Thermodynamics of lipid-protein interactions: Interaction of apolipoprotein A-II from human plasma high-density lipoproteins with dimyristoylphosphatidylcholine

Assessment of the validity of the double superhelix model for reconstituted high density lipoproteins: a combined computational-experimental approach

For several decades, the standard model for high density lipoprotein (HDL) particles reconstituted from apolipoprotein A-I (apoA-I) and phospholipid (apoA-I/HDL) has been a discoidal particle ∼100 Å in diameter and the thickness of a phospholipid bilayer. Recently, Wu et al. (Wu, Z., Gogonea, V., Lee, X., Wagner, M. A., Li, X. M., Huang, Y., Undurti, A., May, R. P., Haertlein, M., Moulin, M., Gutsche, I., Zaccai, G., Didonato, J. A., and Hazen, S. L. (2009) J. Biol. Chem. 284, 36605-36619) used small angle neutron scattering to develop a new model they termed double superhelix (DSH) apoA-I that is dramatically different from the standard model. Their model possesses an open helical shape that wraps around a prolate ellipsoidal type I hexagonal lyotropic liquid crystalline phase. Here, we used three independent approaches, molecular dynamics, EM tomography, and fluorescence resonance energy transfer spectroscopy (FRET) to assess the validity of the DSH model. (i) By using molecular dynamics, two different approaches, all-atom simulated annealing and coarse-grained simulation, show that initial ellipsoidal DSH particles rapidly collapse to discoidal bilayer structures. These results suggest that, compatible with current knowledge of lipid phase diagrams, apoA-I cannot stabilize hexagonal I phase particles of phospholipid. (ii) By using EM, two different approaches, negative stain and cryo-EM tomography, show that reconstituted apoA-I/HDL particles are discoidal in shape. (iii) By using FRET, reconstituted apoA-I/HDL particles show a 28-34-Å intermolecular separation between terminal domain residues 40 and 240, a distance that is incompatible with the dimensions of the DSH model. Therefore, we suggest that, although novel, the DSH model is energetically unfavorable and not likely to be correct. Rather, we conclude that all evidence supports the likelihood that reconstituted apoA-I/HDL particles, in general, are discoidal in shape.

Water-membrane partition thermodynamics of an amphiphilic lipopeptide: an enthalpy-driven hydrophobic effect

To shed light on the driving force for the hydrophobic effect that partitions amphiphilic lipoproteins between water and membrane, we carried out an atomically detailed thermodynamic analysis of a triply lipid modified H-ras heptapeptide anchor (ANCH) in water and in a DMPC (1,2-dimyristoyl-sn-glycero-3-phosphocholine) bilayer. Combining molecular mechanical and continuum solvent approaches with an improved technique for solute entropy calculation, we obtained an overall transfer free energy of approximately -13 kcal mol(-1). This value is in qualitative agreement with free energy changes derived from a potential of mean force calculation and indirect experimental observations. Changes in free energies of solvation and ANCH conformational reorganization are unfavorable, whereas ANCH-DMPC interactions-especially van der Waals-favor insertion. These results are consistent with an enthalpy-driven hydrophobic effect, in accord with earlier calorimetric data on the membrane partition of other amphiphiles. Furthermore, structural and entropic analysis of molecular dynamics-generated ensembles suggests that conformational selection may play a hitherto unappreciated role in membrane insertion of lipid-modified peptides and proteins.

Thermotropic phase transition in soluble nanoscale lipid bilayers

The role of lipid domain size and protein-lipid interfaces in the thermotropic phase transition of dipalmitoylphosphatidylcholine (DPPC) and dimyristoylphosphatidylcholine (DMPC) bilayers in Nanodiscs was studied using small-angle X-ray scattering (SAXS), differential scanning calorimetry (DSC), and generalized polarization (GP) of the lipophilic probe Laurdan. Nanodiscs are water-soluble, monodisperse, self-assembled lipid bilayers encompassed by a helical membrane scaffold protein (MSP). MSPs of different lengths were used to define the diameter of the Nanodisc lipid bilayer from 76 to 108 A and the number of DPPC molecules from 164 to 335 per discoidal structure. In Nanodiscs of all sizes, the phase transitions were broader and shifted to higher temperatures relative to those observed in vesicle preparations. The size dependences of the transition enthalpies and structural parameters of Nanodiscs reveal the presence of a boundary lipid layer in contact with the scaffold protein encircling the perimeter of the disc. The thickness of this annular layer was estimated to be approximately 15 A, or two lipid molecules. SAXS was used to measure the lateral thermal expansion of Nanodiscs, and a steep decrease of bilayer thickness during the main lipid phase transition was observed. These results provide the basis for the quantitative understanding of cooperative phase transitions in membrane bilayers in confined geometries at the nanoscale.

