Structural properties and lipid binding of human apolipoprotein A-IV

The structure of human apolipoprotein A-IV as revealed by stable isotope-assisted cross-linking, molecular dynamics, and small angle x-ray scattering

Apolipoprotein (apo)A-IV plays important roles in dietary lipid and glucose metabolism, and knowledge of its structure is required to fully understand the molecular basis of these functions. However, typical of the entire class of exchangeable apolipoproteins, its dynamic nature and affinity for lipid has posed challenges to traditional high resolution structural approaches. We previously reported an x-ray crystal structure of a dimeric truncation mutant of apoA-IV, which showed a unique helix-swapping molecular interface. Unfortunately, the structures of the N and C termini that are important for lipid binding were not visualized. To build a more complete model, we used chemical cross-linking to derive distance constraints across the full-length protein. The approach was enhanced with stable isotope labeling to overcome ambiguities in determining molecular span of the cross-links given the remarkable similarities in the monomeric and dimeric apoA-IV structures. Using 51 distance constraints, we created a starting model for full-length monomeric apoA-IV and then subjected it to two modeling approaches: (i) molecular dynamics simulations and (ii) fitting to small angle x-ray scattering data. This resulted in the most detailed models yet for lipid-free monomeric or dimeric apoA-IV. Importantly, these models were of sufficient detail to direct the experimental identification of new functional residues that participate in a "clasp" mechanism to modulate apoA-IV lipid affinity. The isotope-assisted cross-linking approach should prove useful for further study of this family of apolipoproteins in both the lipid-free and -bound states.

ApoA-IV promotes the biogenesis of apoA-IV-containing HDL particles with the participation of ABCA1 and LCAT

The objective of this study was to establish the role of apoA-IV, ABCA1, and LCAT in the biogenesis of apoA-IV-containing HDL (HDL-A-IV) using different mouse models. Adenovirus-mediated gene transfer of apoA-IV in apoA-I(-/-) mice did not change plasma lipid levels. ApoA-IV floated in the HDL2/HDL3 region, promoted the formation of spherical HDL particles as determined by electron microscopy, and generated mostly α- and a few pre-β-like HDL subpopulations. Gene transfer of apoA-IV in apoA-I(-/-) × apoE(-/-) mice increased plasma cholesterol and triglyceride levels, and 80% of the protein was distributed in the VLDL/IDL/LDL region. This treatment likewise generated α- and pre-β-like HDL subpopulations. Spherical and α-migrating HDL particles were not detectable following gene transfer of apoA-IV in ABCA1(-/-) or LCAT(-/-) mice. Coexpression of apoA-IV and LCAT in apoA-I(-/-) mice restored the formation of HDL-A-IV. Lipid-free apoA-IV and reconstituted HDL-A-IV promoted ABCA1 and scavenger receptor BI (SR-BI)-mediated cholesterol efflux, respectively, as efficiently as apoA-I and apoE. Our findings are consistent with a novel function of apoA-IV in the biogenesis of discrete HDL-A-IV particles with the participation of ABCA1 and LCAT, and may explain previously reported anti-inflammatory and atheroprotective properties of apoA-IV.

The structure of dimeric apolipoprotein A-IV and its mechanism of self-association

Apolipoproteins are key structural elements of lipoproteins and critical mediators of lipid metabolism. Their detergent-like properties allow them to emulsify lipid or exist in a soluble lipid-free form in various states of self-association. Unfortunately, these traits have hampered high-resolution structural studies needed to understand the biogenesis of cardioprotective high-density lipoproteins (HDLs). We derived a crystal structure of the core domain of human apolipoprotein (apo)A-IV, an HDL component and important mediator of lipid absorption. The structure at 2.4 Å depicts two linearly connected 4-helix bundles participating in a helix swapping arrangement that offers a clear explanation for how the protein self-associates as well as clues to the structure of its monomeric form. This also provides a logical basis for antiparallel arrangements recently described for lipid-containing particles. Furthermore, we propose a "swinging door" model for apoA-IV lipid association.

