Lipid-protein interactions in the plasma lipoproteins

Folding-Mediated DNA Delivery by α-Helical Amphipathic Peptides

The renaissance gene therapy experiences these days requires specialist biomaterials and a systemic understanding of major factors influencing their ability to deliver genetic material. Peptide transfection systems represent a major class of such biomaterials. Several peptidic reagents have been commercialized to date. However, a comparative assessment of peptide sequences alone without auxiliary support or excipients against a common determinant for their ability to complex and deliver DNA has been lacking. This study cross-compares commercial and experimental transfection reagents from the same family of helical amphiphiles. Factors defining the efficacy of DNA delivery including cell uptake and gene expression are assessed along with cytotoxicity and DNA complexation. The results show that despite differences in sequence composition, length, and origin, peptide reagents of the same structural family exhibit similar characteristics and limitations with common variability trends. The cross-comparison revealed that functional DNA delivery is independent of the peptide sequence used but is mediated by the ability of the reagents to co-fold with DNA. Peptide folding proved to be the common determinant for DNA complexation and delivery by peptidic transfection reagents.

From worms to humans: Understanding intestinal lipid metabolism via model organisms

The intestine is responsible for efficient absorption and packaging of dietary lipids before they enter the circulatory system. This review provides a comprehensive overview of how intestinal enterocytes from diverse model organisms absorb dietary lipid and subsequently secrete the largest class of lipoproteins (chylomicrons) to meet the unique needs of each animal. We discuss the putative relationship between diet and metabolic disease progression, specifically Type 2 Diabetes Mellitus. Understanding the molecular response of intestinal cells to dietary lipid has the potential to undercover novel therapies to combat metabolic syndrome.

Peptide Nanoparticles for Gene Packaging and Intracellular Delivery

Efficient gene transfer is necessary for advanced biotechnologies ranging from gene therapy to synthetic biology. Peptide nanoparticles provide suitable packaging systems promoting targeted gene expression or silencing. Though these systems have yet to match the transfection efficacy of viruses, they are typically devoid of drawbacks characteristic of virus-based vectors, including insertional mutagenesis, low packaging capacities, and strong immune responses. Given the promise nanoparticle formulations hold for gene delivery, methods of their preparation and accurate analysis of their physicochemical and biological properties become indispensable for progress toward systems that seek to outperform viral vectors. Herein, we report a comprehensive protocol for the preparation and characterization of archetypal peptide nanoparticles resulting from nonspecific and noncovalent complexation with RNA and DNA.

Interaction of different lipoprotein types with cholesterol at the air/water interface

Cholesterol (Chol) interacts with lipoproteins, in order to be transported through the aqueous bloodstream. High density lipoproteins (HDL) and low density lipoproteins (LDL) transport cholesterol differently, a result that may be due to a difference in their interactions with cholesterol. Here, we investigated how the lipoprotein type affects the interaction with cholesterol by using a Langmuir trough and fluorescence microscope. We studied pure monolayers of 1) Chol, 2) LDL, and 3) HDL, and mixed monolayers of 1) Chol-LDL, and 2) Chol-HDL at air/water interfaces. Images of the Chol-LDL mixed monolayer showed many small sterol domains distributed in the non-sterol molecules (e.g. phospholids, proteins and lipids) of LDL. The sterol domains that were seen in the Chol-HDL mixed monolayer were larger in size but smaller in number than those seen in the Chol-LDL mixed monolayers. These images and the excess area, excess free energy, and free energy of mixing values obtained from the thermodynamic analysis of the surface pressure-area per molecule isotherms suggested that the cholesterol phase separated more from HDL than from LDL. Cholesterol was therefore concluded to interact with LDL better than with HDL. This more favorable interaction was explained by the presence of hydrophobic interactions between cholesterol and Apo-B, the major apoprotein of LDL.

Helminth Defense Molecules as Design Templates for Membrane Active Antibiotics

A design template for membrane active antibiotics against microbial and tumor cells is described. The template is an amino acid sequence that combines the properties of helminth defense molecules, which are not cytolytic, with the properties of host-defense peptides, which disrupt microbial membranes. Like helminth defense molecules, the template folds into an amphipathic helix in both mammalian host and microbial phospholipid membranes. Unlike these molecules, the template exhibits antimicrobial and anticancer properties that are comparable to those of antimicrobial and anticancer antibiotics. The selective antibiotic activity of the template builds upon a functional synergy between three distinctive faces of the helix, which is in contrast to two faces of membrane-disrupting amphipathic structures. This synergy enables the template to adapt pore formation mechanisms according to the nature of the target membrane, inducing the lysis of microbial and tumor cells.

Linear and orthogonal peptide templating of silicified protein fibres

Biomineralisation is essential for biology. Specialist proteins use peptide motifs that catalyse mineral deposition into nano-to-microscale inorganic materials. Unlike in native proteins, the motifs incorporated into self-assembled fibres can persistently propagate on the microscopic scale enabling empirically defined silica nanostructures. Herein we show that the two main modes of motif templating - linear and orthogonal - in self-assembling, fibre-forming peptide sequences effectively silicify protein fibres. We show that the mere charge and morphology of protein fibres are not sufficient for silica deposition, but it is the synergy between fibrillogenesis and silica-specific motifs regularly spaced in fibres that ensures silica templating, regardless of the relative orientation of the motifs.

Autonomously folded α-helical lockers promote RNAi

RNAi is an indispensable research tool with a substantial therapeutic potential. However, the complete transition of the approach to an applied capability remains hampered due to poorly understood relationships between siRNA delivery and gene suppression. Here we propose that interfacial tertiary contacts between α-helices can regulate siRNA cytoplasmic delivery and RNAi. We introduce a rationale of helical amphipathic lockers that differentiates autonomously folded helices, which promote gene silencing, from helices folded with siRNA, which do not. Each of the helical designs can deliver siRNA into cells via energy-dependent endocytosis, while only autonomously folded helices with pre-locked hydrophobic interfaces were able to promote statistically appreciable gene silencing. We propose that it is the amphipathic locking of interfacing helices prior to binding to siRNA that enables RNAi. The rationale offers structurally balanced amphipathic scaffolds to advance the exploitation of functional RNAi.

