Arrangement of apolipoprotein A-I in reconstituted high-density lipoprotein disks: an alternative model based on fluorescence resonance energy transfer experiments

Computational studies of plasma lipoprotein lipids

Plasma lipoproteins are macromolecular assemblies of proteins and lipids found in the blood. The lipid components of lipoproteins are amphipathic lipids such as phospholipids (PLs), and unesterified cholesterols (UCs) and hydrophobic lipids such as cholesteryl esters (CEs) and triglycerides (TGs). Since lipoproteins are soft matter supramolecular assemblies easily deformable by thermal fluctuations and they also exist in varying densities and protein/lipid components, a detailed understanding of their structure/function is experimentally difficult. Molecular dynamics (MD) simulation has emerged as a particularly promising way to explore the structure and dynamics of lipoproteins. The purpose of this review is to survey the current status of computational studies of the lipid components of the lipoproteins. Computational studies aim to explore three levels of complexity for the 3-dimensional structural dynamics of lipoproteins at various metabolic stages: (i) lipoprotein particles consist of protein with minimal lipid; (ii) lipoprotein particles consist of PL-rich discoidal bilayer-like lipid particles; (iii) mature circulating lipoprotein particles consist of CE-rich or TG-rich spheroidal lipid-droplet-like particles. Due to energy barriers involved in conversion between these species, other biomolecules also participate in lipoprotein biological assembly. For example: (i) lipid-poor apolipoprotein A-I (apoA-I) interacts with ATP-binding cassette transporter A1 (ABCA1) to produce nascent discoidal high density lipoprotein (dHDL) particles; (ii) lecithin-cholesterol acyltransferase (LCAT) mediates the conversion of UC to CE in dHDL, driving spheroidal HDL (sHDL) formation; (iii) transfer proteins, cholesterol ester transfer protein (CETP) and phospholipid transfer protein (PLTP), transfer both CE and TG and PL, respectively, between lipoprotein particles. Computational studies have the potential to explore different lipoprotein particles at each metabolic stage in atomistic detail. This review discusses the current status of computational methods including all-atom MD (AAMD), coarse-grain MD (CGMD), and MD-simulated annealing (MDSA) and their applications in lipoprotein structural dynamics and biological assemblies. Results from MD simulations are discussed and compared across studies in order to identify key findings, controversies, issues and future directions. This article is part of a Special Issue entitled: Biosimulations edited by Ilpo Vattulainen and Tomasz Róg.

Structural Insights into High Density Lipoprotein: Old Models and New Facts

The physiological link between circulating high density lipoprotein (HDL) levels and cardiovascular disease is well-documented, albeit its intricacies are not well-understood. An improved appreciation of HDL function and overall role in vascular health and disease requires at its foundation a better understanding of the lipoprotein's molecular structure, its formation, and its process of maturation through interactions with various plasma enzymes and cell receptors that intervene along the pathway of reverse cholesterol transport. This review focuses on summarizing recent developments in the field of lipid free apoA-I and HDL structure, with emphasis on new insights revealed by newly published nascent and spherical HDL models constructed by combining low resolution structures obtained from small angle neutron scattering (SANS) with contrast variation and geometrical constraints derived from hydrogen-deuterium exchange (HDX), crosslinking mass spectrometry, electron microscopy, Förster resonance energy transfer, and electron spin resonance. Recently published low resolution structures of nascent and spherical HDL obtained from SANS with contrast variation and isotopic labeling of apolipoprotein A-I (apoA-I) will be critically reviewed and discussed in terms of how they accommodate existing biophysical structural data from alternative approaches. The new low resolution structures revealed and also provided some answers to long standing questions concerning lipid organization and particle maturation of lipoproteins. The review will discuss the merits of newly proposed SANS based all atom models for nascent and spherical HDL, and compare them with accepted models. Finally, naturally occurring and bioengineered mutations in apoA-I, and their impact on HDL phenotype, are reviewed and discuss together with new therapeutics employed for restoring HDL function.

Surface Density-Induced Pleating of a Lipid Monolayer Drives Nascent High-Density Lipoprotein Assembly

Biogenesis of high-density lipoproteins (HDL) is coupled to the transmembrane protein, ATP-binding cassette transporter A1 (ABCA1), which transports phospholipid (PL) from the inner to the outer membrane monolayer. Using a combination of computational and experimental approaches, we show that increased outer lipid monolayer surface density, driven by excess PL or membrane insertion of amphipathic helices, results in pleating of the outer monolayer to form membrane-attached discoidal bilayers. Apolipoprotein (apo)A-I accelerates and stabilizes the pleats. In the absence of apoA-I, pleats collapse to form vesicles. These results mimic cells overexpressing ABCA1 that, in the absence of apoA-I, form and release vesicles. We conclude that the basic driving force for nascent discoidal HDL assembly is a PL pump-induced surface density increase that produces lipid monolayer pleating. We then argue that ABCA1 forms an extracellular reservoir containing an isolated pressurized lipid monolayer decoupled from the transbilayer density buffering of cholesterol.

Amyloidogenic propensity of a natural variant of human apolipoprotein A-I: stability and interaction with ligands

A number of naturally occurring mutations of human apolipoprotein A-I (apoA-I) have been associated with hereditary amyloidoses. The molecular mechanisms involved in amyloid-associated pathology remain largely unknown. Here we examined the effects of the Arg173Pro point mutation in apoA-I on the structure, stability, and aggregation propensity, as well as on the ability to bind to putative ligands. Our results indicate that the mutation induces a drastic loss of stability, and a lower efficiency to bind to phospholipid vesicles at physiological pH, which could determine the observed higher tendency to aggregate as pro-amyloidogenic complexes. Incubation under acidic conditions does not seem to induce significant desestabilization or aggregation tendency, neither does it contribute to the binding of the mutant to sodium dodecyl sulfate. While the binding to this detergent is higher for the mutant as compared to wt apoA-I, the interaction of the Arg173Pro variant with heparin depends on pH, being lower at pH 5.0 and higher than wt under physiological pH conditions. We suggest that binding to ligands as heparin or other glycosaminoglycans could be key events tuning the fine details of the interaction of apoA-I variants with the micro-environment, and probably eliciting the toxicity of these variants in hereditary amyloidoses.

A model of lipid-free apolipoprotein A-I revealed by iterative molecular dynamics simulation

Apolipoprotein A-I (apo A-I), the major protein component of high-density lipoprotein, has been proven inversely correlated to cardiovascular risk in past decades. The lipid-free state of apo A-I is the initial stage which binds to lipids forming high-density lipoprotein. Molecular models of lipid-free apo A-I have been reported by methods like X-ray crystallography and chemical cross-linking/mass spectrometry (CCL/MS). Through structural analysis we found that those current models had limited consistency with other experimental results, such as those from hydrogen exchange with mass spectrometry. Through molecular dynamics simulations, we also found those models could not reach a stable equilibrium state. Therefore, by integrating various experimental results, we proposed a new structural model for lipid-free apo A-I, which contains a bundled four-helix N-terminal domain (1–192) that forms a variable hydrophobic groove and a mobile short hairpin C-terminal domain (193–243). This model exhibits an equilibrium state through molecular dynamics simulation and is consistent with most of the experimental results known from CCL/MS on lysine pairs, fluorescence resonance energy transfer and hydrogen exchange. This solution-state lipid-free apo A-I model may elucidate the possible conformational transitions of apo A-I binding with lipids in high-density lipoprotein formation.