Local/bulk determinants of conformational stability of exchangeable apolipoproteins

GuHCl-induced denaturation of human plasma apoA-I, apoA-II, apoA-IV, apoE3 and three recombinant apoE isoforms in solution and discoidal complexes with phosphatidylcholine (only plasma proteins) was studied. The protein conformational stability (ΔG(H(2)O)) and a slope of linear dependence of free energy of unfolding on GuHCl concentration (m-value) were estimated with the three equilibrium schemes. The data for all proteins, except apoA-II, fit with the three-state model, thus evidencing two-domain structure. The predicted folding rate of the four apoE in solution correlated with conformational stability. The dependence disappeared at the inclusion of apoA-I and apoA-IV into analysis and the m-values, adjusted for residue number in helices (m(rh)), differed between those for apoE and apoA-I/apoA-IV. However, the m(rh)-values for six proteins correlated positively with the fractional change in accessible surface area at unfolding for Phe, Lys and Asn, while negatively for Arg, Ala and Gly residues. The difference between the adjusted ΔG(rh)(H(2)O) values for apolipoproteins in complexes and in solution decreased at the increase of reduced temperature (T(obs)-T(t))/T(t). The induction of intrinsic disorder by arginine residues may be of primary importance in metabolism and function of exchangeable apolipoproteins, while their stability in nascent discoidal HDL is controlled by the physical state of phosphatidylcholine.

Distinctive structure and interfacial activity of the human apolipoprotein A-IV 347S isoprotein

The T347S polymorphism in the human apolipoprotein (apo) A-IV gene is present at high frequencies among all the world's populations. Carriers of a 347S allele exhibit faster clearance of triglyceride-rich lipoproteins, greater adiposity, and increased risk for developing atherosclerosis, which suggests that this conservative amino acid substitution alters the structure of apo A-IV. Herein we have used spectroscopic and surface chemistry techniques to examine the structure, stability, and interfacial properties of the apo A-IV 347S isoprotein. Circular dichroism spectroscopy revealed that the 347S isoprotein has similar alpha-helical structure but lower thermodynamic stability than the 347T isoprotein. Fluorescence spectroscopy found that the 347S isoprotein exhibits an enhanced tyrosine emission and reduced tyrosine-->tryptophan energy transfer, and second derivative UV absorption spectra noted increased tyrosine exposure, suggesting that the 347S isoprotein adopts a looser tertiary conformation. Surface chemistry studies found that although the 347S isoprotein bound rapidly to the lipid interface, it has a lower interfacial exclusion pressure and lower elastic modulus than the 347T isoprotein. Together, these observations establish that the T347S substitution alters the conformation of apo A-IV and lowers its interfacial activity-changes that could account for the effect of this polymorphism on postprandial lipid metabolism.

Role of phosphatidic acid in the coupling of the ERK cascade

The production of phosphatidic acid plays a crucial role in the activation of the ERK cascade. This role was linked to the binding of phosphatidate to a specific polybasic site within the kinase domain of Raf-1. Here we show that phosphatidate promotes ERK phosphorylation in intact cells but does not activate Raf in vitro. The kinase suppressor of Ras (KSR) contains a sequence homologous to the phosphatidate binding site of Raf-1. Direct binding of phosphatidate to synthetic peptides derived from the sequences of the binding domains of Raf-1 and KSR was demonstrated by spectroscopic techniques. The specificity of these interactions was confirmed using synthetic lipids and mutated peptides in which the core of the phosphatidic acid binding domain was disrupted. Insulin and exogenous dioleoyl phosphatidate induced a rapid translocation of a mouse KSR1-EGFP construct to the plasma membrane of HIRcB cells. Mutation of two arginines located in the core of the putative phosphatidate binding site abolished dioleoyl phosphatidate- and insulin-induced translocation of KSR1. Overexpression of the mutant KSR1 in HIRcB cells inhibited insulin-dependent MEK and ERK phosphorylation. The addition of dioleoyl phosphatidate or insulin increased the co-localization of KSR1 and H-Ras and promoted the formation of plasma membrane patches enriched in both proteins and phosphatidic acid. These results, in conjunction with our previous work, suggest the formation of phosphatidate-enriched membrane microdomains that contain all components of the ERK cascade. We propose that these domains act as molecular scaffolds in the coupling of signaling events.