Visualization and analysis of apolipoprotein A-I interaction with binary phospholipid bilayers

Apolipoprotein A-I (apoA-I) interaction with specific cell lipid domains was suggested to trigger cholesterol and phospholipid efflux. We analyzed here apoA-I interaction with dimyristoylphosphatidylcholine/distearoylphosphatidylcholine (DMPC/DSPC) bilayers at a temperature showing phase coexistence. Solid and liquid-crystalline domains were visualized by two-photon fluorescence microscopy on giant unilamellar vesicles (GUVs) labeled with 6-dodecanoyl-2-dimethyl-amino-naphthalene (Laurdan). A decrease of vesicle size was detected as long as they were incubated with lipid-free apoA-I, together with a shape deformation and a relative enrichment in DSPC. Selective lipid removal mediated by apoA-I from different domains was followed in real time by changes in the Laurdan generalized polarization. The data show a selective interaction of apoA-I with liquid-crystalline domains, from which it removes lipids, at a molar ratio similar to the domain compositions. Next, apoA-I was incubated with DMPC/DSPC small unilamellar vesicles, and products were isolated and quantified. Protein solubilized both lipids but formed complexes relatively enriched in the liquid component. We also show changes in the GUV morphology when cooling down. Our results suggest that the most efficient reaction between apoA-I and DMPC/DSPC occurs in particular bilayer conditions, probably when small fluid domains are nucleated within a continuous gel phase and interfacial packing defects are maximal.

Kinetic stabilization and fusion of apolipoprotein A-2:DMPC disks: comparison with apoA-1 and apoC-1

Denaturation studies of high-density lipoproteins (HDL) containing human apolipoprotein A-2 (apoA-2) and dimyristoyl phosphatidylcholine indicate kinetic stabilization. Circular dichroism (CD) and light-scattering melting curves show hysteresis and scan rate dependence, indicating thermodynamically irreversible transition with high activation energy E(a). CD and light-scattering data suggest that protein unfolding triggers HDL fusion. Electron microscopy, gel electrophoresis, and differential scanning calorimetry show that such fusion involves lipid vesicle formation and dissociation of monomolecular lipid-poor protein. Arrhenius analysis reveals two kinetic phases, a slower phase with E(a,slow) = 60 kcal/mol and a faster phase with E(a,fast) = 22 kcal/mol. Only the fast phase is observed upon repetitive heating, suggesting that lipid-poor protein and protein-containing vesicles have lower kinetic stability than the disks. Comparison of the unfolding rates and the melting data recorded by differential scanning calorimetry, CD, and light scattering indicates the rank order for the kinetic disk stability, apoA-1 > apoA-2 > apoC-1, that correlates with protein size rather than hydrophobicity. This contrasts with the tighter association of apoA-2 than apoA-1 with mature HDL, suggesting different molecular determinants for stabilization of model discoidal and plasma spherical HDL. Different effects of apoA-2 and apoA-1 on HDL fusion and stability may reflect different metabolic properties of apoA-2 and/or apoA-1-containing HDL.

Apolipoprotein A-II regulates HDL stability and affects hepatic lipase association and activity

The effect of apolipoprotein A-II (apoA-II) on the structure and stability of HDL has been investigated in reconstituted HDL particles. Purified human apoA-II was incorporated into sonicated, spherical LpA-I particles containing apoA-I, phospholipids, and various amounts of triacylglycerol (TG), diacylglycerol (DG), and/or free cholesterol. Although the addition of PC to apoA-I reduces the thermodynamic stability (free energy of denaturation) of its alpha-helices, PC has the opposite effect on apoA-II and significantly increases its helical stability. Similarly, substitution of apoA-I with various amounts of apoA-II significantly increases the thermodynamic stability of the particle alpha-helical structure. ApoA-II also increases the size and net negative charge of the lipoprotein particles. ApoA-II directly affects apoA-I conformation and increases the immunoreactivity of epitopes in the N and C termini of apoA-I but decreases the exposure of central domains in the molecule (residues 98-186). ApoA-II appears to increase HL association with HDL and inhibits lipid hydrolysis. ApoA-II mildly inhibits PC hydrolysis in TG-enriched particles but significantly inhibits DG hydrolysis in DG-rich LpA-I. In addition, apoA-II enhances the ability of reconstituted LpA-I particles to inhibit VLDL-TG hydrolysis by HL. Therefore, apoA-II affects both the structure and the dynamic behavior of HDL particles and selectively modifies lipid metabolism.

Phospholipid phase transitions in homogeneous nanometer scale bilayer discs

Nanoscale protein supported phospholipid bilayer discs, or Nanodiscs, were produced for the purpose of studying the phase transition behavior of the incorporated lipids. Nanodiscs and vesicles were prepared with two phospholipids, dipalmitoyl phosphatidylcholine and dimyristoyl phosphatidylcholine, and the phase transition of each was analyzed using laurdan fluorescence and differential scanning calorimetry. Laurdan is a fluorescent probe sensitive to the increase of hydration in the lipid bilayer that accompanies the gel to liquid crystalline phase transition. The emission intensity profile can be used to derive the generalized polarization, a measure of the relative amount of each phase present. Differential scanning calorimetry was used to further quantitate the phase transition of the phospholipids. Both methods revealed broader transitions for the lipids in Nanodiscs compared to those in vesicles. Also, the transition midpoint was shifted 3-4 degrees C higher for both lipids when incorporated into Nanodiscs. These findings are explained by a loss of cooperativity in the lipids of Nanodiscs which is attributable to the small size of the Nanodiscs as well as the interaction of boundary lipids with the protein encircling the discs. The broad transition of the Nanodisc lipid bilayer better mimics the phase behavior of cellular membranes than vesicles, making Nanodiscs a 'native-like' lipid environment in which to study membrane associated proteins.