An experimentally robust model of monomeric apolipoprotein A-I created from a chimera of two X-ray structures and molecular dynamics simulations

High-density lipoprotein (HDL) retards atherosclerosis by accepting cholesterol from the artery wall. However, the structure of the proposed acceptor, monomeric apolipoprotein A-I (apoA-I), the major protein of HDL, is poorly understood. Two published models for monomeric apoA-I used cross-linking distance constraints to derive best fit conformations. This approach has limitations. (i) Cross-linked peptides provide no information about secondary structure. (ii) A protein chain can be folded in multiple ways to create a best fit. (iii) Ad hoc folding of a secondary structure is unlikely to produce a stable orientation of hydrophobic and hydrophilic residues. To address these limitations, we used a different approach. We first noted that the dimeric apoA-I crystal structure, (Δ185-243)apoA-I, is topologically identical to a monomer in which helix 5 forms a helical hairpin, a monomer with a hydrophobic cleft running the length of the molecule. We then realized that a second crystal structure, (Δ1-43)apoA-I, contains a C-terminal structure that fits snuggly via aromatic and hydrophobic interactions into the hydrophobic cleft. Consequently, we combined these crystal structures into an initial model that was subjected to molecular dynamics simulations. We tested the initial and simulated models and the two previously published models in three ways: against two published data sets (domains predicted to be helical by H/D exchange and six spin-coupled residues) and against our own experimentally determined cross-linking distance constraints. We note that the best fit simulation model, superior by all tests to previously published models, has dynamic features of a molten globule with interesting implications for the functions of apoA-I.

The conformation of lipid-free human apolipoprotein A-I in solution

Apolipoprotein AI (apoA-I) is the principal acceptor of lipids from ATP-binding cassette transporter A1, a process that yields nascent high density lipoproteins. Analysis of lipidated apoA-I conformation yields a belt or twisted belt in which two strands of apoA-I lie antiparallel to one another. In contrast, biophysical studies have suggested that a part of lipid-free apoA-I was arranged in a four-helix bundle. To understand how lipid-free apoA-I opens from a bundle to a belt while accepting lipid it was necessary to have a more refined model for the conformation of lipid-free apoA-I. This study reports the conformation of lipid-free human apoA-I using lysine-to-lysine chemical cross-linking in conjunction with disulfide cross-linking achieved using selective cysteine mutations. After proteolysis, cross-linked peptides were verified by sequencing using tandem mass spectrometry. The resulting structure is compact with roughly four helical regions, amino acids 44-186, bundled together. C- and N-terminal ends, amino acids 1-43 and 187-243, respectively, are folded such that they lie close to one another. An unusual feature of the molecule is the high degree of connectivity of lysine40 with six other lysines, lysines that are close, for example, lysine59, to distant lysines, for example, lysine239, that are at the opposite end of the primary sequence. These results are compared and contrasted with other reported conformations for lipid-free human apoA-I and an NMR study of mouse apoA-I.

Human apolipoprotein A-I natural variants: molecular mechanisms underlying amyloidogenic propensity

Human apolipoprotein A-I (apoA-I)-derived amyloidosis can present with either wild-type (Wt) protein deposits in atherosclerotic plaques or as a hereditary form in which apoA-I variants deposit causing multiple organ failure. More than 15 single amino acid replacement amyloidogenic apoA-I variants have been described, but the molecular mechanisms involved in amyloid-associated pathology remain largely unknown. Here, we have investigated by fluorescence and biochemical approaches the stabilities and propensities to aggregate of two disease-associated apoA-I variants, apoA-IGly26Arg, associated with polyneuropathy and kidney dysfunction, and apoA-ILys107-0, implicated in amyloidosis in severe atherosclerosis. Results showed that both variants share common structural properties including decreased stability compared to Wt apoA-I and a more flexible structure that gives rise to formation of partially folded states. Interestingly, however, distinct features appear to determine their pathogenic mechanisms. ApoA-ILys107-0 has an increased propensity to aggregate at physiological pH and in a pro-inflammatory microenvironment than Wt apoA-I, whereas apoA-IGly26Arg elicited macrophage activation, thus stimulating local chronic inflammation. Our results strongly suggest that some natural mutations in apoA-I variants elicit protein tendency to aggregate, but in addition the specific interaction of different variants with macrophages may contribute to cellular stress and toxicity in hereditary amyloidosis.

Structure of saposin A lipoprotein discs

The saposins are small, membrane-active proteins that exist in both soluble and lipid-bound states. Saposin A has roles in sphingolipid catabolism and transport and is required for the breakdown of galactosylceramide by β-galactosylceramidase. In the absence of lipid, saposin A adopts a closed monomeric apo conformation typical of this family. To study a lipid-bound state of this protein, we determined the crystal structure of saposin A in the presence of detergent to 1.9 Å resolution. The structure reveals two chains of saposin A in an open conformation encapsulating 40 internally bound detergent molecules organized in a highly ordered bilayer-like hydrophobic core. The complex provides a high-resolution view of a discoidal lipoprotein particle in which all of the internalized acyl chains are resolved. Saposin A lipoprotein discs exhibit limited selectivity with respect to the incorporated lipid, and can solubilize phospholipids, sphingolipids, and cholesterol into discrete, monodisperse particles with mass of approximately 27 kDa. These discs may be the smallest possible lipoprotein structures that are stabilized by lipid self-assembly.

The "beta-clasp" model of apolipoprotein A-I--a lipid-free solution structure determined by electron paramagnetic resonance spectroscopy

Apolipoprotein A-I (apoA-I) is the major protein component of high density lipoproteins (HDL) and plays a central role in cholesterol metabolism. The lipid-free/lipid-poor form of apoA-I is the preferred substrate for the ATP-binding cassette transporter A1 (ABCA1). The interaction of apoA-I with ABCA1 leads to the formation of cholesterol laden high density lipoprotein (HDL) particles, a key step in reverse cholesterol transport and the maintenance of cholesterol homeostasis. Knowledge of the structure of lipid-free apoA-I is essential to understanding its critical interaction with ABCA1 and the molecular mechanisms underlying HDL biogenesis. We therefore examined the structure of lipid-free apoA-I by electron paramagnetic resonance spectroscopy (EPR). Through site directed spin label EPR, we mapped the secondary structure of apoA-I and identified sites of spin coupling as residues 26, 44, 64, 167, 217 and 226. We capitalize on the fact that lipid-free apoA-I self-associates in an anti-parallel manner in solution. We employed these sites of spin coupling to define the central plane in the dimeric apoA-I complex. Applying both the constraints of dipolar coupling with the EPR-derived pattern of solvent accessibility, we assembled the secondary structure into a tertiary context, providing a solution structure for lipid-free apoA-I. This article is part of a Special Issue entitled Advances in High Density Lipoprotein Formation and Metabolism: A Tribute to John F. Oram (1945-2010).