Structural and dynamic interfacial properties of the lipoprotein initiating domain of apolipoprotein B

To better understand the earliest steps in the assembly of triglyceride (TG)-rich lipoproteins, we compared the biophysical and interfacial properties of two closely related apolipoprotein B (apoB) truncation mutants, one of which contains the complete lipoprotein initiating domain (apoB20.1; residues 1-912), and one of which, by virtue of a 50 amino acid C-terminal truncation, is incapable of forming nascent lipoproteins (apoB19; residues 1-862). Spectroscopic studies detected no major differences in secondary structure, and only minor differences in conformation and thermodynamic stability, between the two truncation mutants. Monolayer studies revealed that both apoB19 and apoB20.1 bound to and penetrated egg phosphatidylcholine (EPC) monolayers; however, the interfacial exclusion pressure of apoB20.1 was higher than apoB19 (25.1 mN/m vs. 22.8 mN/m). Oil drop tensiometry revealed that both proteins bound rapidly to the hydrophobic triolein/water interface, reducing interfacial tension by approximately 20 mN/m. However, when triolein drops were first coated with phospholipids (PL), apoB20.1 bound with faster kinetics than apoB19 and also displayed greater interfacial elasticity (26.9 +/- 0.8 mN/m vs. 22.9 +/- 0.8 mN/m). These data establish that the transition of apoB to assembly competence is accompanied by increases in surface activity and elasticity, but not by significant changes in global structure.

A three-dimensional homology model of lipid-free apolipoprotein A-IV using cross-linking and mass spectrometry

Human apolipoprotein A-IV (apoA-IV) is a 46-kDa exchangeable plasma protein with many proposed functions. It is involved in chylomicron assembly and secretion, protection from atherosclerosis through a variety of mechanisms, and inhibition of food intake. There is little structural basis for these proposed functions due to the lack of a solved three-dimensional structure of the protein by x-ray crystallography or NMR. Based on previous studies, we hypothesized that lipid-free apoA-IV exists in a helical bundle, like other apolipoprotein family members and that regions near the N and C termini may interact. Utilizing a homobifunctional lysine cross-linking agent, we identified 21 intramolecular cross-links by mass spectrometry. These cross-links were used to constrain the building of a sequence threaded homology model using the I-TASSER server. Our results indicate that lipid-free apoA-IV does indeed exist as a complex helical bundle with the N and C termini in close proximity. This first structural model of lipid-free apoA-IV should prove useful for designing studies aimed at understanding how apoA-IV interacts with lipids and possibly with unknown protein partners.

The biotin-capture lipid affinity assay: a rapid method for determining lipid binding parameters for apolipoproteins

The lipid affinity of plasma apolipoproteins is an important modulator of lipoprotein metabolism. Mutagenesis techniques have been widely used to modulate apolipoprotein lipid affinity for studying biological function, but the approach requires rapid and reliable lipid affinity assays to compare the mutants. Here, we describe a novel method that measures apolipoprotein binding to a standardized preparation of small unilamellar vesicles (SUVs) containing trace biotinylated and fluorescent phospholipids. After a 30 min incubation at various apolipoprotein concentrations, vesicle-bound protein is rapidly separated from free protein on columns of immobilized streptavidin in a 96-well microplate format. Vesicle-bound protein and lipid are eluted and measured in a fluorescence microplate reader for calculation of a dissociation constant and the maximum number of potential binding sites on the SUVs. Using human apolipoprotein A-I (apoA-I), apoA-IV, and mutants of each, we show that the assay generates binding constants that are comparable to other methods and is reproducible across time and apolipoprotein preparations. The assay is easy to perform and can measure triplicate binding parameters for up to 10 separate apolipoproteins in 3.5 h, consuming only 120 microg of apolipoprotein in total. The benefits and potential drawbacks of the assay are discussed.

Specific Sequences in the N and C Termini of Apolipoprotein A-IV Modulate Its Conformation and Lipid Association

Apolipoprotein (apoA-IV) is a 376-residue exchangeable apolipoprotein that may play a number of important roles in lipid metabolism, including chylomicron assembly, reverse cholesterol transport, and appetite regulation. In vivo, apoA-IV exists in both lipid-poor and lipid-associated forms, and the balance between these states may determine its function. We examined the structural elements that modulate apoA-IV lipid binding by producing a series of deletion mutants and determining their ability to interact with phospholipid liposomes. We found that the deletion of residues 333-343 strongly increased the lipid association rate versus native apoA-IV. Additional mutagenesis revealed that two phenylalanine residues at positions 334 and 335 mediated this lipid binding inhibitory effect. We also observed that residues 11-20 in the N terminus were required for the enhanced lipid affinity induced by deletion of the C-terminal sequence. We propose a structural model in which these sequences can modulate the conformation and lipid affinity of apoA-IV.