Interfacial zippering-up of coiled-coil protein filaments

Protein self-assembled materials find increasing use in medicine and nanotechnology. A challenge remains in our ability to tailor such materials at a given length scale. Here we report a de novo self-assembly topology which enables the engineering of filamentous protein nanostructures under morphological control. The rationale is exemplified by a ubiquitous self-assembly motif - an α-helical coiled-coil stagger. The stagger incorporates regularly spaced interfacial tryptophan residues, which allows it to zipper up into discrete filaments that bundle together without thickening by maturation. Using a combination of spectroscopy, microscopy, X-ray small-angle scattering and fibre diffraction methods we show that the precise positioning of tryptophan residues at the primary and secondary structure levels defines the extent of coiled-coil packing in resultant filaments. Applicable to other self-assembling systems, the rationale holds promise for the construction of advanced protein-based architectures and materials.

Tryptophan probes reveal residue-specific phospholipid interactions of apolipoprotein C-III

Apolipoproteins are essential human proteins for lipid metabolism. Together with phospholipids, they constitute lipoproteins, nm to μm sized particles responsible for transporting cholesterol and triglycerides throughout the body. To investigate specific protein-lipid interactions, we produced and characterized three single-Trp containing apolipoprotein C-III (ApoCIII) variants (W42 (W54F/W65F), W54 (W42F/W65F), W65 (W42F/W54F)). Upon binding to phospholipid vesicles, wild-type ApoCIII adopts an α-helical conformation (50% helicity) as determined by circular dichroism spectroscopy with an approximate apparent partition constant of 3×10(4) M(-1). Steady-state and time-resolved fluorescence measurements reveal distinct residue-specific behaviors with W54 experiencing the most hydrophobic environment followed by W42 and W65. Interestingly, time-resolved anisotropy measurements show a converse trend for relative Trp mobility with position 54 being the least immobile. To determine the relative insertion depths of W42, W54, and W65 in the bilayer, fluorescence quenching experiments were performed using three different brominated lipids. W65 had a clear preference for residing near the headgroup while W54 and W42 sample the range of depths ~8-11 Å from the bilayer center. On average, W54 is slightly more embedded than W42. Based on Trp spectral differences between ApoCIII binding to phospholipid vesicles and sodium dodecyl sulfate micelles, we suggest that ApoCIII adopts an alternate helical conformation on the bilayer which could have functional implications.

Supramolecular amphipathicity for probing antimicrobial propensity of host defence peptides

Supramolecular amphipathicity exposes antimicrobial propensity of host defence peptides.

Filming protein fibrillogenesis in real time

Protein fibrillogenesis is a universal tool of nano-to-micro scale construction supporting different forms of biological function. Its exploitable potential in nanoscience and technology is substantial, but the direct observation of homogeneous fibre growth able to underpin a kinetic-based rationale for building customized nanostructures in situ is lacking. Here we introduce a kinetic model of de novo protein fibrillogenesis which we imaged at the nanoscale and in real time, filmed. The model helped to reveal that, in contrast to heterogeneous amyloid assemblies, homogeneous protein recruitment is principally characterized by uniform rates of cooperative growth at both ends of growing fibers, bi-directional growth, with lateral growth arrested at a post-seeding stage. The model provides a foundation for in situ engineering of sequence-prescribed fibrous architectures.

Peculiarities of one-carbon metabolism in the strict carnivorous cat and the role in feline hepatic lipidosis

Research in various species has indicated that diets deficient in labile methyl groups (methionine, choline, betaine, folate) produce fatty liver and links to steatosis and metabolic syndrome, but also provides evidence of the importance of labile methyl group balance to maintain normal liver function. Cats, being obligate carnivores, rely on nutrients in animal tissues and have, due to evolutionary pressure, developed several physiological and metabolic adaptations, including a number of peculiarities in protein and fat metabolism. This has led to specific and unique nutritional requirements. Adult cats require more dietary protein than omnivorous species, maintain a consistently high rate of protein oxidation and gluconeogenesis and are unable to adapt to reduced protein intake. Furthermore, cats have a higher requirement for essential amino acids and essential fatty acids. Hastened use coupled with an inability to conserve certain amino acids, including methionine, cysteine, taurine and arginine, necessitates a higher dietary intake for cats compared to most other species. Cats also seemingly require higher amounts of several B-vitamins compared to other species and are predisposed to depletion during prolonged inappetance. This carnivorous uniqueness makes cats more susceptible to hepatic lipidosis.

Anti-antimicrobial peptides: folding-mediated host defense antagonists

Antimicrobial or host defense peptides are innate immune regulators found in all multicellular organisms. Many of them fold into membrane-bound α-helices and function by causing cell wall disruption in microorganisms. Herein we probe the possibility and functional implications of antimicrobial antagonism mediated by complementary coiled-coil interactions between antimicrobial peptides and de novo designed antagonists: anti-antimicrobial peptides. Using sequences from native helical families such as cathelicidins, cecropins, and magainins we demonstrate that designed antagonists can co-fold with antimicrobial peptides into functionally inert helical oligomers. The properties and function of the resulting assemblies were studied in solution, membrane environments, and in bacterial culture by a combination of chiroptical and solid-state NMR spectroscopies, microscopy, bioassays, and molecular dynamics simulations. The findings offer a molecular rationale for anti-antimicrobial responses with potential implications for antimicrobial resistance.

Relationship between apolipoprotein C-III concentrations and high-density lipoprotein subclass distribution

High-density lipoprotein (HDL) subclasses have different antiatherogenic potentials and functional properties. This work presents our findings and discussions on their metabolic implications on apolipoprotein (apo) C-III together with other apolipoprotein levels and HDL subclass distribution profile. Apolipoprotein A-I contents of plasma HDL subclasses were quantitated by 2-dimensional gel electrophoresis coupled with immunodetection in 511 subjects. Concentrations of triglycerides and of apo B-100, C-II, and C-III were higher, whereas those of HDL cholesterol were lower, for subjects in the highest tertile of apo C-III levels group, which presented a typical hypertriglyceridemic lipid profile. Subjects in the middle and highest tertile of apo C-III levels groups had increased prebeta(1)-HDL, HDL(3c), HDL(3b) (only in the highest tertile of apo C-III group), and HDL(3a), but decreased HDL(2a) and HDL(2b) contents compared with subjects in the lowest tertile of apo C-III levels group. With the elevation of apo C-III together with apo C-II levels, contents of small-sized prebeta(1)-HDL increased successively and significantly; but those of large-sized HDL(2b) reduced successively and significantly. With a rise in apo C-III and apo A-I levels, those of prebeta(1)-HDL increased significantly. Moreover, subjects with high apo A-I levels showed a substantial increase in HDL(2b); on the contrary, HDL(2b) declined progressively and obviously for subjects in the low apo A-I levels with the elevation of apo C-III levels. Correlation analysis illustrated that apo C-III levels were positively associated with prebeta(1)-HDL, prebeta(2)-HDL, and HDL(3a). The particle size of HDL shifted toward smaller sizes with the increase of plasma apo C-III levels, and the shift was more remarkable when the elevation of apo C-III and apo C-II was simultaneous; and besides, higher apo A-I concentrations could modify the effect of apo C-III on HDL subclass distribution profile. Large-sized HDL(2b) particles decreased greatly for hypertriglyceridemic subjects who were characterized by elevated apo C-III and C-II accompanied with significantly lower apo A-I, which, in turn, blocked the maturation of HDL.