Stoichiometry of reconstituted high-density lipoproteins in the hydrated state determined by photon antibunching

Apolipoprotein A-I plays a central role in the solution structure of high-density lipoproteins. Determining the stoichiometry of lipid-bound apo A-I in the hydrated state is therefore fundamental to understanding how high-density lipoproteins form and function. Here, we use the quantum optical phenomenon of photon antibunching to determine the number of apo A-I molecules bound to discoidal lipoproteins and compare this with values obtained by photon-counting histogram analysis. Both the photon antibunching and photon-counting analyses show that reconstituted high-density lipoprotein particles contain two apo A-I molecules, which is in agreement with the commonly accepted double-belt model.

Rotational and hinge dynamics of discoidal high density lipoproteins probed by interchain disulfide bond formation

To develop a detailed double belt model for discoidal HDL, we previously scored inter-helical salt bridges between all possible registries of two stacked antiparallel amphipathic helical rings of apolipoprotein (apo) A-I. The top score was the antiparallel apposition of helix 5 with 5 followed closely by appositions of helix 5 with 4 and helix 5 with 6. The rationale for the current study is that, for each of the optimal scores, a pair of identical residues can be identified in juxtaposition directly on the contact edge between the two antiparallel helical belts of apoA-I. Further, these residues are always in the '9th position' in one of the eighteen 11-mer repeats that make up the lipid-associating domain of apoA-I. To illustrate our terminology, 129j (LL5/5) refers to the juxtaposition of the Cα atoms of G129 (in a '9th position') in the pairwise helix 5 domains. We reasoned that if identical residues in the double belt juxtapositions were mutated to a cysteine and kept under reducing conditions during disc formation, we would have a precise method for determining registration in discoidal HDL by formation of a disulfide-linked apoA-I homodimer. Using this approach, we conclude that 129j (LL5/5) is the major rotamer orientation for double belt HDL and propose that the small ubiquitous gap between the pairwise helix 5 portions of the double belt in larger HDL discoidal particles is significantly dynamic to hinge off the disc edge under certain conditions, e.g., in smaller particles or perhaps following binding of the enzyme LCAT. This article is part of a Special Issue entitled Advances in High Density Lipoprotein Formation and Metabolism: A Tribute to John F. Oram (1945-2010).

Human apolipoprotein A-I-derived amyloid: its association with atherosclerosis

Amyloidoses constitute a group of diseases in which soluble proteins aggregate and deposit extracellularly in tissues. Nonhereditary apolipoprotein A-I (apoA-I) amyloid is characterized by deposits of nonvariant protein in atherosclerotic arteries. Despite being common, little is known about the pathogenesis and significance of apoA-I deposition. In this work we investigated by fluorescence and biochemical approaches the impact of a cellular microenvironment associated with chronic inflammation on the folding and pro-amyloidogenic processing of apoA-I. Results showed that mildly acidic pH promotes misfolding, aggregation, and increased binding of apoA-I to extracellular matrix elements, thus favoring protein deposition as amyloid like-complexes. In addition, activated neutrophils and oxidative/proteolytic cleavage of the protein give rise to pro amyloidogenic products. We conclude that, even though apoA-I is not inherently amyloidogenic, it may produce non hereditary amyloidosis as a consequence of the pro-inflammatory microenvironment associated to atherogenesis.

Conformational transitions in the membrane scaffold protein of phospholipid bilayer nanodiscs

Phospholipid bilayer nanodiscs are model membrane systems that provide an environment where membrane proteins are highly stable and monodisperse without the use of detergents or liposomes. Nanodiscs consist of a discoidal phospholipid bilayer encircled by two copies of an amphipathic alpha helical membrane scaffold protein, which is modeled from apolipoprotein A-1. Hydrogen exchange mass spectrometry was used to probe the structure and dynamics of the scaffold protein in the presence and absence of lipid. On nanodisc self-assembly, the entire scaffold protein gained significant protection from exchange, consistent with a large, protein-wide, structural rearrangement. This protection was short-lived and the scaffold protein was highly deuterated within 2 h. Several regions of the scaffold protein, in both the lipid-free and lipid-associated states, displayed EX1 unfolding kinetics. The rapid deuteration of the scaffold protein and the presence of correlated unfolding events both indicate that nanodiscs are dynamic rather than rigid bodies in solution. This work provides a catalog of the expected scaffold protein peptic peptides in a nanodisc-hydrogen exchange mass spectrometry experiment and their deuterium uptake signatures, data that can be used as a benchmark to verify correct assembly and nanodisc structure. Such reference data will be useful control data for all hydrogen exchange mass spectrometry experiments involving nanodiscs in which transmembrane or lipid-associated proteins are the primary molecule(s) of interest.

Membrane insertion topology of the central apolipoprotein A-I region. Fluorescence studies using single tryptophan mutants

Apolipoprotein A-I (apoAI) contains several amphipathic α-helices. To carry out its function, it exchanges between lipid-free and different lipidated states as bound to membranes or to lipoprotein complexes of different morphology, size, and composition. When bound to membranes or to spherical lipoprotein surfaces, it is thought that most α-helices arrange with their long axis parallel to the membrane surface. However, we previously found that a central region spanning residues 87-112 is exclusively labeled by photoactivable reagents deeply located into the membrane (Córsico et al. (2001) J. Biol. Chem. 276, 16978-16985). A pair of amphipathic α-helical repeats with a particular charge distribution is predicted in this region. In order to study their insertion topology, three single tryptophan mutants, each one containing the tryptophan residue at a selected position in the hydrophobic face of the central Y-helices (W@93, W@104, and W@108), were used. From the accessibility to quenchers located at different membrane depths, distances from the bilayer center of 13.4, 10.5, and 15.7 Å were estimated for positions 93, 104, and 108, respectively. Reported data also indicate that distances between homologous positions (in particular for W@93 and W@104) are very short in dimers in aqueous solution, but they are larger in membrane-bound dimers. Data indicate that an intermolecular central Y-helix bundle would penetrate the membrane perpendicularly to the membrane surface. Intermolecular helix-helix interactions would occur through the hydrophilic helix faces in the membrane-bound bundle but through the hydrophobic faces in the case of dimers in solution.

Morphology and structure of lipoproteins revealed by an optimized negative-staining protocol of electron microscopy

Plasma lipoprotein levels are predictors of risk for coronary artery disease. Lipoprotein structure-function relationships provide important clues that help identify the role of lipoproteins in cardiovascular disease. The compositional and conformational heterogeneity of lipoproteins are major barriers to the identification of their structures, as discovered using traditional approaches. Although electron microscopy (EM) is an alternative approach, conventional negative staining (NS) produces rouleau artifacts. In a previous study of apolipoprotein (apo)E4-containing reconstituted HDL (rHDL) particles, we optimized the NS method in a way that eliminated rouleaux. Here we report that phosphotungstic acid at high buffer salt concentrations plays a key role in rouleau formation. We also validate our protocol for analyzing the major plasma lipoprotein classes HDL, LDL, IDL, and VLDL, as well as homogeneously prepared apoA-I-containing rHDL. High-contrast EM images revealed morphology and detailed structures of lipoproteins, especially apoA-I-containing rHDL, that are amenable to three-dimensional reconstruction by single-particle analysis and electron tomography.