Influence of apoE domain structure and polymorphism on the kinetics of phospholipid vesicle solubilization

We examined the effects of apolipoprotein E (apoE) domain structure and polymorphism on the kinetics of solubilization (clearance) of dimyristoyl-phosphatidylcholine multilamellar vesicles. This second order reaction consisted of two simultaneous kinetic phases; it also exhibited saturable kinetics when the apolipoprotein concentration was increased at a constant lipid concentration. Rigid connections between alpha-helices in the 4-helix bundle formed by the 22 kDa N-terminal domain of apoE reduced the reaction rate. In contrast, the more flexible interhelical connections in apoA-I and the 10 kDa C-terminal domain of apoE promoted rapid solubilization of dimyristoyl-phosphatidylcholine (DMPC) multilamellar vesicles (mLV). Full-length apoE-3 reacted at about half the rate of the C-terminal domain alone. This decrease occurred because the hinge region probably decreased the interhelical flexibility of the 10 kDa domain and because both domains are conformationally restricted when covalently linked. Furthermore, the mLV surface affinities and reaction rates of the N-terminal domain fragments of the three common apoE isoforms tended to vary inversely with the stabilities of these fragments. These results confirm the importance of apoE's structure on the kinetics of lipid interaction. They suggest that flexibility in an apolipoprotein molecule increases the time-averaged exposure of hydrophobic surface area, thereby increasing the rate of phospholipid solubilization.

Effect of inhalation anaesthetics on the phase behaviour, permeability and order of phosphatidylcholine bilayers

We have used differential scanning calorimetry and fluorescence anisotropy measurements to investigate the effect of five inhalation anaesthetics of diverse chemical structure (halothane, enflurane, n-pentane, chloroform and diethylether) on the phase behaviour of liposomes prepared from dimyristoylphosphatidylcholine (DMPC) and dipalmitoylphosphatidylcholine (DPPC), respectively. The incorporation of these anaesthetics induced a decrease of the phase transition temperature and/or a broadening of the phase transition peak depending on the transverse localisation of the investigated anaesthetic. At high anaesthetic concentrations we observed the disappearance of the pretransition peak and the appearance of a shoulder on the main phase transition peak due to the domain formation of the anaesthetics. An anaesthetic induced carboxyfluorescein efflux from the vesicle lumen was completed within a few minutes after the addition of the anaesthetics, probably resulting from a transient formation of membrane holes. All results are discussed with regard to the physicochemical properties of the anaesthetics applied.

The surface properties of phospholipid liposome systems and their characterisation

The field of liposome (vesicle) research has expanded considerably over the last 30 years. In physical chemical terms liposomes have many of the characteristics of colloidal particles and their stability is determined in part by the classical surface forces. It is now possible to engineer a wide range of liposomes varying in size, phospholipid composition and surface characteristics. The surfaces of liposomes can be modified by the choice of bilayer lipid as well as by the incorporation and covalent linkage of proteins (e.g. antibodies and sugar binding proteins [lectins]), glycoproteins and synthetic polymers. Much of the impetus for liposome design has come from their potential value as drug delivery systems. The development of technologies for the production of such a range of liposome systems has presented interesting problems in the characterisation of their properties. The review addresses the progress that has been made in characterising the surfaces of different types of liposomes with specific reference to their electrophoretic properties and their interpretation and the physical interactions between liposomal bilayers.

Purification and characterization of recombinant human apolipoprotein A-II expressed in Escherichia coli

We have expressed recombinant human apolipoprotein A-II (apoA-II) in Escherichia coli, as a fusion protein with Schistosoma japonicum glutathione-S-transferase (GST). The GST-AII fusion protein was recovered by affinity chromatography using glutathione as a ligand. After thrombin cleavage and removal of the GST carrier, recombinant apoA-II was obtained in a highly purified form and was exclusively composed of dimeric apoA-II. Kinetics of association to dimyristoylglycerophosphocholine (Myr2GroPCho) vesicles showed that recombinant apoA-II exhibited the same pattern of association as human plasma apoA-II. Electron microscopic analysis of the complexes showed a typical pattern of rouleaux, characteristic of stacked discs, with a diameter similar to that determined by gradient-gel electrophoresis. Circular dichroism measurements showed that the alpha-helical content of both plasma and recombinant apoA-II increased similarly when the proteins associated with Myr2GroPCho vesicles, at the expense of a random-coil structure. Lipid-bound apoA-II consisted of 70-72% alpha helices, suggesting the presence of three 18-residue alpha helices/apoA-II monomer. Cross-linking experiments indicated that Myr2GroPCho complexes contained two molecules dimeric apoA-II/vesicle. Recombinant apoA-II was as efficient as plasma apoA-II in associating with HDL subclasses, and in displacing apoA-I from dipalmitoylglycerophosphocholine/cholesterol/apoA-I complexes, most likely due to its highly ordered secondary structure when associated with Myr2GroPCho vesicles. These findings demonstrate that recombinant apoA-II exhibits the same structural and functional properties as human plasma apoA-II. Thus, the expression system utilized is appropriate to produce mutagenized forms to further structure/function analysis.

