytochrome c molecule and the effect of covalent coupling of the heme group to the protein for the translocation across a model-membrane was studied by using chemically and enzymatically prepared fragments of horse heart apo- and holocytochrome c and model-membranes composed of phosphatidylserine and phosphatidylcholine. Binding experiments showed that fragments of apocytochrome c with the highest net positive charge have the highest affinity for negatively charged large unilamellar phosphatidylserine vesicles. Monolayer experiments demonstrated that the amino-terminal fragments were only able to penetrate into a phosphatidylserine monolayer whereas the carboxyl-terminal fragments, in addition, penetrated into a phosphatidylcholine monolayer although with a lower efficiency. The covalent coupling of the heme group to both a small amino-terminal fragment residue numbers 1-38 and to the entire precursor protein resulted in a marked decrease in the ability to penetrate into a phosphatidylserine monolayer. Translocation experiments with trypsin enclosed in vesicles, showed that only the amino-terminal fragments of the precursor protein and not carboxyl-terminal peptides or the heme-containing fragments of the mature protein were able to cross the bilayer and become digestible by trypsin at the opposite side of the bilayer. Circular dichroism measurements with the various peptides both in an aqueous and lipidic environment were performed to investigate the conformation of apocytochrome c after interaction with model-membranes. Implications of these data for the import of apocytochrome c into mitochondria will be discussed.

The lipoprotein lipase-catalyzed hydrolysis of triacylglycerol was determined in a lipid monolayer containing egg phosphatidylcholine and tri[14C]oleoylglycerol. In the presence of purified bovine milk lipoprotein lipase and fatty acid-free albumin, the rate of hydrolysis of tri[14C]oleoylglycerol, as determined by the decrease in surface activity, was dependent upon enzyme concentration and was enhanced by the addition of apolipoprotein C-II, the activator protein for the enzyme. Increasing the triacylglycerol content of the phospholipid monolayer from 1 to 6 mol% (relative to phospholipid) enhanced the rate of catalysis in the presence and absence of apolipoprotein C-II. However, at low substrate concentrations (less than 4 mol% tri[14C]oleoylglycerol), the activation factor for apolipoprotein C-II was greater than at high (4-6 mol%) triacylglycerol concentrations. The addition of sphingomyelin to the phosphatidylcholine monolayer decreased lipoprotein lipase activity. Based on these monolayer studies, we conclude that lipoprotein lipase catalyzes the hydrolysis of triacylglycerol at a phospholipid interface and that the rate of catalysis is dependent on the lipid composition of the monolayer.

Monolayers have proved to be a valuable model system for studying the physical properties of membrane constituents and the dynamic properties of biological membranes.

Current concepts on the structure of biological membranes depict the membrane as a liquid crystalline lipid bilayer with embedded protein structures. It is assumed that the proteins are responsible for specific properties as signal transduction and membrane transport whereas the lipids provide a fluid matrix and impermeable barrier. Although one suitable type of lipid could accomplish the requirements of a fluid matrix biological membranes are built of complex mixtures of lipids, showing variations in polar headgroups, charge, length and unsaturation of paraffin chains and sterol content. Large differences in composition can be noted between membranes with different functions. This raises the question whether a complex lipid composition is a funcitonal requirement and if lipids contribute in a more specific way to membrane functions.

(1) The interaction of synthetic and natural phospholipids with spectrin, purified from human erythrocyte membranes, was studied using the monolayer technique at constant surface pressure. Spectrin penetration into the lipid monolayer was recorded as the rate of surface area increase on a two-compartment trough. (2) High spectrin penetration rates were observed with negatively charged phospholipids while zwitterionic or neutral lipids showed only poor spectrin affinity. This penetration rate was strongly affected by the subphase pH. At pH 5.5, maximal pentration rates wre obsreved for phosphatidylglycerol and phosphatidylserine but not for phosphatidylcholine. (3) In comparing the penetration rates for phospholipids with a natural fatty acid composition and the dimyristoyl species of phosphatidic acid, phosphatidylglycerol, phosphatidylserine and phosphatidylcholine, the lipid fatty acid composition proved to be an important parameter. The differences are collelated with the area per lipid molecule. (4) Other parameters affecting the area per lipid molecule such as surface pressure, pH and salt concentration also strongly influenced spectrin penetration rates for negatively charged phospholipids. Spectrin penetration into phosphatidylcholine monolayers is only slightly affected by variation of these conditions. (5) The effect of Ca2+ on spectrin-lipid interactions was studied for several phosphatidylglycerol and phosphatidylserine species. Both lipids condensed upon the addition of Ca2+, but only in the case of the phosphatidyleserine was this accompanied by extrusion of the spectrin from the interface, which is in agreement with earlier calorimetric experiments with bilayer systems of analogous composition (Mombers, C., Verkleij, A.J., de Gier, J. and van Deenen, L.L.M. (1979) Biochim. Biophys. Acta 551, 271-281). For this phenomenon a model is presented.