Novel N-terminal mutation of human apolipoprotein A-I reduces self-association and impairs LCAT activation

We have identified a novel mutation in apoA-I (serine 36 to alanine; S36A) in a human subject with severe hypoalphalipoproteinemia. The mutation is located in the N-terminal region of the protein, which has been implicated in several functions, including lipid binding and lecithin:cholesterol acyltransferase (LCAT) activity. In the present study, the S36A protein was produced recombinantly and characterized both structurally and functionally. While the helical content of the mutant protein was lower compared with wild-type (WT) apoA-I, it retained its helical character. The protein stability, measured as the resistance to guanidine-induced denaturation, decreased significantly. Interestingly, native gel electrophoresis, cross-linking, and sedimentation equilibrium analysis showed that the S36A mutant was primarily present as a monomer, notably different from the WT protein, which showed considerable oligomeric forms. Although the ability of S36A apoA-I to solubilize phosphatidylcholine vesicles and bind to lipoprotein surfaces was not altered, a significantly impaired LCAT activation compared with the WT protein was observed. These results implicate a region around S36 in apoA-I self-association, independent of the intact C terminus. Furthermore, the region around S36 in the N-terminus of human apoA-I is necessary for LCAT activation.

Conformation of dimeric apolipoprotein A-I milano on recombinant lipoprotein particles

Apolipoprotein A-I Milano (apoA-I(Milano)) is a naturally occurring human mutation of wild-type apolipoprotein A-I (apoA-I(WT)) having cystine substituted for arginine(173). Two molecules of apo-I(WT) form disks with phospholipid having a defined relationship between the apoA-I(WT) molecules. ApoA-I(Milano) forms cystine homodimers that would not allow the protein to adopt the conformation reported for apoA-I(WT). The conformational constraints for dimeric apoA-I(Milano) recombinant high-density lipoprotein (rHDL) disks made with phospholipid were deduced from a combination of chemical cross-linking and mass spectrometry. Lysine-selective homobifunctional cross-linkers were reacted with homogeneous rHDL having diameters of 78 and 125 A. After reduction, cross-linked apoA-I(Milano) was separated from monomeric apoprotein by gel electrophoresis and then subjected to in-gel trypsin digest. Cross-linked peptides were confirmed by MS/MS sequencing. The cross-links provided distance constraints that were used to refine models of lipid-bound dimeric apoA-I(Milano). These studies suggest that a single dimeric apoA-I(Milano) on 78 A diameter rHDL girdles the edge of a phospholipid disk assuming a "belt" conformation similar to the "belt" region of apoA-I(WT) on rHDL. However, the C-terminal end of dimeric apoA-I(Milano) wraps around the periphery of the particle to shield the fatty acid chains from water rather than folding back onto the "belt" as does apoA-I(WT). The two apoA-I(Milano) dimers on a 125 A diameter rHDL do not encircle the periphery of a phospholipid disk but appear to reside on the surface of a laminar micelle.

Lipid packing determines protein-membrane interactions: challenges for apolipoprotein A-I and high density lipoproteins

Protein and protein-lipid interactions, with and within specific areas in the cell membrane, are critical in order to modulate the cell signaling events required to maintain cell functions and viability. Biological bilayers are complex, dynamic platforms, and thus in vivo observations usually need to be preceded by studies on model systems that simplify and discriminate the different factors involved in lipid-protein interactions. Fluorescence microscopy studies using giant unilamellar vesicles (GUVs) as membrane model systems provide a unique methodology to quantify protein binding, interaction, and lipid solubilization in artificial bilayers. The large size of lipid domains obtainable on GUVs, together with fluorescence microscopy techniques, provides the possibility to localize and quantify molecular interactions. Fluorescence Correlation Spectroscopy (FCS) can be performed using the GUV model to extract information on mobility and concentration. Two-photon Laurdan Generalized Polarization (GP) reports on local changes in membrane water content (related to membrane fluidity) due to protein binding or lipid removal from a given lipid domain. In this review, we summarize the experimental microscopy methods used to study the interaction of human apolipoprotein A-I (apoA-I) in lipid-free and lipid-bound conformations with bilayers and natural membranes. Results described here help us to understand cholesterol homeostasis and offer a methodological design suited to different biological systems.

The helix bundle: a reversible lipid binding motif

Apolipoproteins are the protein components of lipoproteins that have the innate ability to inter convert between a lipid-free and a lipid-bound form in a facile manner, a remarkable property conferred by the helix bundle motif. Composed of a series of four or five amphipathic alpha-helices that fold to form a helix bundle, this motif allows the en face orientation of the hydrophobic faces of the alpha-helices in the protein interior in the lipid-free state. A conformational switch then permits helix-helix interactions to be substituted by helix-lipid interactions upon lipid binding interaction. This review compares the apolipoprotein high-resolution structures and the factors that trigger this switch in insect apolipophorin III and the mammalian apolipoproteins, apolipoprotein E and apolipoprotein A-I, pointing out the commonalities and key differences in the mode of lipid interaction. Further insights into the lipid-bound conformation of apolipoproteins are required to fully understand their functional role under physiological conditions.

Apolipoprotein AI tertiary structures determine stability and phospholipid-binding activity of discoidal high-density lipoprotein particles of different sizes

Human high-density lipoprotein (HDL) plays a key role in the reverse cholesterol transport pathway that delivers excess cholesterol back to the liver for clearance. In vivo, HDL particles vary in size, shape and biological function. The discoidal HDL is a 140-240 kDa, disk-shaped intermediate of mature HDL. During mature spherical HDL formation, discoidal HDLs play a key role in loading cholesterol ester onto the HDL particles by activating the enzyme, lecithin:cholesterol acyltransferase (LCAT). One of the major problems for high-resolution structural studies of discoidal HDL is the difficulty in obtaining pure and, foremost, homogenous sample. We demonstrate here that the commonly used cholate dialysis method for discoidal HDL preparation usually contains 5-10% lipid-poor apoAI that significantly interferes with the high-resolution structural analysis of discoidal HDL using biophysical methods. Using an ultracentrifugation method, we quickly removed lipid-poor apoAI. We also purified discoidal reconstituted HDL (rHDL) into two pure discoidal HDL species of different sizes that are amendable for high-resolution structural studies. A small rHDL has a diameter of 7.6 nm, and a large rHDL has a diameter of 9.8 nm. We show that these two different sizes of discoidal HDL particles display different stability and phospholipid-binding activity. Interestingly, these property/functional differences are independent from the apoAI alpha-helical secondary structure, but are determined by the tertiary structural difference of apoAI on different discoidal rHDL particles, as evidenced by two-dimensional NMR and negative stain electron microscopy data. Our result further provides the first high-resolution NMR data, demonstrating a promise of structural determination of discoidal HDL at atomic resolution using a combination of NMR and other biophysical techniques.