Time course effect of thyroxine on serum lipoprotein concentrations in hypothyroid subjects

In order to analyse the time course effect of thyroxine replacement therapy on lipoprotein concentrations, 12 hypothyroid subjects were studied before and repeatedly during therapy. The substitution dose was increased gradually every fourth week until the plasma level of TSH became normal. The concentrations of cholesterol and triglycerides were analysed in whole serum and in the three major lipoprotein fractions VLDL, LDL and HDL. The main abnormality before treatment was an increased LDL cholesterol concentration. Normalization of serum LDL was established at a dose level which normalized TSH. The onset of the effect of thyroxine on LDL cholesterol was rapid and could be noted already after two weeks.

Reconsideration of hydrophobic lipid distributions in lipoprotein particles

Lipoprotein particles are commonly known as micellar aggregates with hydrophobic lipids located within the core and amphipathic molecules in the surface. Using a new structural model for optimizing the distribution of hydrophobic lipids, namely triglyceride (TG) and cholesterol ester (CE) molecules, we reveal that particle size-dependent proportion of these 'core lipids' may locate in the surface of lipoprotein particles. The composition of the particles also strongly influences the actual molecular content of the surface. For example, in high-density lipoprotein (HDL) particles the percentage of CEs of all surface lipids is between 13% and 27% due to the high tendency of CEs to locate in the surface and the high concentration of CEs in the particles. Conversely, although the percentage of TG molecules in the surface of HDL particles is also high, approximately 60% as for CE, the percentage of TGs of all surface lipids is low, only up to 5%, because HDL particles have a low-TG concentration. These structural models provide an intuitive and coherent structural rationale for various metabolic cascades in lipoprotein metabolism with the catalytic enzyme action and molecular binding for transport proteins taking place at the surface of the particles.

A high density lipoprotein from Piaractus mesopotamicus, pacu, (Osteichthyes, Characidae), is associated with paraoxonase activity

We have characterized the serum lipoprotein profile and localized the serum paraoxonase activity of pacu, Piaractus mesopotamicus, a tropical fish species. The total lipoprotein profile of pacu serum obtained after KBr density ultracentrifugation shows the predominance of HDL (1.1267 g/mL). SDS-PAGE electrophoresis revealed a negligible amount of LDL. Pacu HDL was purified by gel filtration column on HPLC, and its molecular mass was estimated to be 246 kDa. Protein composition was 35%, and comprised four protein components with molecular masses of 45, 38, 25 and 12.5 kDa. Lipids represent 58% of total HDL, comprising 40% neutral lipids and 18% phospholipids by weight. The HDL contains 7% of carbohydrates, and mannose was the only sugar detected by paper chromatography in HDL hydrolysates. HDL-containing fractions showed the major paraoxonase activity. Purification of HDL resulted in a 23-fold enrichment of this activity. This is the first experimental evidence demonstrating the association of paraoxonase activity with a HDL in fish.

Mutagenesis and computer modelling approach to study determinants for recognition of signal peptides by the mitochondrial processing peptidase

Determinants for the recognition of a mitochondrial presequence by the mitochondrial processing peptidase (MPP) have been investigated using mutagenesis and bioinformatics approaches. All plant mitochondrial presequences with a cleavage site that was confirmed by experimental studies can be grouped into three classes. Two major classes contain an arginine residue at position -2 or -3, and the third class does not have any conserved arginines. Sequence logos revealed loosely conserved cleavage motifs for the first two classes but no significant amino acid conservation for the third class. Investigation of processing determinants for a class III precursor, Nicotiana plumbaginifolia F1beta precursor of ATP synthase (pF1beta), was performed using a series of pF1beta presequence mutants and mutant presequence peptides derived from the C-terminal portion of the presequence. Replacement of -2 Gln by Arg inhibited processing, whereas replacement of either the most proximally located -5 Arg or -15 Arg by Leu had only a low inhibitory effect. The C-terminal portion of the pF1beta presequence forms a helix-turn-helix structure. Mutations disturbing or prolonging the helical element upstream of the cleavage site inhibited processing significantly. Structural models of potato MPP and the C-terminal pF1beta presequence peptide were built by homology modelling and empirical conformational energy search methods, respectively. Molecular docking of the pF1beta presequence peptide to the MPP model suggested binding of the peptide to the negatively charged binding cleft formed by the alpha-MPP and beta-MPP subunits in close proximity to the H111XXE114H115X(116-190)E191 proteolytic active site on beta-MPP. Our results show for the first time that the amino acid at the -2 position, even if not an arginine, as well as structural properties of the C-terminal portion of the presequence are important determinants for the processing of a class III precursor by MPP.

Intestinal sterol absorption mediated by scavenger receptors is competitively inhibited by amphipathic peptides and proteins

Exchangeable serum apolipoproteins and amphipathic alpha-helical peptides are effective inhibitors of sterol (free and esterified cholesterol) uptake at the small-intestinal brush border membrane. The minimal structural requirement of an inhibitor is an amphipathic alpha-helix of 18 amino acids. The inhibition is competitive, indicating that the inhibitor binds to scavenger receptor class B type I (SR-BI) present in the brush border membrane and responsible for sterol uptake. Binding of apolipoprotein A-I to SR-BI of rabbit brush border membrane is cooperative, characterized by a dissociation constant K(d) = 0.45 microM and a Hill coefficient of n = 2.8. The cooperativity of the interaction is due to binding of the inhibitor molecule to a dimeric or oligomeric form of SR-BI held together by disulfide bridges. Consistent with the competitive nature of the inhibition, the K(d) value agrees within experimental error with the IC(50) value of inhibition and with the inhibition constant K(I). After proteinase K treatment of brush border membrane vesicles, the affinity of the interaction of apolipoprotein A-I expressed as K(d) is reduced by a factor of 20, and the cooperativity is lost. The interaction of proteinase K-treated brush border membrane vesicles with apolipoprotein A-I is nonspecific partitioning of the apolipoprotein into the lipid bilayer of brush border membrane vesicles.