Interaction of apolipoprotein AII with the putative high-density lipoprotein receptor

There is strong evidence to indicate that binding of HDL by cells is due to recognition of apoproteins residing on the surface of the lipoprotein by the putative HDL receptor(s). Although both of the major HDL apoproteins, AI and AII, are recognized by the putative receptor, the nature of the binding interaction and the domains of the apoproteins involved are largely unknown. Previous data from this laboratory led to the proposal of a model to explain how HDL particles containing AII interacted with the HDL receptor in a different manner as compared to HDL particles which contain apoAI but not apoAII [Vadiveloo, P. K., & Fidge, N. H. (1992) Biochem. J. 284, 145-151]. The model predicted that each chain of the apoAII homodimer contained a binding domain capable of interacting with the HDL receptor. This model was tested in the current study by preparing apoAII monomers, complexing them with phospholipid, and determining the ability of these complexes to bind to putative HDL receptors in rat liver plasma membranes (RLPM) and bovine aortic endothelial cell membranes (BAECM) by ligand blotting. The data showed that these complexes were bound by HB1 and HB2 from RLPM, and to the 110-kDa HDL binding protein from BAECM, providing critical evidence to support the model. Further investigation into the binding interaction revealed that apoAII complexed with phospholipid (apoAII-PC) bound more than delipidated apoAII, which bound more than delipidated apoAII monomers. Thus, optimum binding required the presence of lipid.(ABSTRACT TRUNCATED AT 250 WORDS)

Alpha-helical requirements for free apolipoproteins to generate HDL and to induce cellular lipid efflux

The structural requirement has been studied for apolipoproteins in their free form to interact with cells, to generate high density lipoprotein (HDL), and to cause cellular lipid efflux (J. Biol. Chem. 266, 3080-3086, 1991). It is shown that human apolipoprotein (apo) A-IV and apolipophorin III of Manduca sexta cause cholesterol efflux from cholesterol-loaded mouse peritoneal macrophages and reduce intracellularly accumulated cholesteryl ester as a result of forming HDL-like particles with cellular lipids, as do apoA-I, A-II and E. On the other hand, similar to apoC-III, reduced-and-carboxymethylated human apoA-II had no such effect. Thus, apolipoproteins seem to require at least four amphiphilic helical segments per molecule to express this function.

Kinetics of tryptic hydrolysis as a probe of the structure of human plasma apolipoprotein A-II

As a model system to understand apolipoprotein structure-function and their relationships to proteolytic events, the kinetics of tryptic hydrolysis of apolipoprotein A-II (apo A-II) was investigated in solution and in association with phospholipid. The rates of appearance and identities of specific peptides were determined by reversed-phase high-performance liquid chromatography and amino acid analysis, respectively. For the kinetics of hydrolysis of apo A-II in solution, the carboxyl-terminal peptides of residues 55-77 and 56-77 appeared first, followed by peptides of residues 4-23, 29-39, 40-44 and 45-54, which appeared at nearly identical rates. The kinetics of hydrolysis of apo A-II associated with 1,2-dimyristoyl-sn-glycero-3-phosphocholine showed several differences. First, a 100-fold larger amount of trypsin was needed to obtain a similar rate of product formation; second, a new peptide appeared, eluting earlier than apo A-II but having a similar amino acid composition; and third, the relative rates of appearance of peptides were different. The secondary structure surrounding the bonds susceptible to trypsin cleavage was determined by several predictive algorithms. The lysine amino acid bonds were found to be in regions defined by a high helical amphipathic moment. The reduced susceptibility to tryptic hydrolysis of apo-II associated with phospholipid appears to be due to a higher free energy of stabilization of protein secondary structure. As a consequence, the lysine amino acid bonds are in folded regions of the protein where they are conformationally inaccessible to enzymatic hydrolysis. By use of structure-prediction methods, it is possible to designate which regions of apolipoproteins may be important in proteolysis.

Spectroscopic studies of the tyrosine residues of human plasma apolipoprotein A-II

Human apolipoprotein A-II (apo A-II) in solution and associated with 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) was investigated by a combination of absorbance and fluorescence methods. Each apo A-II polypeptide chain contains four tyrosine residues but no tryptophan residues. Two and three tyrosine residues, respectively, appear to be buried for apo A-II in aqueous solution and in the lipid-associated protein. The spectroscopic properties of the tyrosine residues of lipid-associated apo A-II were also investigated. Plots of fluorescence intensity against temperature revealed a discontinuity in the region of the phase transition; however, over the same temperature range, there was no change in the exposure of tyrosine residues to the aqueous environment or in their mobility as measured by fluorescence polarization. Near-ultraviolet circular dichroic measurements demonstrated that the environments of the tyrosine residues of lipid-associated apo A-II and nitrated apo A-II were different from that of the apo A-II in solution or in a denatured state. Similar measurements also revealed that the microenvironments around tyrosines of apo A-II bound to DMPC in the gel phase are different from those observed in the liquid crystalline phase. Using environmentally sensitive fluorescence lipid probes, we have previously demonstrated that the polarity of the lipid/water interface of DMPC changes through a phase transition. The observations presented here indicate that these environmental changes also occur at the lipid/protein interface.