Three-dimensional models of HDL apoA-I: implications for its assembly and function

The purpose of this review is to highlight recent advances toward the refinement of a three-dimensional structure for lipid-bound apolipoprotein A-I (apoA-I) on recombinant HDL. Recently, X-ray crystallography has yielded a new structure for full-length, lipid-free apoA-I. Although this approach has not yet been successful in solving the three-dimensional structure of lipid-bound apoA-I, analysis of the X-ray structures has been of immense help in the interpretation of structural data obtained from other methods that yield structural information. Recent studies emphasize the use of mass spectrometry to unambiguously identify cross-linked peptides or to quantify solvent accessibility using hydrogen-deuterium exchange. The combination of mass spectrometry, molecular modeling, molecular dynamic analysis, and small-angle X-ray diffraction has provided additional structural information on apoA-I folding that complements previous approaches.

An analysis of the role of a retroendocytosis pathway in ABCA1-mediated cholesterol efflux from macrophages

The ATP binding cassette transporter A-1 (ABCA1) is critical for apolipoprotein-mediated cholesterol efflux, an important mechanism employed by macrophages to avoid becoming lipid-laden foam cells, the hallmark of early atherosclerotic lesions. It has been proposed that lipid-free apolipoprotein A-I (apoA-I) enters the cell and is resecreted as a lipidated particle via a retroendocytosis pathway during ABCA1-mediated cholesterol efflux from macrophages. To determine the functional importance of such a pathway, confocal microscopy was used to characterize the internalization of a fully functional apoA-I cysteine mutant containing a thiol-reactive fluorescent probe in cultured macrophages. ApoA-I was also endogenously labeled with (35)S-methionine to quantify cellular uptake and to determine the metabolic fate of the internalized protein. It was found that apoA-I was specifically taken inside macrophages and that a small amount of intact apoA-I was resecreted from the cells. However, a majority of the label that reappeared in the media was degraded. We estimate that the mass of apoA-I retroendocytosed is not sufficient to account for the HDL produced by the cholesterol efflux reaction. Furthermore, we have demonstrated that lipid-free apoA-I-mediated cholesterol efflux from macrophages can be pharmacologically uncoupled from apoA-I internalization into cells. On the basis these findings, we present a model in which the ABCA1-mediated lipid transfer process occurs primarily at the membrane surface in macrophages, but still accounts for the observed specific internalization of apoA-I.

Rapid incorporation of functional rhodopsin into nanoscale apolipoprotein bound bilayer (NABB) particles

Human apolipoprotein A-I (apo A-I) and its engineered constructs form discoidal lipid bilayers upon interaction with lipids in vitro. We now report the cloning, expression, and purification of apo A-I derived from zebrafish (Danio rerio), which combines with phospholipids to form similar discoidal bilayers and may prove to be superior to human apo A-I constructs for rapid reconstitution of seven-transmembrane helix receptors into nanoscale apolipoprotein bound bilayers (NABBs). We characterized NABBs by gel-filtration chromatography, native polyacrylamide gradient gel electrophoresis, UV-visible photobleaching difference spectroscopy, and fluorescence spectroscopy. We used electron microscopy to determine the stoichiometry and orientation of rhodopsin (rho)-containing NABBs prepared under various conditions and correlated stability and signaling efficiency of rho in NABBs with either one or two receptors. We discovered that the specific activity of G protein coupling for single rhos sequestered in individual NABBs was nearly identical with that of two rhos per NABB under conditions where stoichiometry and orientation could be inferred by electron microscopy imaging. Thermal stability of rho in NABBs was superior to that of rho in various commonly used detergents. We conclude that the NABB system using engineered zebrafish apo A-I is a native-like membrane mimetic system for G-protein-coupled receptors and discuss strategies for rapid incorporation of expressed membrane proteins into NABBs.

Computer modeling in biotechnology: a partner in development

Computational modeling can be a useful partner in biotechnology, in particular, in nanodevice engineering. Such modeling guides development through nanoscale views of biomolecules and devices not available through experimental imaging methods. We illustrate the role of computational modeling, mainly of molecular dynamics, through four case studies: development of silicon bionanodevices for single molecule electrical recording, development of carbon nano-tube-biomolecular systems as in vivo sensors, development of lipoprotein nanodiscs for assays of single membrane proteins, and engineering of oxygen tolerance into the enzyme hydrogenase for photosynthetic hydrogen gas production. The four case studies show how molecular dynamics approaches were adapted to the specific technical uses through (i) multi-scale extensions, (ii) fast quantum chemical force field evaluation, (iii) coarse graining, and (iv) novel sampling methods. The adapted molecular dynamics simulations provided key information on device behavior and revealed development opportunities, arguing that the "computational microscope" is an indispensable nanoengineering tool.

Role of extracellular domain dimerization in agonist-induced activation of natriuretic peptide receptor A

Natriuretic peptide receptor (NPR) A is composed of an extracellular domain (ECD) with a ligand binding site, a single transmembrane region, a kinase homology domain, and a guanylyl cyclase domain. The natural agonists atrial and brain natriuretic peptides (ANP, BNP) bind and activate NPRA, leading to cyclic GMP production, which is responsible for their role in cardiovascular homeostasis. Previous studies suggested that stabilization of a dimeric form of NPRA by agonist is essential for receptor activation. However, ligand specificity and sequential steps of this dimerization process have not been investigated. We used radioligand binding, fluorescence resonance energy transfer homoquenching, and molecular modeling to characterize the interaction of human NPRA-ECD with ANP, BNP, the superagonist (Arg(10),Leu(12),Ser(17),Leu(18))-rANP-(1-28), the minimized analog mini-ANP and the antagonist (Arg(6),beta-cyclohexyl-Ala(8),d-Tic(16),Arg(17),Cys(18))-rANP-(6-18)-amide (A71915). ANP binds to preformed ECD dimers and spontaneous dimerization is the rate-limiting step of the ligand binding process. All the studied peptides, including A71915 antagonist, induce a dose-dependent fluorescence homoquenching, specific to dimerization, with potencies highly correlated with their binding affinities. A71915 induced more quenching than other peptides, suggesting stabilization by the antagonist of ECD dimer in a distinct inactive conformation. In summary, these results indicate that the ligand-induced dimerization process of NPRA is different from that for cytokine receptor model. Agonists or antagonists bind to preformed dimeric ECD, leading to dimer stabilization in an active or inactive conformation, respectively. Furthermore, the highly sensitive fluorescence assay designed to assess dimerization could serve as a powerful tool for further detailing the kinetic steps involved in natriuretic peptide receptor binding and activation.

Assembly of lipids and proteins into lipoprotein particles

The self-assembly of reconstituted discoidal high-density lipoproteins, known as nanodiscs, was studied using coarse-grained molecular dynamics and small-angle X-ray scattering. In humans, high-density lipoprotein particles transport cholesterol in the blood and facilitate the removal of excess cholesterol from the body. Native high-density lipoprotein exhibits a wide variety of shapes and sizes, forming lipid-free/poor, nascent discoidal, and mature spherical particles. Little is known about how these lipoprotein particles assemble and transform from one state to another. Multiple 10 micros coarse-grained simulations reveal the assembly of discoidal high-density lipoprotein particles from disordered protein-lipid complexes. Small-angle X-ray scattering patterns were calculated from the final assembled structures and compared with experimental measurements carried out for this study to verify the accuracy of the coarse-grained simulations. Results show that hydrophobic interactions assemble, within several microseconds, the amphipathic helical proteins and lipids into roughly discoidal particles, while the proteins assume a final approximate double-belt configuration on a slower time scale.