Characterization of lipid efflux particles generated by seminal phospholipid-binding proteins

We reported recently that the choline phospholipid-binding proteins (BSP-A1/-A2, BSP-A3 and BSP-30-kDa) of bovine seminal plasma (BSP) stimulate cholesterol and choline phospholipid efflux from fibroblasts. In this study, we characterized the lipid efflux particles generated by BSP proteins. The density gradient ultracentrifugation of the efflux medium from radiolabeled fibroblasts incubated with BSP proteins showed a single peak of [3H]cholesterol between density (d) 1.12 and 1.14 g/ml, which is in the range of high-density lipoproteins. Size-exclusion chromatographic and immunoblot analysis revealed that the efflux particles have a large size equal to or bigger than very low-density lipoproteins and contained BSP proteins. Lipid analysis of density gradient and gel filtration fractions from efflux medium of simultaneously labeled fibroblasts ([3H]cholesterol and [3H]choline) incubated with BSP proteins showed that the efflux particles were homogeneous and composed of cholesterol and choline phospholipids. The lipid particles contained BSP proteins, cholesterol and choline phospholipids in molar ratio of 0.05:1.21:1, respectively. Agarose gel electrophoresis showed that the BSP-generated lipid particles had a gamma migration pattern which is slower than low-density lipoproteins. The sonication of cholesterol and BSP proteins followed by gel filtration chromatographic analysis indicated no direct binding of cholesterol to BSP proteins. These results taken together indicate that BSP proteins induce a concomitant cholesterol and choline phospholipid efflux and generate large protein-lipid particles.

Binding steps of apolipoprotein A-I with phospholipid monolayers: adsorption and penetration

In whole HDL particles, the arrangement of apoA-I relative to phospholipids is of crucial interest with respect to the physiological formations of HDL. We report here new data concerning the nature of the interaction of apoA-I with condensed phospholipid (PL) monolayers (phosphatidylcholine and phosphatidylserine). The use of alternative current polarography allowed the detection for the first time of different binding steps which are dependent on apoA-I concentration. At low concentration (below 10 micrograms/mL), apoA-I adsorbs on PL polar headgroups, through electrostatic interactions. Above this threshold concentration, apoA-I penetrates within the monolayer (i.e., part of apoA-I crosses the PL polar headgroup/hydrocarbon chain interface). The process of penetration described here brings experimental evidence supporting Segrest's "snorkel" model. Penetrated helices are lying at the interface, their apolar face in contact with PL hydrocarbon chains and their polar face in contact with PL polar headgroups. In the absence of cholesterol, a second level of penetration was detected at higher apoA-I concentrations. It was facilitated in the presence of phosphatidylserine in comparison to phosphatidylcholine and disappeared in the presence of cholesterol. It is proposed that the C-terminal domain is involved in the first binding steps and that hinged domains may also be implicated. Furthermore, we propose that the apoA-I binding states stabilize the protein/phospholipid layer complex. These different binding states are discussed with respect to their roles in HDL metabolism.

Studies of synthetic peptides of human apolipoprotein A-I containing tandem amphipathic alpha-helixes

In mature human apolipoprotein A-I (apo A-I), the amino acid residues 1-43 are encoded by exon 3, whereas residues 44-243 are encoded by exon 4 of the apo A-I gene. The region encoded by exon 4 of the apo A-I gene contains 10 tandem amphipathic alpha-helixes; their location and the class to which they belong are as follows: helix 1 (44-65, class A1), helix 2 (66-87, class A1), helix 3 (88-98, class Y), helix 4 (99-120, class Y), helix 5 (121-142, class A1), helix 6 (143-164, class A1), helix 7 (165-186, class A1), helix 8 (187-208, class A1), helix 9 (209-219, class Y), and helix 10 (220-241, class Y). To examine the effects of multiple tandem amphipathic helixes compared to individual helixes of apo A-I on lipid association, we have studied lipid-associating properties of the following peptides: Ac-44-87-NH2 (peptide 1-2), Ac-66-98-NH2 (peptide 2-3), Ac-66-120-NH2 (peptide 2-3-4), Ac-88-120-NH2 (peptide 3-4), Ac-99-142-NH2 (peptide 4-5), Ac-121-164-NH2 (peptide 5-6), Ac-143-186-NH2 (peptide 6-7), Ac-165-208-NH2 (peptide 7-8), Ac-187-219-NH2 (peptide 8-9), and Ac-209-241-NH2 (peptide 9-10). To study lipid-associating properties of the region encoded by exon 3 of the apo A-I gene, 1-33-NH2 (peptide G) has also been studied. The results of the present study indicate that, among the peptides studied, peptides 1-2 and 9-10 possess significantly higher lipid affinity than the other peptides, with peptide 9-10 having higher lipid affinity than peptide 1-2, as evidenced by (i) higher helical content in the presence of 1, 2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), (ii) faster rate of association with DMPC multilamellar vesicles (MLV), (iii) greater reduction in the enthalpy of gel to liquid-crystalline phase transition of DMPC MLV, (iv) higher exclusion pressure from an egg yolk phosphatidylcholine monolayer, and (v) higher partitioning into 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine MLV. A comparison of the free energies of lipid association (DeltaG) of the peptides studied here with those studied previously by us [Palgunachari, M. N. , et al. (1996) Arterioscler. Thromb. Vasc. Biol. 16, 328-338] indicates that, except for the peptides 4-5 and 5-6, other peptides possess higher lipid affinities compared to constituent helixes. However, the lipid affinities of the peptides studied here are neither higher than nor equal to the sum of the lipid affinities of the constituent helixes. This indicates the absence of cooperativity among the adjacent amphipathic helical domains of apo A-I for lipid association. As indicated by DeltaG, the lipid affinity of peptide 4-5 is higher than peptide 5 but lower than peptide 4; the lipid affinity of peptide 5-6 is lower than both peptides 5 and 6. Implications of these results for the structure and function of apo A-I are discussed.

The use of sodium dodecyl sulfate to model the apolipoprotein environment. Evidence for peptide-SDS complexes using pulsed-field-gradient NMR spectroscopy

Pulsed-field-gradient NMR spectroscopy was used to measure translational diffusion coefficients (Ds) for a peptide corresponding to a proposed lipid-binding domain of human apolipoprotein C-I, residues 7-24 (apoC-I(7-24)). Diffusion coefficients for apoC-I(7-24) were determined directly by following the decay of the resonance intensity of selected peptide protons at various concentrations of sodium dodecyl sulfate (SDS), a detergent increasingly being used to model the apolipoprotein environment. Previously, diffusion coefficients of peptides in the presence of SDS have been determined indirectly by monitoring the SDS diffusion coefficient. The direct measurement of the diffusion coefficient of the peptide enables one to distinguish whether SDS simply coats the peptide's surface to produce a uniformly charged 'rod' or if the peptide associates with a micelle. Using the direct method, at SDS concentrations above 5 mM (which is below the SDS critical micelle concentration (8.1 mM)), apoC-I(7-24) exhibited diffusion coefficients consistent with the formation of a large-molecular-weight complex. Based on the ratio of the diffusion coefficients for free- and SDS-associated peptide, the molecular weight of the peptide-SDS complex was much larger than a factor of 1. 4, the increase in molecular weight of the free peptide predicted if apoC-I(7-24) was uniformly surface coated with SDS.