The effect of cholesteryl oleate on the lipoprotein lipase-catalyzed hydrolysis of trioleoylglycerol was determined in monolayers of egg phosphatidylcholine at a constant surface pressure of 24 mN m-1. The phospholipid monolayers contained 1.0 to 7.5 mol % trioleoylglycerol and various amounts (0 to 20 mol %) of cholesteryl oleate. The initial rates of trioleoylglycerol hydrolysis were determined with lipoprotein lipase purified from bovine milk. In phospholipid monolayers containing 5.0 or 7.5 mol % trioleoylglycerol, the further addition of cholesteryl oleate caused a decrease in lipoprotein lipase activity. In contrast, addition of cholesteryl oleate to phospholipid monolayers containing 1.0 or 2.5 mol % trioleoylglycerol enhanced enzyme activity; a 3-fold enhancement was observed with 5.0-7.5 mol % cholesteryl oleate. Based on force-area measurements, the cholesteryl ester-mediated decrease in lipoprotein lipase activity observed at high substrate concentrations may be explained by displacement of trioleoylglycerol from the interface, thereby reducing the interfacial trioleoylglycerol concentration available for enzyme catalysis. One explanation for the cholesteryl oleate-mediated enhancement of lipoprotein lipase activity at low trioleoylglycerol concentrations is that the additional spreading of cholesteryl oleate disrupts microemulsions of trioleoylglycerol, thereby increasing the effective monomer substrate concentration available for enzyme catalysis. Based on these monolayer studies with model systems, we suggest that the relative amount of cholesteryl esters in plasma triacylglycerol-rich lipoproteins plays a regulatory role in determining the rate at which triacylglycerols are cleared from the circulation.

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.

An immune response to the peripheral nerve basic protein P2 may be operative in the pathogenesis of the Guillain-Barré syndrome (GBS). A method is described for the purification of P2 of human origin. Purified P2 was used to investigate whether lymphocytes derived from peripheral blood of GBS patients are capable of producing P2-specific antibodies after stimulation with the antigen in vitro. Peripheral blood lymphocytes (PBL) from 5 GBS patients, from 3 patients with chronic idiopathic polyradiculoneuropathy (CIP) and from 3 normal controls were cultured in vitro in the presence of P2. PBL from the 5 GBS patients were shown to generate an antigen (P2)-specific antibody response. Contrariwise, PBL from the 3 CIP patients as well as from the 3 normal controls did not show this specific response.

Purified mitochondrial creatine kinase (Mi-CK) (EC 2.7.3.2) from chicken heart was shown to interact simultaneously with purified inner and outer mitochondrial membranes, thereby creating an intermembrane chondrial membranes, thereby creating an intermembrane were purified from rat liver and thus were fully devoid of Mi-CK. Intermembrane contact formation was demonstrated by measuring the binding of inner membrane vesicles to outer membranes spread at the air-water interface. Mi-CK also mediated intermembrane adhesion when membranes formed with total lipid extracts of both membranes were used, pointing to the role of lipids as potential membrane anchors of Mi-CK in the mitochondrial intermembrane space. Other enzymes of the intermembrane space that (like Mi-CK) are also cationic, as well as cytosolic isoenzymes of creatine kinase, failed to induce contact formation. Thus, of the proteins tested, membrane contact formation was specific for Mi-CK. The two oligomeric forms of Mi-CK (octamer and dimer) differed in their ability to mediate intermembrane adhesion, the octamer being more potent. Highly basic peptides, i.e. poly-L-lysines, were shown to strongly interact with membranes formed with lipid extracts of mitochondrial membranes: they both induced intermembrane binding and fusion. Interestingly, the extent of contact formation mediated by poly-L-lysines was lower than that of octameric Mi-CK. The implications of these findings on the function and localization of Mi-CK and on the structure of the mitochondrial intermembrane compartment are discussed.