Effect of micelle diameter on tryptophan dynamics in an amphipathic helical peptide in phosphatidylcholine

The effect of dimyristoylphosphatidylcholine (DMPC) on the conformation and environment of the single tryptophan residue of a model amphipathic helical polypeptide has been investigated by fluorescence quenching with a water-soluble, neutral quencher (acrylamide) and multiple-frequency phase fluorometry. The peptide H-Ser-Ser-Ala-Asp-Trp-Leu-Lys-Ala-Phe-Tyr-Asp-Lys-Val-Ala-Glu-Lys-Leu-Ly s-Glu- Ala-Phe-Ser-Ser-Ser-OH [18As; Kanellis, P., Romans, A.Y., Johnson, B.J., Kercret, H., Chiovetti, R., Jr., Allen, T.M., & Segrest, S.P. (1980) J. Biol. Chem. 255, 11464] was synthesized by solid-phase techniques. Peptide was incubated at 26 degrees C with DMPC at various peptide:lipid weight ratios. The diameter of the resulting disk-shaped micelles increases with increasing lipid concentration from 12.0 +/- 0.4 nm at a 1:1 weight ratio of peptide to lipid to a maximum of 48.7 +/- 1.0 nm at a 1:13 ratio. At a weight ratio of 1:5, the average diameter is 22.7 +/- 0.6 nm. Decreasing the peptide:lipid ratio of the micelle resulted in a blue-shift in the fluorescence emission maximum (from 337 nm at 1:1 to 334 nm at 1:5), an increase in the fluorescence lifetime of the tryptophan measured by the phase shift method at 18 MHz (from 3.12 ns at 1:1 to 3.61 ns at 1:5), a decrease in the rate of fluorescence quenching by acrylamide (from 0.87 x 10(9) M-1 s-1 at 1:1 to 0.42 x 10(9) M-1 s-1 at 1:5), and an increase in the activation energy for quenching (from 6.7 kcal/mol at 1:1 to 12.7 kcal/mol at 1:5).(ABSTRACT TRUNCATED AT 250 WORDS)

Incorporation of cholesterol into apolipoprotein A-I-dimyristoylphosphatidylcholine recombinants

Apolipoprotein A-I (apoA-I) spontaneously associates with dimyristoylphosphatidylcholine (DMPC) liposomes to form discoidal high-density lipoprotein (HDL) recombinants. The uptake of cholesterol by this model HDL was studied by incubation with Celite-dispersed cholesterol. Separation of the resulting complexes by gradient centrifugation and gel filtration showed a heterogeneous distribution of particle size and composition as a consequence of the disruption and rearrangement of the recombinants. Quantitation of the amount of cholesterol taken up gave values between about 28 and 40 mol% cholesterol for the fractions within the protein peaks; the fractions with the lowest DMPC/apoA-I ratios had the lowest cholesterol contents. In another set of experiments, the association of apoA-I with DMPC-cholesterol liposomes was shown to result in complexes with characteristics similar to those obtained by the cholesterol-uptake experiments. Low concentrations of cholesterol in the liposomes enhanced the rate of lipid-protein association, but larger amounts decreased the yield of complexes by making the process thermodynamically and kinetically unfavorable. The enthalpy of recombinant formation increased with decreasing lipid/protein ratio and increasing cholesterol content, and became endothermic at about 23 mol% cholesterol. The effect of cholesterol on the thermal properties of HDL recombinants suggests that cholesterol is partially excluded from the boundary region adjacent to apoA-I. It is concluded that discoidal HDL recombinants, as a model for 'nascent' HDL, can acquire substantial amounts of cholesterol, which may be of great physiological importance for the reverse cholesterol transport and prevention of atherosclerosis.

Characterization of complexes of egg yolk phosphatidylcholine and apolipoprotein A-II prepared in the absence and presence of sodium cholate

Complexes of apolipoprotein A-II and egg yolk phosphatidylcholine were prepared in mixtures of different composition in the absence and presence of sodium cholate. By gradient gel electrophoresis, complex preparations were polydisperse and particle size distributions were influenced by the composition of the reconstitution mixture. Complexes generally exhibited a discoidal morphology by electron microscopy, but showed increased formation of vesicular complexes at elevated levels of egg yolk PC in the mixtures. By chemical crosslinking, complexes formed in the absence of cholate were shown to consist primarily of discoidal species with three apolipoprotein A-II molecules per particle in the mixtures investigated; complexes formed in the presence of cholate included species ranging from three to five apolipoprotein A-II per particle. The number of apolipoprotein A-II per particle and the sizes of the complexes, prepared in cholate, increased with increase of egg yolk PC in the reconstitution mixture. Relative to the particle size distribution of discoidal complexes formed in the absence of cholate, those prepared in cholate showed a distribution shifted to larger particle sizes. Complexes of similar particle size distribution formed in the presence or absence of cholate showed similar physical-chemical properties. Discoidal complexes with the same number of apolipoprotein A-II per particle but of different size and composition were observed, suggesting the possibility of some conformational adaptation of apolipoprotein A-II leading to stabilization of egg yolk PC bilayers of different diameter. Properties of particle size distributions of discoidal complexes prepared in cholate of apolipoprotein A-II and egg yolk PC were compared with those of complexes of apolipoprotein A-I previously reported (Nichols, A.V., Gong, E.L., Blanche, P.J. and Forte, T.M. (1983) Biochim. Biophys. Acta 750, 353-364).