Two different types of amyloid deposits--apolipoprotein A-IV and transthyretin--in a patient with systemic amyloidosis

Certain forms of systemic amyloidosis have been associated with the pathologic deposition as fibrils of three different apolipoprotein-related proteins--apolipoprotein A-I, apolipoprotein A-II, and serum amyloid A. We have previously reported (Bergström et al, Biochem Biophys Res Commun 2001;285:903-908) that amyloid fibrils extracted from the heart of an elderly male with senile systemic amyloidosis contained, in addition to wild-type transthyretin-related molecules, an N-terminal fragment of yet a fourth apolipoprotein--apolipoprotein A-IV (apoA-IV). We now provide the results of our studies that have established the complete amino-acid sequence of this approximately 70-residue component and, additionally, have shown this protein to be the product of an unmutated apoA-IV gene. Notably, the apoA-IV and transthyretin fibrils were not codeposited but, rather, had anatomically distinct patterns of distribution within the heart and other organs, as evidenced immunohistochemically, by variation in the ultra structural morphology and by differences in the intensity of Congo red birefringence. These findings provide the first conclusive evidence that two separate forms of amyloid, each derived from a wild-type amyloidogenic precursor protein, were present in a patient with systemic amyloidosis.

Contributions of domain structure and lipid interaction to the functionality of exchangeable human apolipoproteins

Exchangeable apolipoproteins function in lipid transport as structural components of lipoprotein particles, cofactors for enzymes and ligands for cell-surface receptors. Recent findings with apoA-I and apoE suggest that the tertiary structures of these two members of the human exchangeable apolipoprotein gene family are related. Characteristically, these proteins contain a series of proline-punctuated, 11- or 22-amino acid, amphipathic alpha-helical repeats that can adopt a helix bundle conformation in the lipid-free state. The amino- and carboxyl-terminal regions form separate domains with the latter being primarily responsible for lipid binding. Interaction with lipid induces changes in the conformation of the amino-terminal domain leading to alterations in function; for example, opening of the amino-terminal four-helix bundle in apolipoprotein E upon lipid binding is associated with enhanced receptor-binding activity. The concept of a two-domain structure for the larger exchangeable apolipoproteins is providing new molecular insights into how these apolipoproteins interact with lipids and other proteins, such as receptors. The ways in which structural changes induced by lipid interaction modulate the functionality of these apolipoproteins are reviewed.

Fibrillar amyloid protein present in atheroma activates CD36 signal transduction

The self-association of proteins to form amyloid fibrils has been implicated in the pathogenesis of a number of diseases including Alzheimer's, Parkinson's, and Creutzfeldt-Jakob diseases. We recently reported that the myeloid scavenger receptor CD36 initiates a signaling cascade upon binding to fibrillar beta-amyloid that stimulates recruitment of microglia in the brain and production of inflammatory mediators. This receptor plays a key role in the pathogenesis of atherosclerosis, prompting us to evaluate whether fibrillar proteins were present in atherosclerotic lesions that could initiate signaling via CD36. We show that apolipoprotein C-II, a component of very low and high density lipoproteins, readily forms amyloid fibrils that initiate macrophage inflammatory responses including reactive oxygen production and tumor necrosis factor alpha expression. Using macrophages derived from wild type and Cd36(-/-) mice to distinguish CD36-specific events, we show that fibrillar apolipoprotein C-II activates a signaling cascade downstream of this receptor that includes Lyn and p44/42 MAPKs. Interruption of this signaling pathway through targeted deletion of Cd36 or blocking of p44/42 MAPK activation inhibits macrophage tumor necrosis factor alpha gene expression. Finally, we demonstrate that apolipoprotein C-II in human atheroma co-localizes to regions positive for markers of amyloid and macrophage accumulation. Together, these data characterize a CD36-dependent signaling cascade initiated by fibrillar amyloid species that may promote atherogenesis.