Fluorescence as a method to reveal structures and membrane-interactions of amyloidogenic proteins

Amyloidogenesis is a characteristic feature of the 40 or so known protein deposition diseases, and accumulating evidence strongly suggests that self-association of misfolded proteins into either fibrils, protofibrils, or soluble oligomeric species is cytotoxic. The most likely mechanism for toxicity is through perturbation of membrane structure, leading to increased membrane permeability and eventual cell death. There have been a rather limited number of investigations of the interactions of amyloidogenic polypeptides and their aggregated states with membranes; these are briefly reviewed here. Amyloidogenic proteins discussed include A-beta from Alzheimer's disease, the prion protein, alpha-synuclein from Parkinson's disease, transthyretin (FAP, SSA amyloidosis), immunoglobulin light chains (primary (AL) amyloidosis), serum amyloid A (secondary (AA) amyloidosis), amylin or IAPP (Type 2 diabetes) and apolipoproteins. This review highlights the significant role played by fluorescence techniques in unraveling the nature of amyloid fibrils and their interactions and effects on membranes. Fluorescence spectroscopy is a valuable and versatile method for studying the complex mechanisms of protein aggregation, amyloid fibril formation and the interactions of amyloidogenic proteins with membranes. Commonly used fluorescent techniques include intrinsic and extrinsic fluorophores, fluorescent probes incorporated in the membrane, steady-state and lifetime measurements of fluorescence emission, fluorescence correlation spectroscopy, fluorescence anisotropy and polarization, fluorescence resonance energy transfer (FRET), fluorescence quenching, and fluorescence microscopy.

Interaction of high density lipoprotein particles with membranes containing cholesterol

In this study, free cholesterol (FC) efflux mediated by human HDL was investigated using fluorescence methodologies. The accessibility of FC to HDL may depend on whether it is located in regions rich in unsaturated phospholipids or in domains containing high levels of FC and sphingomyelin, known as "lipid rafts." Laurdan generalized polarization and two-photon microscopy were used to quantify FC removal from different pools in the bilayer of giant unilamellar vesicles (GUVs). GUVs made of POPC and FC were observed after incubation with reconstituted particles containing apolipoprotein A-I and POPC [78A diameter reconstituted high density lipoprotein (rHDL)]. Fluorescence correlation spectroscopy data show an increase in rHDL size during the incubation period. GUVs made of two "raft-like" mixtures [DOPC/DPPC/FC (1:1:1) and POPC/SPM/FC (6:1:1)] were used to model liquid-ordered/liquid-disordered phase coexistence. Through these experiments, we conclude that rHDL preferentially removes cholesterol from the more fluid phases. These data, and their extrapolation to in vivo systems, show the significant role that phase separation plays in the regulation of cholesterol homeostasis.

Coating of open tubular capillaries with discoidal and spherical high-density lipoprotein particles in electrochromatography

A new method was developed for the coating of fused-silica capillaries with human high-density lipoproteins (HDLs) for use in electrochromatography. The HDL particles used for the coating differed in particle shape and composition. Both discoidal and spherical particles formed a monolayer on the inner silica wall as confirmed by atomic force microscopy. The effect of coating conditions, such as HDL concentration and coating time, was investigated with spherical HDL particles. Examination of the influence of pH on the coating stability also allowed the determination of pI values for the HDL particles attached to the capillary wall. The pI values for spherical and discoidal HDL particles were close to 5.0. The repeatabilities of the EOF mobility and the retention factors of the uncharged steroid hormones used as model compounds were exploited in the evaluation of the coating stability. The optimal coating was achieved with 0.1 mg/mL HDL protein and 50 min flushing with coating solution followed by 15 min standing time. Electrochromatography with HDL-coated open tubular capillaries offers a new tool for the study of HDL particle structure and transformations.

Disassembly of nanodiscs with cholate

Nanodiscs are protein-lipid particles that furnish a nanometer-sized membrane environment for the investigation of membrane proteins. Nanodiscs assemble spontaneously upon the removal of cholate from an initial mixture of cholate, lipids, and engineered amphipathic proteins. A combined experimental-computational approach is applied here to study the disassembly of nanodiscs through the addition of cholate to preformed particles. For this purpose, small-angle X-ray scattering experiments and coarse-grained molecular dynamics simulations were performed and compared. The study offers a detailed view of nanodisc dynamics.

A model structure for the heterodimer apoA-IMilano-apoA-II supports its peculiar susceptibility to proteolysis

In this study, we propose a structure for the heterodimer between apolipoprotein A-I(Milano) and apolipoprotein A-II (apoA-I(M)-apoA-II) in a synthetic high-density lipoprotein (HDL) containing L-alpha-palmitoyloleoyl phosphatidylcholine. We applied bioinformatics/computational tools and procedures, such as molecular docking, molecular and essential dynamics, starting from published crystal structures for apolipoprotein A-I and apolipoprotein A-II. Structural and energetic analyses onto the simulated system showed that the molecular dynamics produced a stabilized synthetic HDL. The essential dynamic analysis showed a deviation from the starting belt structure. Our structural results were validated by limited proteolysis experiments on HDL from apoA-I(M) carriers in comparison with control HDL. The high sensitivity of apoA-I(M)-apoA-II to proteases was in agreement with the high root mean-square fluctuation values and the reduction in secondary structure content from molecular dynamics data. Circular dichroism on synthetic HDL containing apoA-I(M)-apoA-II was consistent with the alpha-helix content computed on the proposed model.

The use of chemical cross-linking and mass spectrometry to elucidate the tertiary conformation of lipid-bound apolipoprotein A-I

PURPOSE OF REVIEW

The purpose of this review is to highlight recent advances in mass spectrometry and its use for identifying the lipid-bound conformation of apolipoprotein A-I. Given the current interest in understanding the structure of HDL apolipoprotein A-I, this approach seems ideal in assessing its dual role as mediator of lipid efflux and modulator of cellular inflammation.

RECENT FINDINGS

A large number of different technical approaches have been employed over the past 25 years in attempts to solve the lipid-bound conformation of apolipoprotein A-I. Since the X-ray crystal structure of lipid-free Delta43 apolipoprotein A-I was reported in 1997, a 'double belt' model describing lipid-bound apolipoprotein A-I conformation for recombinant HDL has prevailed. Recent studies have focused on determining the exact helix-helix registry and salt-bridging partners found on a two apolipoprotein A-I molecule disc as well as on spherical HDL particles. Investigations are all aimed at defining the conformation of lipid-bound apolipoprotein A-I which may provide an explanation for how specific domains of apolipoprotein A-I interact with important HDL-modifying proteins that ultimately determine the apolipoprotein's fate in circulation.