Lecithin:cholesterol acyltransferase activation by synthetic amphipathic peptides

The amphipathic helical theory of Segrest and colleagues (FEBS Lett.:38:247-253, 1974) proposes that the lipid-binding segments of serum apolipoproteins are in an alpha helical conformation. Furthermore the helices have a hydrophobic face and a hydrophilic face with a specific distribution of positively and negatively charged residues. The importance of the pattern of the charged residues in the lipid binding and lecithin:cholesterol acyltransferase (LCAT) activation by the segments is still debated. We designed a 30-residue peptide, GALA, which in the alpha helical conformation has a hydrophilic face composed of glutamic acid residues (Sabbarao et al.: Biochemistry 26:2964-2972, 1987). GALA behaves like the serum apolipoproteins in its interaction with dimyristoylphosphatidylcholine (DMPC) at neutral pH; the amino terminal tryptophan of GALA undergoes a blue shift in its fluorescence emission spectrum, and the circular dichroism (CD) spectrum indicates that GALA acquires alpha helical structure in the presence of DMPC. A DMPC-GALA:19/1 (molar ratio) complex can be isolated by gel-permeation chromatography. This complex has a discoidal structure with the approximate dimensions of 44-A edge thickness and a 170- to 350-A diameter. GALA activates LCAT with DMPC but not with unsaturated phospholipids as the substrate. The apparent partition coefficient of GALA into DMPC vesicles is 100-fold larger than into egg phosphatidylcholine vesicles. The interaction of GALA with unsaturated lipids at neutral pH is so weak that no detectable change in the spectroscopic properties of GALA or the structure of the liposomes can be detected under the conditions used here. The sequence of GALA differs from previously studied model Apo A1 peptides by the absence of positively charged residues on the hydrophilic face. This indicates that positive charges in Apo A1-like peptides are not required in order to form discoidal structures with saturated phospholipids or to activate LCAT with such lipid substrates.

Morphological changes of phosphatidylcholine bilayers induced by melittin: vesicularization, fusion, discoidal particles

Morphological changes induced by the melittin tetramer on bilayers of egg phosphatidylcholine and dipalmitoylphosphatidylcholine have been studied by quasi-elastic light scattering, gel filtration and freeze-fracture electron microscopy. It is concluded that melittin similarly binds and changes the morphology of both single and multilamellar vesicles, provided that their hydrocarbon chains have a disordered conformation, i.e., at temperatures higher than that of the transition, Tm. When the hydrocarbon chains are ordered (gel phase), only small unilamellar vesicles are morphologically affected by melittin. However after incubation at T greater than Tm, major structural changes are detected in the gel phase, regardless of the initial morphology of the lipids. Results from all techniques agree on the following points. At low melittin content, phospholipid-to-peptide molar ratios, Ri greater than 30, heterogeneous systems are observed, the new structures coexisting with the original ones. For lipids in the fluid phase and Ri greater than 12, the complexes formed are large unilamellar vesicles of about 1300 +/- 300 A diameter and showing on freeze-fracture images rough fracture surfaces. For lipids in the gel phase, T less than Tm after passage above Tm, and for 5 less than Ri less than 50, disc-like complexes are observed and isolated. They have a diameter of 235 +/- 23 A and are about one bilayer thick; their composition corresponds to one melittin for about 20 +/- 2 lipid molecules. It is proposed that the discs are constituted by about 1500 lipid molecules arranged in a bilayer and surrounded by a belt of melittin in which the mellitin rods are perpendicular to the bilayer. For high amounts of melittin, Ri less than 2, much smaller and more spherical objects are observed. They are interpreted as corresponding to lipid-peptide co-micelles in which probably no more bilayer structure is left. It is concluded that melittin induces a reorganization of lipid assemblies which can involve different processes, depending on experimental conditions: vesicularization of multibilayers; fusion of small lipid vesicles; fragmentation into discs and micelles. Such processes are discussed in connexion with the mechanism of action of melittin: the lysis of biological membranes and the synergism between melittin and phospholipases.

Role of peptide structure in lipid-peptide interactions: high-sensitivity differential scanning calorimetry and electron spin resonance studies of the structural properties of dimyristoylphosphatidylcholine membranes interacting with pentagastrin-related pentapeptides

The effects of amino acid substitutions in the pentapeptide pentagastrin on the nature of its interactions with dimyristoylphosphatidylcholine (DMPC) are assessed by differential scanning calorimetry and electron spin resonance. In two peptide analogues, the Asp at position 4 in pentagastrin (N-t-Boc-beta-Ala-Trp-Met-Asp-Phe-NH2) is replaced by Gly or Phe. These uncharged, more hydrophobic peptides have little effect on the transition temperature of DMPC, but they broaden the transition and lower the transition enthalpy as do integral membrane proteins. These peptides also mimic the behavior of integral membrane proteins in decreasing the order of a 5-doxylstearic acid spin probe below the transition temperature and in exhibiting a second immobilized lipid component using a 16-doxylstearic acid spin probe in DMPC. Three charged peptides were studied: pentagastrin, an analogue with positions 4 and 5 reversed (i.e., ending in Phe-Asp-NH2), and one with Asp replaced by Arg at position 4. All three of these charged peptides altered the phase transition behavior of DMPC to give two components, one above and one below the transition temperature of the pure lipid. With increasing peptide concentration, the higher melting transition became more prominent. The arginine-containing peptide produced the largest shifts in melting temperature followed by pentagastrin and then the "reversed" peptide. The arginine-containing peptide also increased the enthalpy of the transition. These peptides also increased the ordering of DMPC below the phase transition as measured with both 5- and 16-doxylstearic acid. The ordering effect was most pronounced with the arginine-containing peptide using the 5-doxylstearic acid probe. The results demonstrate that even the zwitterionic DMPC can interact more strongly with positively charged peptides than with negatively charged ones. In addition, peptide sequence as well as composition is important in determining the nature of peptide-lipid interactions. The markedly different effects of these pentagastrin peptides on the phase transition and motional properties of DMPC occur despite the similar depth of burial of these peptides with DMPC.