Inter-molecular coiled-coil formation in human apolipoprotein E C-terminal domain

Human apolipoprotein E (apoE) is composed of an N-terminal (NT) domain (residues 1-191) that bears low-density lipoprotein receptor-binding sites, and a C-terminal (CT) domain (residues 210-299), which houses lipoprotein binding and apoE self-association sites. The NT domain is comprised of a four-helix bundle, while the structural organization of the CT domain is not known. Secondary structural algorithms predict that the apoE CT domain adopts an amphipathic alpha-helical conformation. On the basis of further sequence predictions, we identified a segment (residues 218-266) in the apoE CT domain that bears a high propensity to form a coiled-coil helix, which coincides with the putative lipoprotein-binding surface. An apoE construct bearing residues 201-299 that encompasses the entire CT domain was designed, expressed in Escherichia coli and purified by affinity chromatography. Circular dichroism (CD) spectroscopy of the apoE CT domain reveals spectra characteristic of coiled-coil helices, with the ratio of molar ellipticities at 222 nm and 208 nm ([theta](222)/[theta](208)) of 1.03. Trifluoroethanol (TFE) stabilized the secondary structure of the apoE CT domain and disrupted coiled-coil helix formation as determined by CD and tryptophan fluorescence analysis. Analytical ultracentrifugation and lysine-specific cross-linking analysis of the apoE CT domain revealed predominant formation of dimeric and tetrameric species in aqueous buffers, and monomeric forms in 50% TFE. Guanidine hydrochloride-induced denaturation studies reveal that, at low concentrations of denaturant, the apoE CT domain maintains the [theta](222)/[theta](208) ratio at approximately 1.0 and elicits an altered tertiary environment with a shift in oligomeric state towards a dimer, indicative of the role of coiled-coil helix formation in inter molecular interactions. Further, coiled-coil formation is disrupted by protonation below pH 6.0, with a corresponding decrease in Trp fluorescence emission intensity, demonstrating that salt-bridge interactions play a critical role in maintaining the structural integrity of the apoE CT domain. The data support the concept that inter molecular coiled-coil helix formation is an essential structural feature of the apoE CT domain, which likely plays a role in clustering heparin-binding sites and/or sequestering the lipid-binding surface in lipid-free states.

Testing the role of apoA-I, HDL, and cholesterol efflux in the atheroprotective action of low-level apoE expression

Low levels of transgenic mouse apolipoprotein E (apoE) suppress atherosclerosis in apoE knockout (apoE-/-) mice without normalizing plasma cholesterol. To test whether this is due to facilitation of cholesterol efflux from the vessel wall, we produced apoA-I-/-/apoE-/- mice with or without the transgene. Even without apoA-I and HDL, apoA-I-/-/apoE-/- mice had the same amount of aorta cholesteryl ester as apoE-/- mice. Low apoE in the apoA-I-/-/apoE-/- transgenic mice reduced aortic lesions by 70% versus their apoA-I-/-/apoE-/- siblings. To define the free cholesterol (FC) efflux capacity of lipoproteins from the various genotypes, sera were assayed on macrophages expressing ATP-binding cassette transporter A1 (ABCA1). Surprisingly, ABCA1 FC efflux was twice as high to sera from the apoA-I-/-/apoE-/- or apoE-/- mice compared with wild-type mice, and this activity correlated with serum apoA-IV. Immunodepletion of apoA-IV from apoA-I-/-/apoE-/- serum abolished ABCA1 FC efflux, indicating that apoAI-V serves as a potent acceptor for FC efflux via ABCA1. With increasing apoE expression, apoA-IV and FC acceptor capacity decreased, indicating a reciprocal relationship between plasma apoE and apoA-IV. Low plasma apoE (1-3 x 10(-8) M) suppresses atherosclerosis by as yet undefined mechanisms, not dependent on the presence of apoA-I or HDL or an increased capacity of serum acceptors for FC efflux.

Structure and interfacial properties of human apolipoprotein A-V

Apolipoprotein A-V (apoA-V), the newest member of the plasma apolipoprotein family, was recently discovered by comparison of the mouse and human genomes. Studies in rodents and population surveys of human apoA-V polymorphisms have noted a strong effect of apoA-V on plasma triglyceride levels. Toward the elucidation of the biologic function of apoA-V, we used spectroscopic and surface chemistry techniques to probe its structure and interfacial activity. Computer-assisted sequence analysis of apoA-V predicts that it is very hydrophobic, contains a significant amount of alpha-helical secondary structure, and probably is composed of discrete structural regions with varying degrees of lipid affinity. Fluorescence spectroscopy of recombinant human apoA-V provided evidence of tertiary folding, and light scattering studies indicated that apoA-V transforms dimyristoylphosphatidylcholine vesicles into discoidal complexes with an efficiency similar to that of apoA-I. Surface chemistry techniques revealed that apoA-V displays high affinity, low elasticity, and slow binding kinetics at hydrophobic interfaces, properties we propose may retard triglyceride-rich particle assembly. Metabolic labeling and immunofluorescence studies of COS-1 cells transfected with human apoA-V demonstrated that apoA-V is poorly secreted, remains associated with the endoplasmic reticulum, and does not traffic to the Golgi. Given that overexpression of the apoA-V gene lowers plasma triglycerides in mice, these data together suggest that apoA-V may function intracellularly to modulate hepatic VLDL synthesis and/or secretion.