SUMMARY

Recent advances in mass spectrometric sequencing of cross-linked peptides provide an excellent tool to help define protein tertiary structure. This approach has provided refined structural information on apolipoprotein A-I folding which had eluded all previous approaches.

Apolipoprotein A-I assumes a "looped belt" conformation on reconstituted high density lipoprotein

Apolipoprotein A-I (apoA-I) plays a central role in the reverse cholesterol transport pathway; however, the structural basis for its antiatherogenic effects remains poorly understood. Here we employ EPR spectroscopy and fluorescence resonance energy transfer to elucidate the conformation and relative alignment of apoA-I monomers on discoidal (9.4 nm) reconstituted high density lipoprotein (rHDL). EPR spectroscopy provided evidence for an extended helical secondary structure. Position 139 since it was the only residue examined to display a dynamic motional character consistent with a flexible loop structure. The EPR spectra of nitroxide probes at positions 133 and 146 exhibit spin coupling, indicating that these positions are proximal to an apoA-I paired counterpart on the perimeter of rHDL. fluorescence resonance energy transfer studies employing engineered apoA-I variants possessing a single tryptophan (energy donor) and/or a single cysteine (whose thiol moiety was covalently labeled with an extrinsic energy acceptor) provided evidence that paired apoA-I molecules around the perimeter of rHDL align in an extended antiparallel conformation. Taken together with the observation that the EPR spectra of nitroxide probes positioned at intervening sequence positions (134-145) do not exhibit spin coupling, this has led us to propose a "looped belt" model, wherein residues 133-146 comprise a flexible loop segment that confers to apoA-I an intrinsic ability to adapt its structure to accommodate changing particle lipid content. Specifically, in the looped belt model, with the exception of amino acids 134-145, apoA-I aligns with its counterpart in a helix 5-helix 5 registry, centered at position 139.

Structure and stability of apolipoprotein a-I in solution and in discoidal high-density lipoprotein probed by double charge ablation and deletion mutation

To identify residues and segments in the central region of apolipoprotein A-I (apoA-I) that are important for the protein structure and stability, we studied the effects of four double charge ablations, D102A/D103A, E110A/E111A, R116V/K118A, and R160V/H162A, and two deletion mutations, Delta(61-78) and Delta(121-142), on the conformation and stability of apoA-I in the lipid-free state and in reconstituted discoidal phospholipid-cholesterol-apoA-I particles (rHDL). The findings suggest that D102/D103 and E110/E111 located in helix 4 and segment(s) between residues 61 and 78 are involved in maintenance of the conformation and stability of apoA-I in both the lipid-free state and in rHDL. R116/K118 located in helix 4 are essential for the conformation and stabilization of apoA-I in rHDL but not vital for the lipid-free state of the protein. The R160V/H162A substitutions in helix 6 lead to a less compact tertiary structure of lipid-free apoA-I without notable effects on the lipid-free or lipid-bound secondary conformation, suggesting involvement of R160/H162 in important interhelical interactions. The results on the Delta(121-142) mutant, together with our earlier findings, suggest disordered structure of a major segment between residues 121 and 143, likely including residues 131-143, in lipid-free apoA-I. Our findings provide the first experimental evidence for stabilization of rHDL by specific electrostatic interhelical interactions, in agreement with the double belt model. The effects of alterations in the conformation and stability of the apoA-I mutants on in vitro and in vivo functions of apoA-I and lipid homeostasis are discussed.

Homologous versus heterologous interactions in the bicomponent staphylococcal gamma-haemolysin pore

Staphylococcal gamma-haemolysin HlgA-HlgB forms a beta-barrel transmembrane pore in cells and in model membranes. The pore is formed by the oligomerization of two different proteins and a still debated number of monomers. To clarify the topology of the pore, we have mutated single residues - placed near the right and left interfaces of each monomer into cysteine. The mutants were labelled with fluorescent probes, forming a donor-acceptor pair for FRET (fluorescence resonance energy transfer). Heterologous couples (labelled on complementary left and right interfaces) displayed a marked FRET, suggesting extensive HlgA-HlgB or HlgB-HlgA contacts. Heterologous control couples (with both components labelled on the same side) showed absent or low FRET. We found the same result for the homologous couple formed by HlgA [i.e. HlgA-HlgA in the presence of wt (wild-type) HlgB]. The homologous HlgB couple (HlgB-HlgB labelled on left and right interfaces and in the presence of wt HlgA) displayed a transient, declining FRET, which may indicate fast formation of an intermediate that is consumed during pore formation. We conclude that bicomponent pores are assembled by alternating heterologous monomers.

Intermolecular contact between globular N-terminal fold and C-terminal domain of ApoA-I stabilizes its lipid-bound conformation: studies employing chemical cross-linking and mass spectrometry

The structure of apoA-I on discoidal high density lipoprotein (HDL) was studied using a combination of chemical cross-linking and mass spectrometry. Recombinant HDL particles containing 145 molecules of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine and two molecules of apoA-I with a 96-A diameter were treated with the lysine-specific cross-linker, dithiobis(succinimidylpropionate) at varying molar ratios from 2:1 to 200:1. At low molar ratios of dithiobis(succinimidylpropionate) to apoA-I, two products were obtained corresponding to approximately 53 and approximately 80 kDa. At high molar ratios, these two products merged, yielding a product of approximately 59 kDa, close to the theoretical molecular mass of dimeric apoA-I. To identify the intermolecular cross-links giving rise to the two different sized products, bands were excised from the gel, digested with trypsin, and then analyzed by liquid chromatography-electrospray-tandem mass spectrometry. In addition, tandem mass spectrometry of unique cross-links found in the 53- and 80-kDa products suggested that a distinct conformation exists for lipid-bound apoA-I on 96-A recombinant HDL, emphasizing the inherent flexibility and malleability of the N termini and its interaction with its C-terminal domain.

Apolipoprotein structure and dynamics

PURPOSE OF REVIEW

This review highlights recent advances in structural studies of exchangeable human apolipoproteins and the insights they provide into lipoprotein action in cardiovascular and amyloid diseases.

RECENT FINDINGS

The high-resolution X-ray crystal structure of free apoA-II reveals a parallel helical array that may represent other lipid-poor apolipoproteins, and the structure in complex with detergent substantiates the belt model for the protein arrangement on lipoproteins. Nuclear magnetic resonance structures of apolipoprotein-detergent complexes show a repertoire of curved helical conformations, suggesting multiple helical arrangements on the lipid. Low-resolution spectroscopic analyses, interface studies and molecular modeling provide new insights into the 'hinge-domain' mechanism of apolipoprotein adaptation at variable lipoprotein surfaces. A kinetic mechanism for lipoprotein stabilization is proposed.

SUMMARY

Cumulative evidence supports the belt model that provides a general structural basis for understanding the molecular mechanisms of functional apolipoprotein reactions, such as binding to lipoprotein receptors, lipid transporters, and the activation of lipophilic enzymes. However, the detailed protein and lipid conformations on lipoproteins and the underlying molecular interactions are unclear. New insights will hopefully emerge once the first detailed lipoprotein structure is solved.