Differential membrane-binding and activation mechanisms of protein kinase C-alpha and -epsilon

To elucidate the mechanisms of membrane binding and activation of conventional and novel protein kinase C (PKC), we measured the interactions of rat PKC-alpha and -epsilon with phospholipid monolayers and vesicles of various compositions. Besides the established difference in calcium requirement, the two isoforms showed major differences in their membrane-binding and activation mechanisms. For PKC-alpha, diacylglycerol (DG) specifically enhanced the binding of PKC-alpha to phosphatidylserine (PS)-containing vesicles by 2 orders of magnitude, allowing PKC-alpha high specificity for PS. Also, PKC-alpha could penetrate into the phospholipid monolayer with a packing density comparable to that of the cell membrane only in the presence of Ca2+ and PS. When compared to PKC-alpha, PKC-epsilon had lower binding affinity for PS-containing vesicles both in the presence and in the absence of DG. As a result, PKC-epsilon did not show pronounced specificity for PS. Also, PKC-epsilon showed reduced penetration into PS-containing monolayers, which was comparable to the Ca2+-independent penetration of PKC-alpha into the same monolayers. Taken together, these results suggest the following: (1) The role of Ca2+ in the membrane binding of PKC-alpha is to expose a specific PS-binding site. (2) Once bound to membrane surfaces, PS specifically induces the partial membrane penetration of PKC-alpha that allows its optimal interactions with DG, hence the enhanced membrane binding and activation. (3) PKC-epsilon, due to the lack of Ca2+ binding, cannot specifically interact with PS and DG, which implies the presence of other physiological activator(s) for this isoform.

Crystal structure of truncated human apolipoprotein A-I suggests a lipid-bound conformation

The structure of truncated human apolipoprotein A-I (apo A-I), the major protein component of high density lipoprotein, has been determined at 4-A resolution. The crystals comprise residues 44-243 (exon 4) of apo A-I, a fragment that binds to lipid similarly to intact apo A-I and that retains the lipid-bound conformation even in the absence of lipid. The molecule consists almost entirely of a pseudo-continuous, amphipathic alpha-helix that is punctuated by kinks at regularly spaced proline residues; it adopts a shape similar to a horseshoe of dimensions 125 x 80 x 40 A. Four molecules in the asymmetric unit associate via their hydrophobic faces to form an antiparallel four-helix bundle with an elliptical ring shape. Based on this structure, we propose a model for the structure of apo A-I bound to high density lipoprotein.

Role of phospholipids in the protein stability of an insect lipoprotein, lipophorin from Rhodnius prolixus

Lipophorin (Lp) is the major lipoprotein in insect hemolymph. The structural organization proposed for Lp is basically the same as that suggested for vertebrate lipoproteins, consisting of a hydrophobic core containing neutral lipids, stabilized in the aqueous environment by surrounding polar moieties of protein and phospholipids at the particle surface. After complete removal of phospholipids from Lp by phospholipase A2, the particle remains soluble [Gondim, K. C., Atella, G. C., Kawooya, J. K., & Masuda, H. (1992) Arch. Insect Biochem. Physiol. 20, 303-314]. However, studies on the roles of phospholipid on the structural stability of Lp are still lacking. In the present work, we have studied the structure and stability of dephospholipidated lipophorin (d-Lp). Trypsinolysis of d-Lp indicated no exposure of new cleavage sites on the protein when compared to Lp. However, an enhanced rate of proteolysis of the apoproteins (especially apolipophorin II) was observed in d-Lp. Circular dichroism analysis indicated that the secondary structure of Lp was not significantly affected by phospholipid removal. Furthermore, the exposure of tryptophan residues to the aqueous solvent in d-Lp was the same as in Lp, as indicated by intrinsic fluorescence emission spectra and fluorescence quenching experiments. Interestingly, d-Lp was more resistant to denaturation by guanidine hydrochloride than Lp. d-Lp was also found to be less sensitive than Lp to structural changes induced by hydrostatic pressure. Taken together, these results indicate that, although changes in its structural organization were subtle, dephospholipidated lipophorin may have additional protein-protein and/or protein-neutral lipid interactions that are responsible for the observed increase in stability. Therefore, phospholipids are not only not essential for Lp stability, but their presence in the particle seems to result in a less stable structure in the aqueous environment.

Interaction of model class A1, class A2, and class Y amphipathic helical peptides with membranes

To test the hypothesis that differences in the lipid affinity of exchangeable apolipoproteins are due to the presence of different classes of amphipathic alpha-helical motifs which differ primarily in the distribution of charged amino acid residues, we designed and synthesized model peptides mimicking class A1, class A2, and class Y amphipathic helices present in these apolipoproteins. Both class A1 and class A2 helices have positive residues at the polar-nonpolar interface and negative residues at the center of the polar face. However, clustering of positive and negative residues is less exact in class A1 compared to class A2 helices. The class Y helices have two negative residue clusters on the polar face separating the two arms and the base of the Y motif formed by three positive residue clusters. The lipid affinities of three 18 residue model peptides representing these classes, Ac-18A1-NH2 (Ac-ELLEKWAEKLAALKEALK-NH2), Ac-18A2-NH2 (Ac-ELLEKWKEALAALAEKLK-NH2), and Ac-18Y-NH2 (Ac-ELLKAWKEALEALKEKLA-NH2), were determined by right-angle light scattering, circular dichroism spectroscopy, differential scanning calorimetry, and fluorescence spectroscopy. The observed rank order of lipid affinity of these three peptides is: Ac-18A2-NH2 > Ac-18Y-NH2 > Ac-18A1-NH2. This order is consistent with the known lipid affinity of exchangeable apolipoproteins containing class A1, class A2, and class Y helices (class A2 > class Y > class A1). Results of this study illustrate the important role of interfacial lysine residues in modulating the lipid affinity of amphipathic helices and suggest that the effect of interfacial lysine residues in increasing lipid affinity is additive. We propose that interfacial lysine residues, in addition to widening the hydrophobic face because of snorkeling, also help anchor the amphipathic helix in the lipid bilayer.