High-density lipoprotein and its apolipoproteins inhibit cytolytic activity of complement. Studies on the nature of inhibitory moiety

Human high-density lipoprotein (HDL) and its apolipoproteins A-I and A-II inhibit complement-mediated lysis of human and sheep erythrocytes. This inhibitory activity under study is exerted after C9 is bound to membrane-associated C5b-8 complexes but prior to completed assembly and insertion of the C5b-9 complex. In this paper, we define some structure-activity relationships of the inhibitory moiety. With the exception of weak lytic inhibitory activity found in LDL/VLDL pools and in some unconcentrated minor fractions of plasma obtained by hydrophobic chromatography, all inhibitor activity was found in fractions which contained either apolipoprotein A-I, apolipoprotein A-II, or both. Intact HDL has a high level of inhibitor activity but delipidation by chloroform-methanol extraction was associated with an increase in activity on a protein-weight basis. Purified apolipoprotein A-I and apolipoprotein A-II exhibited equal inhibitory activity, greater than that exhibited by intact HDL. Nevertheless, ultracentrifugal fractions in which no free apolipoproteins could be demonstrated still possessed inhibitory activity. These experiments suggest that delipidation of HDL is not necessary for expression of inhibitor activity, although we could not rule out the possibility that apolipoproteins in dynamic equilibrium with HDL are responsible for the inhibitor activity observed in whole serum and plasma and in HDL preparations. Limited proteinase digestion completely abolished the inhibitory activity of partially delipidated HDL. Phospholipase C had little or no effect on the inhibitory activity of delipidated HDL, apolipoprotein A-I or apolipoprotein A-II, but reduced the inhibitory activity of intact HDL. These data suggest that the phospholipid polar headgroups are not necessary for inhibitory activity. However, the loss of these headgroups is associated with decreased activity, possibly due to increased hydrophobicity of HDL, or increased association among HDL micelles, and subsequent decrease in effective molar concentration of the inhibitory moiety.

Lipid-apolipoprotein interactions in surfactant studied by reassembly

Recent evidence suggests that the structure of tubular myelin, an extracellular form of pulmonary surfactant, is dependent on the interaction of lipids with certain proteins specific for this material and with calcium ions. In order to investigate how protein and calcium may affect the surfactant complex, we studied the composition and properties of reassembly materials formed with a major surfactant apolipoprotein (35,000-38,000 molecular weight) and the principal lipids found in the natural material. We were interested in three questions: 1) Does this apolipoprotein preferentially associate with certain of the lipids in surfactant? 2) What forces are involved in the binding? 3) Does the interaction result in changes in the physical state of the lipid? We found that this apolipoprotein binds phosphatidylcholines that are in a gel phase with much greater affinity than it does phosphatidylcholines that are liquid-crystalline. However, maximum binding does not occur with the pure phosphatidylcholines but rather with mixtures of phosphatidylcholines and 15% phosphatidylglycerol. Calcium ions have no effect on the amount of apolipoprotein that is bound, but they do modify the physical state of the reassembly lipoprotein and the stoichiometry of lipid to protein. These results indicate that the binding of the apolipoprotein to the lipid does not primarily involve ionic bonds. However, apolar interactions, which are influenced by the state of the lamellar phospholipid, appear to be important. Small amounts of phosphatidylglycerol and other glycolipids, which probably disrupt the regularity of a gel-phase lamellar structure when mixed with saturated phosphatidylcholines, may provide binding sites favoring the interaction. Indirect evidence, based on thermodynamic analyses, suggests that phosphatidylcholines may be partially immobilized about the protein in the formation of the complex. This conclusions is reinforced by the preliminary findings obtained from the differential scanning calorimetry of the reassembly materials.

Thermodynamics of lipid-protein association. Enthalphy of association of apolipoprotein A-II with dimyristoylphosphatidylcholine-cholesterol mixtures

Apolipoprotein A-II spontaneously associates with dimyristoylphosphatidylcholine (DMPC)-cholesterol mixtures to give products whose composition is a sensitive function of temperature and cholesterol content. At most temperatures, the lipid-to-protein stoichiometry of the product recombinant increases with increasing mol% cholesterol. Up to about 18 mol% cholesterol, the complexes have the same average sterol/DMPC ratio as that of the starting mixtures. At 24 mol% cholesterol or higher, no detectable lipid/protein complex formed. At 37 degrees C, the lipid-to-protein stoichiometry is essentially constant, irrespective of the cholesterol content and substitution of unsaturated phospholipids for DMPC. The enthalpy of lipid-protein association is a function of cholesterol content and, at 25 degrees C, increases linearly with the mol% cholesterol in the reaction mixture until it becomes endothermic between 15 and 20 mol% cholesterol. The results fit a model in which cholesterol is excluded from phospholipids in the 'boundary' layer, which is perturbed by the protein. At high cholesterol concentrations, the formation of a recombinant is thermodynamically unfavorable.