Apolipoprotein structural organization in high density lipoproteins: belts, bundles, hinges and hairpins

PURPOSE OF REVIEW

To summarize recent advances towards an understanding of the three-dimensional structures of the apolipoprotein components of HDL with a specific focus on high resolution models of apolipoprotein A-I.

RECENT FINDINGS

Since the primary sequence was first reported, various models have been advanced for the structure of apolipoprotein A-I, the major protein constituent of HDL, in its lipid-free and lipid-bound forms. Unfortunately, the generation of experimental data capable of distinguishing among the competing models has lagged far behind. However, recent experimental strategies, including X-ray crystallography, applications of resonance energy transfer and mass spectrometry, have combined with sophisticated theoretical approaches to develop three-dimensional structural models of apolipoprotein A-I with previously unavailable resolution.

SUMMARY

The recent synergy of sophisticated computer modeling techniques with hard experimental data has generated new models for apolipoprotein A-I in certain subclasses of HDL produced in vitro. The challenge now is to adapt and test these models in the more complex forms of HDL isolated directly from human plasma.

Interaction of human apolipoprotein A-I with model membranes exhibiting lipid domains

Several mechanisms for cell cholesterol efflux have been proposed, including membrane microsolubilization, suggesting that the existence of specific domains could enhance the transfer of lipids to apolipoproteins. In this work isothermal titration calorimetry, circular dichroism spectroscopy, and two-photon microscopy are used to study the interaction of lipid-free apolipoprotein A-I (apoA-I) with small unilamellar vesicles (SUVs) of 1-palmitoyl, 2-oleoyl phosphatidylcholine (POPC) and sphingomyelin (SM), with and without cholesterol. Below 30 degrees C the calorimetric results show that apoA-I interaction with POPC/SM SUVs produces an exothermic reaction, characterized as nonclassical hydrophobic binding. The heat capacity change (DeltaCp degrees ) is small and positive, whereas it was larger and negative for pure POPC bilayers, in the absence of SM. Inclusion of cholesterol in the membranes induces changes in the observed thermodynamic pattern of binding and counteracts the formation of alpha-helices in the protein. Above 30 degrees C the reactions are endothermic. Giant unilamellar vesicles (GUVs) of identical composition to the SUVs, and two-photon fluorescence microscopy techniques, were utilized to further characterize the interaction. Fluorescence imaging of the GUVs indicates coexistence of lipid domains under 30 degrees C. Binding experiments and Laurdan generalized-polarization measurements suggest that there is no preferential binding of the labeled apoA-I to any particular domain. Changes in the content of alpha-helix, binding, and fluidity data are discussed in the framework of the thermodynamic parameters.

Structural and functional properties of V156K and A158E mutants of apolipoprotein A-I in the lipid-free and lipid-bound states

Val156 of apolipoprotein A-I (apoA-I) was found to be a key amino acid in the structure and function of high density lipoprotein (HDL) (J. Biol. Chem., 275: 26821-26827, 2000). To determine more precisely the functions of the individual amino acids proximal to Val156, serial point mutants of proapoA-I, including V156K, D157K, and A158E, were overexpressed and purified to at least 95% purity. In the lipid-free state, A158E exhibited the most profound self-associative patterns and the least pronounced dimyristoyl phosphatidylcholine (DMPC) clearance activities. In the lipid-bound state, A158E formed a larger reconstituted HDL (rHDL) with palmitoyloleoyl phosphatidylcholine (POPC), approximately 120 A, whereas other mutants and the wild type (WT) formed 97 A of POPC-rHDL. Cross-linking analysis revealed that A158E-rHDL harbored at least four protein molecules in the particle, while other rHDL conformations contained only two protein molecules. All of the POPC-rHDL produced smaller HDL, around 78 A, after 24 h of incubation in the presence of low density lipoprotein at 37 degrees C. V156K and A158E exhibited decreased lecithin:cholesterol acyltransferase activation activity in the POPC-rHDL state, showing <2% of WT reactivity (apparent Vmax/Km). A158E also displayed markedly different properties in secondary structure, and its accessibility to proteolytic enzymes is different. These results suggest that the two amino acids in helix 6, Val156 and Ala158, are critical to both the structure and function of rHDL.

Double belt structure of discoidal high density lipoproteins: molecular basis for size heterogeneity

We recently proposed an all-atom model for apolipoprotein (apo) A-I in discoidal high-density lipoprotein in which two monomers form stacked antiparallel helical rings rotationally aligned by interhelical salt-bridges. The model can be derived a priori from the geometry of a planar bilayer disc that constrains the hydrophobic face of a continuous amphipathic alpha helix in lipid-associated apoA-I to a plane inside of an alpha-helical torus. This constrains each apoA-I monomer to a novel conformation, that of a slightly unwound, curved, planar amphipathic alpha 11/3 helix (three turns per 11 residues). Using non-denaturing gradient gel electrophoresis, we show that dimyristoylphosphocholine discs containing two apoA-I form five distinct particles with maximal Stokes diameters of 98 A (R2-1), 106 A (R2-2), 110 A (R2-3), 114 A (R2-4) and 120 A (R2-5). Further, we show that the Stokes diameters of R2-1 and R2-2 are independent of the N-terminal 43 residues (the flexible domain) of apoA-I, while the flexible domain is necessary and sufficient for the formation of the three larger complexes. On the basis of these results, the conformation of apoA-I on the R2-2 disc can be modeled accurately as an amphipathic helical double belt extending the full length of the lipid-associating domain with N and C-terminal ends in direct contact. The smallest of the discs, R2-1, models as the R2-2 conformation with an antiparallel 15-18 residue pairwise segment of helixes hinged off the disc edge. The conformations of full-length apoA-I on the flexible domain-dependent discs (R2-3, R2-4 and R2-5) model as the R2-2 conformation extended on the disc edge by one, two or three of the 11-residue tandem amphipathic helical repeats (termed G1, G2 and G3), respectively, contained within the flexible domain. Although we consider these results to favor the double belt model, the topographically very similar hairpin-belt model cannot be ruled out entirely.

Functional independence of a peptide with the sequence of human apolipoprotein A-I central region

Previous results [J. Biol. Chem. 276 (2001) 16978] indicated that an apolipoprotein A-I (apoAI) central region swings away from lipid contact in discoidal high density lipoproteins (HDL), but it is able to penetrate into the bilayer of lipid vesicles. In this work, we have studied the interaction with lipid membranes of a synthetic peptide with the sequence of apoAI region between residues 77 and 120 (AI 77-120). Like apoAI, AI 77-120 binds to phospholipid vesicles and shows selectivity for cholesterol-containing membranes. Moreover, AI 77-120 promotes cholesterol desorption from membranes in a similar fashion as apoAI and can stimulate cholesterol efflux from Chinese hamster ovary cells. AI 77-120 has a considerable alpha-helical content in water solution, and its secondary structure is not largely modified after binding to membranes. Both apoA-I and AI 77-120 are oligomeric in the lipid-bound state, suggesting that dimerization of the central domain could be required for the membrane binding activity of apoA-I in HDL.