Conformational studies of an amphipathic peptide corresponding to human apolipoprotein A-II residues 18-30 with a C-terminal lipid binding motif EWLNS

A peptide was designed and synthesized to enhance the lipid binding properties of a 13-residue fragment of apolipoprotein A-II. The peptide, VTDYGKDLMEKVKEWLNS [apoA-II(18-30)+], contains a five-residue amphipathic motif, EWLNS, at the C-terminus of apolipoprotein A-II residues 18-30. The lipid binding properties of apoA-II(18-30)+ were assessed using optical spectroscopy in the presence of sodium dodecyl sulfate (SDS), dodecylphosphocholine (DPC), tetradecyltrimethyl ammonium chloride (TMA) and dimyristoylphosphatidylcholine (DMPC). The fluorescence emission spectra and the circular dichroism data suggested that apoA-II(18-30)+ interacted most strongly with SDS and most weakly with DMPC. An ensemble of structures for apoA-II(18-30)+ in aqueous solution containing SDS was calculated using distance geometry/simulated annealing methods from 308 NOE-based distance restraints. The backbone (N-C-C = O) RMSD from the average structure of an ensemble of 15 out of 20 calculated structures was 0.54 +/- 0.16 A. Apart from some dynamic fraying at both termini, the distance geometry and simulated annealing calculations showed that apoA-II(18-30)+ adopted a well defined amphipathic helix with distinct hydrophobic and hydrophilic faces.

Conformational features of a synthetic cyclic peptide corresponding to the complete V3 loop of the RF HIV-1 strain in water and water/trifluoroethanol solutions

The disulfide-bridge-closed cyclic peptide corresponding to the whole V3 loop of the RF HIV-1 strain was examined by proton two-dimensional NMR spectroscopy in water and water/trifluoroethanol solutions. Although most of the peptide is conformationally averaged in water, the NOE data support a beta-turn conformation for the central conservative GPGR region and the presence of nascent helix. Upon addition of trifluoroethanol, helix formation in the C-terminal part becomes apparent. This is confirmed by CD data. NOEs indicative of multiple and transient beta-turns around the Asn6 glycosylation site and NOEs fitting X-ray data on a linear V3 peptide-Fab complex also emerge. The C-terminal helix is shown to have amphipathic character and might thus assist in the infection process.

Design and synthesis of amphipathic antimicrobial peptides

A large proportion of antimicrobial peptides share a common structural feature that is critical to their antimicrobial activity, i.e. amphipathic alpha-helices. The amphipathy of a polypeptide chain can be quantitated through the value of the hydrophobic moment. Generally, antimicrobial peptides are characterized by high hydrophobic moment and low hydrophobicity values. Using these criteria we have identified two short segments that possess hydrophobic moment properties associated with known antimicrobial peptides. Using in vitro assays the segment derived from the protein perforin displays no antifungal or antibacterial activity and, while showing no alpha-helicity in buffer or liposomes, exhibits a modest degree of alpha-helical structure in the presence of the alpha-helical inducer, 2,2,2-trifluoroethanol. However, rational modifications result in a derivative which assumes an alpha-helical conformation in the presence of liposomes, exhibits potent antifungal activity against plant fungal pathogens, has significant antibacterial activity, effects leakage of a fluorescent dye from acidic liposomes and is devoid of hemolytic activity. Results are also presented for a segment derived from the human immunodeficiency virus envelope protein. We suggest that the identification of putative amphipathic structures in proteins may provide a useful starting strategy in the design and synthesis of antimicrobial peptides.

Effect of the arrangement of tandem repeating units of class A amphipathic alpha-helixes on lipid interaction

Exchangeable apolipoproteins possess tandem repeating units of class A amphipathic helical segments and many of them are linked together by proline residues. To understand the optimal arrangement of the amphipathic helixes for lipid association, we have studied the interactions of three model class A amphipathic helical peptides with lipids. The three peptides are: 37pA, a dimer of 18A (DWLKAFYDKVAEKLKEAF) linked together by a Pro (18A-Pro-18A); 37aA, a dimer of 18A linked together by an Ala (18A-Ala-18A); and 36A, a dimer of 18A without any linker residue (18A-18A). Circular dichroism (CD) spectra showed that the peptides are predominantly alpha-helical in aqueous and lipid environments. Temperature dependent CD studies indicated that in buffer helix stability decreases in the order 36A > 37aA > 37pA; however, in the presence of dimyristoyl phosphatidylcholine (DMPC), the above order is reversed. The retention times of the peptides on a C18 reversed-phase high performance liquid chromatography column decreased in the order 36A > 37aA > 37pA, consistent with the lengths of the nonpolar faces of the alpha-helixes being in the same order; the retention time of the parent 18A was shorter than 37pA. While 37pA adsorbed to egg phosphatidylcholine monolayers most strongly, the degree and rate of association of 36A were significantly lower. Differential scanning calorimetry indicated that, while 37pA was most effective in reducing the enthalpy of the gel to liquid-crystalline phase transition of DMPC multilamellar vesicles, 36A was least effective; 36A was even less effective than 18A. Fluorescence quenching experiments with iodide and acrylamide indicated that, in the presence of DMPC, Trp residues in 36A are most exposed to the quenchers while in 37pA they are least exposed. In the presence of DMPC, shielding of Trp in 18A from the quenchers was more than that observed with Trp residues in 36A. The results of this study suggest that the arrangement of tandem repeating amphipathic helical units which results in the formation of a class A amphipathic helix with a nonpolar face longer than five or six turns reduces the ability of the helix to associate with phospholipid.

Plasma lipoprotein alterations in patients with chronic hepatocellular liver disease resulting from alcohol abuse: effects of alcohol intake cessation

Cholesterol and triglyceride in plasma and lipoprotein fractions and serum apoprotein concentrations were measured in 51 chronic alcoholic subjects; 23 had minimal or mild hepatic changes (steatosis and/or fibrosis) and 28 had cirrhosis. Of the latter, 16 had stopped alcohol consumption at least 3 months before the study, while the other 12 and all the mildly affected patients had continued drinking. None of the patients presented with cholestasis or alcoholic hepatitis. The control group was composed of 15 healthy, non-drinking volunteers selected from the hospital staff with an age- and sex-distribution similar to that of the alcoholic group. Patients with minimal hepatic changes had plasma total cholesterol concentrations within the ranges of the normal population but with increased high density lipoprotein and decreased low density lipoprotein fractions. Total plasma triglyceride values were not significantly elevated but the distributions in the low density lipoprotein and high density lipoprotein fractions were significantly increased in patients compared to controls. This alteration was accompanied by a consistent increase in serum apolipoprotein C-III concentration. Conversely, in patients with cirrhosis, serum concentrations of apolipoproteins A-I and B were significantly lower and were reflected in the cholesterol concentrations in the lipoprotein fractions. Comparisons between abstainers and non-abstainers within the group with cirrhosis indicated that cessation of alcohol intake was not sufficient to rectify lipoprotein dysfunction following damage from cirrhosis.