A review of plasma apolipoprotein A-I interactions with phosphatidylcholines

This review starts with a brief introduction to the properties of plasma high-density lipoproteins and their major protein component, apolipoprotein A-I, followed, in the following sections, by an account of experimental work from our laboratory on the interactions of apolipoprotein A-I with synthetic and natural phosphatidylcholines. The spontaneous reactions of phosphatidylcholine vesicles with apolipoprotein A-I are described in terms of the methods of observation, the properties of the reaction products (vesicular or micellar complexes of protein and lipid), and the kinetic controlling factors in the formation of the micellar products. A general detergent reconstitution method for the preparation of micellar complexes is presented, and applications of these particles in studies of the apolipoprotein-lipid interface and of enzymatic reactions are discussed.

Thermodynamics and kinetics of protein incorporation into membranes

The free energy and enthalpy of protein incorporation into membranes are calculated with special emphasis on the hitherto neglected effects of immobilization of protein and perturbation of lipid order in the membrane. The free energy change is found to be determined by the hydrophobic effect as the driving force for incorporation and the protein immobilization effect which leads to a considerable reduction of the free energy gained from the hydrophobic effect. For incorporation of a hydrophobic, bilayer-spanning alpha-helix, the free energy change obtained is of the order of -15 kcal/mol (1 cal = 4.184 J) in agreement with experimental results. The lipid perturbation effect yields only a small contribution to the free energy change due to an energy/entropy compensation inherent in lipid order. This effect dominates the enthalpy change, giving rise to values on the order of 100 kcal/mol with a pronounced temperature dependence around the lipid phase transition as observed experimentally. The kinetics of protein incorporation are even more strongly affected by the lipid perturbation effect, leading to an abrupt decrease of the rate of incorporation below the lipid phase transition.

Time-dependent fluorescence intensity and depolarization of diphenylhexatriene in micellar complexes of apolipoprotein C-I and dimyristoylglycerophosphocholine

The lipophilic fluorescent probe diphenylhexatriene was used to probe the lipid order and dynamics in apolipoprotein C-I . dimyristoylglycerophosphocholine (Myr2Gro-P-Cho) complexes. These complexes contain on the average 25 mol Myr2Gro-P-Cho/mol of apolipoprotein C-I, have a molecular weight around 200 000, and appear as discoidal, stacked particles by negative-stain electron microscopy. Steady-state fluorescence polarization of diphenylhexatriene as a function of temperature gives a broadened and shifted phase transition for Myr2Gro-P-Cho from the gel to liquid-crystalline state, with a mid-point around 27 degrees C. Time-dependent fluorescence intensity and anisotropy measurements of the diphenylhexatriene probe at 15 degrees C and 35 degrees C give fluorescence decay curves which can best be fit by two exponential functions, in each case. The fluorescence lifetimes and their fractional amplitudes approach the corresponding parameters in Myr2Gro-P-Cho vesicles and suggest insignificant effects of the protein on the microenvironment and conformations of the probe. The rotational correlation times and their fractional anisotropies indicate similar local motions of the probe in complexes and in vesicles, but reveal a significant ordering effect of the protein at both temperatures. The overall complex rotation at 15 degrees C has a correlation time of 136 +/- 13 ns, consistent with the size (approximately equal to 200 kDa) and shape (disc approximately equal to 5 x 15 nm) of the particle.

Comparison of the enthalpy state of vesicles of different size by their interaction with alpha-lactalbumin

The characteristics of small unilamellar, large unilamellar and large multilamellar vesicles of dimyristoylphosphatidylcholine and their interaction with alpha-lactalbumin are compared at pH 4. (1) By differential scanning calorimetry and from steady-state fluorescence anisotropy data of the lipophilic probe 1,6-diphenyl-1,3,5-hexatriene it is shown that the transition characteristics of the phospholipids in the large unilamellar vesicles resemble more those of the multilamellar vesicles than of the small unilamellar vesicles. (2) The size and composition of the lipid-protein complex formed with alpha-lactalbumin around the transition temperature of the lipid are independent of the vesicle type used. Fluorescence anisotropy data indicate that in this complex the motions of the lipid molecules are strongly restricted in the presence of alpha-lactalbumin. (3) The previous data and a comparison of the enthalpy changes, delta H, of the interaction of the three vesicle types with alpha-lactalbumin allow us to derive that the enthalpy state of the small unilamellar vesicles just below 24 degrees C is about 24 kJ/mol lipid higher than the enthalpy state of both large vesicle types at the same temperature. The abrupt transition from endothermic to exothermic delta H values around 24 degrees C for large vesicles approximates the transition enthalpy of the pure phospholipid.

Action of lecithin:cholesterol acyltransferase on model lipoproteins. Preparation and characterization of model nascent high density lipoprotein

Apolipoprotein A-I, the major protein of human plasma high density lipoprotein, is the primary activator of plasma lecithin:cholesterol acyltransferase. In vitro, the association of apolipoprotein A-I with physiological phosphatidylcholines can be catalyzed by mixing the protein and lipid with sodium cholate, which is removed by chromatography. The apolipoprotein A-I/phospholipid complex has the physical properties of an HDL, and when cholesterol is present the complex is a highly reactive substrate in the lecithin:cholesterol acyltransferase-catalyzed reaction. The relative reactivity of this complex compared with a number of other lipid-protein complexes is presented and discussed.