Apolipoprotein A-I conformation in discoidal particles: evidence for alternate structures

To define the conformation of apolipoprotein A-I in discoidal particles, the immunoreactivity of a series of epitopes distributed along the apoA-I sequence has been evaluated in lipid-free apoA-I and in lipid-bound form. To this end, reconstituted discoidal lipoproteins, here called LpA-I, and defined by number of apoA-I per particle (e.g., Lp2A-I), have been prepared with palmitoyloleoylphosphatidylcholine, cholesterol, and apoA-I. Four LpA-I have been obtained and studied: two in the Lp2A-I class, 7.8 and 9.6 nm in diameter, and two in the Lp3A-I class, 10.8 and 13.4 nm. The immunoreactivity of all the epitopes tested was significantly different in LpA-I particles compared to lipid-free apoA-I, demonstrating that binding to lipids produces a drastic change in apoA-I conformation. Specific domains in the primary sequence become highly exposed while others are masked. Although the variation in immunoreactivity of the epitopes between various LpA-I was not drastic, significant differences in the calculated ED50 values were observed for a number of antibodies in small versus large particles within each class (Lp2A-I or Lp3A-I), indicating that particle size can modulate apoA-I conformation. In addition, when the competition between pairs of mAbs was analyzed in order to understand the relative position of epitopes, highly significant differences were observed as a function of particle size within each class. In particular, the competition between mAbs recognizing epitopes in the central region of apoA-I was greater in the larger particles than in their small counterparts.(ABSTRACT TRUNCATED AT 250 WORDS)

Plasma triglycerides determine low density lipoprotein composition, physical properties, and cell-specific binding in cultured cells

The relationship between the plasma triglycerides and the LDL triglycerides of 30 normal and 48 hypertriglyceridemic subjects has been quantified; the data fit a simple adsorption isotherm, LDL triglyceride/(LDL triglyceride+LDL cholesterol ester) = 0.65 plasma triglyceride/(464 + plasma triglyceride). In vitro transfer of triglyceride from concentrated VLDL to VLDL-depleted plasma produced triglyceride-rich LDL that had similar properties. LDL uptake by HepG2 cells increased with LDL triglyceride content whereas the reverse was found with skin fibroblasts. At 37 degrees C, the cores of both normal and hypertriglyceridemic LDL were isotropic liquids. Circular dichroic spectra revealed no difference in the secondary structure of normal and triglyceride-rich LDL. The affinity of monoclonal antibody MB47, which binds to the receptor ligand of apo B-100 was independent of LDL triglyceride content. MB3, which binds near residue 1022 of apo B-100, showed a triglyceride-dependent decrease in affinity for LDL from hypertriglyceridemic subjects and from in vitro incubations. LDL with an elevated triglyceride content formed in vitro had reduced proteolytic cleavage of apo B-100 by Staphylococcus aureus V8 protease. From these data, we infer that (a) LDL triglyceride is a predictable function of plasma triglyceride, (b) triglyceride induces subtle changes in apo B-100 structure at a site that is remote from the putative receptor binding ligand, and (c) the triglyceride-dependent receptor-binding determinants of apo B-100 are recognized differently by fibroblasts and HepG2 cells.

Kinetics of the interaction of amphipathic alpha-helical peptides with phosphatidylcholines

The rate of association of three amphipathic alpha-helical peptides with phosphatidylcholine liposomes was examined to provide more detailed information on the relationship between peptide length and the kinetics of lipid interactions. When added to dimyristoylphosphatidylcholine (DMPC) or dipalmitoylphosphatidylcholine (DPPC) liposomes from a guanidine-HCl solution, a ten residue peptide rapidly decreased the turbidity of the liposomes. However, a related 17-residue peptide had only a minimal effect on liposome turbidity. A 14-residue peptide was intermediate in effectiveness. Similarly, when liposomes were added to peptides dissolved in an aqueous buffer, the ten residue peptide but not the 17-residue peptide cleared the turbid liposomes and the 14-residue peptide was intermediate in efficacy. The rate of binding to the liposomes was compared with the three peptides by measurements of the kinetics of energy transfer from the single tryptophan residue of the peptides to a fluorescent probe in the bilayer interior. The tryptophan residue of the ten residue peptide effectively transferred energy to the probe, while that of the 14-residue peptide was less effective. Little or no energy transfer was observed with the 17-residue peptide. The binding of the 10 residue peptide was rapid and complete within < 100 ms. The 14-residue peptide bound more slowly, but still within seconds. The time frames for binding are an order of magnitude shorter than those observed for lipid clearing. The relationship between peptide length, liposome clearing and lipid binding kinetics is discussed in terms of a possible competing peptide-peptide interaction in the aqueous phase and a slow rearrangement of the lipid bilayer.

Promotion of beta-structure by interaction of diabetes associated polypeptide (amylin) with phosphatidylcholine

The interaction of the diabetes associated polypeptide (amylin) with dimyristoylphosphatidylcholine (DMPC) was assessed by measurements of turbidity (absorbance at 400 nm) and secondary structure by CD spectroscopy. In trifluoroethanol, human amylin adopts a highly alpha-helical conformation while the rat peptide is less structured. In water, the rat peptide is largely disordered and the human peptide exhibits a combination of alpha- and beta-structures. Mixtures of DMPC and the rat peptide have no effect on either the turbidity of the DMPC or the CD spectrum of the peptide. By contrast, mixtures of the human peptide with DMPC form relatively clear mixtures similar to those observed with amphipathic alpha-helical peptides, but the structure adopted, based on the CD spectrum, is largely beta. These data demonstrate that fundamental differences in the structures adopted by amylins from human and rat species exist in mixtures with DMPC and suggest that these differences may be related to the formation of amyloid fibrils in the human amylin peptide which are not observed in the rat peptide.

Amino acid sequence of a non-specific wheat phospholipid transfer protein and its conformation as revealed by infrared and Raman spectroscopy. Role of disulfide bridges and phospholipids in the stabilization of the alpha-helix structure

A wheat non specific phospholipid transfer protein has been isolated from wheat seeds and its amino acid sequence reveals that it is composed of 90 residues for a molecular weight of 9607. From the comparison of its sequence with those of the eight known proteins of the same family, hypotheses on the role of some conserved residues in the transfer activity can be made. The conformation of this protein has been studied by Raman and Fourier transform infrared spectroscopy and this is the first report on the structure of non specific plant phospholipid transfer proteins. As opposed to previous studies on the structure prediction from the amino acid sequence, the results obtained show that plant non specific phospholipid transfer proteins are not almost entirely composed of beta-sheets. Instead, infrared results show that the wheat protein contains 41% alpha-helix and 19% beta-sheet structures, while 40% of the conformation is undefined or composed of turns. Raman spectroscopy shows that three disulfide bridges adopt a gauche-gauche-gauche conformation while the other exhibits a gauche-gauche-trans conformation, and that the two tyrosine residues are hydrogen bonded to water molecules. The cleavage of the disulfide bonds affects significantly the conformation of the protein, the extended confirmation being increased by 15% at the expense of the alpha-helix content. On the other hand, the binding of 1-palmitoyllysophosphatidylcholine to the protein leads to an increase of 8% of the alpha-helix content compared to the free protein. Secondary structure predictions from the amino acid sequence suggest that the binding of a phospholipid stabilizes helicity of the amphipathic helices while the reduction of disulfide bonds would affect the stability of the N-terminal helix. The extended structure located at the C-terminus is not affected. Finally, the wheat phospholipid transfer protein has no effect on the thermotropic behavior of large unilamellar vesicles of dimyristoylphosphatidylcholine while it increases the conformational order of the acyl chains of large unilamellar vesicles of dimyristoylphosphatidylglycerol in the liquid-crystalline state. No major conformational changes of the protein are observed when it is adsorbed to phospholipid vesicles. These results suggest that the helical structure is essential for the transfer activity without excluding a possible role of the C-terminal extended structure on the adsorption to phospholipid vesicles.

Sequence specific 1H-NMR assignments and secondary structure of a carboxy-terminal functional fragment of apolipoprotein CII

The structural properties of a synthetic fragment of human apolipoprotein CII (apoCII) has been studied by circular dichroism and proton nuclear magnetic resonance. The fragment corresponds to the carboxy-terminal 30 amino acid residues and retains the ability of apoCII to activate lipoprotein lipase. Like native apoCII, the fragment has a tendency to self-associate in pure aqueous solution. Addition of 1,1,1,3,3,3-hexafluoro-2-isopropanol to aqueous solvent dissolves the aggregates and leads to an increase in the alpha-helical content of the peptide, probably by stabilizing transient helical structures. The resonances in the 1H-NMR spectrum of the fragment in 35% (CF3)2CHOH were assigned through standard procedures from nuclear Overhauser enhancement spectroscopy, correlated spectroscopy and total correlated spectroscopy experiments. The NMR data indicates the formation of a stable alpha helix spanning Ile66-Gly77. Another alpha helical turn may be formed between Lys55 and Ala59 and possibly span even further towards the carboxyl terminus. These structural elements are different from those previously predicted for this part of the sequence of apoCII.

Effect of dietary fats on erythrocyte membrane lipid composition and membrane-bound enzyme activities

Four different oil-based diets were used in a feeding study involving rats to assess the relationship between the fatty acid composition of the dietary fat and its influence on erythrocyte membrane (EM) lipid composition and the activities of membrane-bound enzymes. Nutritionally adequate diets containing 20% groundnut (GNO), coconut (CO), safflower (SO), or mustard oil (MO) were fed to weanling CFY rats for 4 months. EMs were analyzed for total cholesterol, phospholipids, fatty acid profiles, and sialic acid content. Activities of membrane-bound enzymes such as Na+, K(+)-adenosine triphosphatase (ATPase), Mg(2+)-ATPase, Ca2+, Mg(2+)-ATPase, and acetylcholinesterase were also assayed. The activities of all membrane-bound enzymes, except Mg(2+)-ATPase, and sialic acid content were higher in the MO-fed group than in the rest of the groups. Ca2+, Mg(2+)-ATPase activity was distinctly lower in the SO-fed group than in the other groups. Cholesterol to phospholipid molar ratio was similar in all the groups. However, SO- and MO-fed groups displayed an increased cholesterol content and a higher degree of unsaturation in the membrane fatty acid composition. The higher membrane fatty acid unsaturation in the SO-fed group was principally due to linoleic (18:2) and arachidonic (20:4) acids, while in the MO-fed group it was mainly due to oleic (18:1), eicosenoic (20:1), erucic (22:1), and linoleic (18:2) acids. These results suggest a relationship between the quality of dietary fat, EM fatty acyl composition, and the activities of membrane-bound enzymes.

Examination of the peptide sequence requirements for lipid-binding. Alternative pathways for promoting the interaction of amphipathic alpha-helical peptides with phosphatidylcholine

To examine the relationship between peptide sequence and the interaction of amphipathic alpha-helical peptides with phosphatidylcholines, various methods of mixing the peptide and lipid were explored. A series of amphipathic alpha-helical peptides containing from 10 to 18 residues were synthesized by solid-phase techniques. An 18-residue peptide and two relatively hydrophobic 10-residue peptides did not disrupt dimyristoylphosphatidylcholine liposomes when added to the lipid in buffer. However, when the peptides were premixed with lipid in a suitable organic solvent and then reconstituted with aqueous buffer, clear micelles were formed, indicating association of the amphipathic alpha-helical peptide with lipid. In general, the best solvent for this purpose was trifluoroethanol. The circular dichroic and fluorescence spectra of peptides which readily formed clear mixtures when mixed in buffer with dimyristoylphosphatidylcholine liposomes were similar when prepared either by the alternative pathway technique using trifluoroethanol or by a cholate removal technique. For the peptides which did not clear liposomes in buffer, first mixing with dimyristoylphosphatidylcholine in trifluoroethanol resulted in an increase in the alpha-helicity of the peptides as judged by circular dichroic spectra and a blue-shift in the fluorescence emission maxima of the single tryptophan residue in each peptide. These data are consistent with formation of an amphipathic alpha-helix in lipid by peptides which based on mixing experiments with dimyristoylphosphatidylcholine liposomes in buffer at the phase transition temperature of the lipid would be considered ineffective in lipid binding. Thus, simple mixing of peptides with liposomes may give misleading results concerning the intrinsic affinity of a particular peptide sequence for lipid. In addition, the data demonstrate that relatively hydrophobic amphipathic alpha-helical peptides which do not form small micelles with dimyristoylphosphatidylcholine spontaneously in aqueous solution may interact with lipid as typical amphipathic alpha-helices when mixed by an alternative pathway.