Domain structure and lipid interaction in human apolipoproteins A-I and E, a general model

Detailed structural information on human exchangeable apolipoproteins (apo) is required to understand their functions in lipid transport. Using a series of deletion mutants that progressively lacked different regions along the molecule, we probed the structural organization of lipid-free human apoA-I and the role of different domains in lipid binding, making comparisons to apoE, which is a member of the same gene family and known to have two structural domains. Measurements of alpha-helix content by CD in conjunction with tryptophan and 8-anilino-1-naphthalenesulfonic acid fluorescence data demonstrated that deletion of the amino-terminal or central regions disrupts the tertiary organization, whereas deletion of the carboxyl terminus has no effect on stability and induces a more cooperative structure. These data are consistent with the lipid-free apoA-I molecule being organized into two structural domains similar to apoE; the amino-terminal and central parts form a helix bundle, whereas the carboxyl-terminal alpha-helices form a separate, less organized structure. The binding of the apoA-I variants to lipid emulsions is modulated by reorganization of the helix bundle structure, because the rate of release of heat on binding is inversely correlated with the stability of the helix bundle. Based on these observations, we propose that there is a two-step mechanism for lipid binding of apoA-I: apoA-I initially binds to a lipid surface through amphipathic alpha-helices in the carboxyl-terminal domain, followed by opening of the helix bundle in the amino-terminal domain. Because apoE behaves similarly, this mechanism is probably a general feature for lipid interaction of other exchangeable apolipoproteins, such as apoA-IV.

Interfacial exclusion pressure determines the ability of apolipoprotein A-IV truncation mutants to activate cholesterol ester transfer protein

We used a panel of recombinant human apolipoprotein (apo) A-IV truncation mutants, in which pairs of 22-mer alpha-helices were sequentially deleted along the primary sequence, to examine the impact of protein structure and interfacial activity on the ability of apoA-IV to activate cholesterol ester transfer protein. Circular dichroism and fluorescence spectroscopy revealed that the secondary structure, conformation, and molecular stability of recombinant human apoA-IV were identical to the native protein. However, deletion of any of the alpha-helical domains in apoA-IV disrupted its tertiary structure and impaired its molecular stability. Surprisingly, determination of the water/phospholipid interfacial exclusion pressure of the apoA-IV truncation mutants revealed that, for most, deletion of amphipathic alpha-helical domains increased their affinity for phospholipid monolayers. All of the truncation mutants activated the transfer of fluorescent-labeled cholesterol esters between high and low density lipoproteins at a rate higher than native apoA-IV. There was a strong positive correlation (r = 0.790, p = 0.002) between the rate constant for cholesterol ester transfer and interfacial exclusion pressure. We conclude that molecular interfacial exclusion pressure, rather than specific helical domains, determines the degree to which apoA-IV, and likely other apolipoproteins, facilitate cholesterol ester transfer protein-mediated lipid exchange.

Apolipoprotein A-IV polymorphisms and diet-gene interactions

Apolipoprotein A-IV is a 46kDa glycoprotein that is synthesized by intestinal enterocytes and is incorporated into the surface of nascent chylomicrons. Considerable evidence suggests that apolipoprotein A-IV plays a role in intestinal lipid absorption and chylomicron assembly. We have proposed that polymorphisms that alter the interfacial behavior of apolipoprotein A-IV may modulate the physical properties and metabolic fate of plasma chylomicrons. Of the reported genetic polymorphisms of apolipoprotein A-IV, two, Q360H and T347S, are known to occur at high frequencies among the world populations. Biophysical studies have established that the Q360H isoprotein displays higher lipid affinity; conversely the T347S isoprotein is predicted to be less lipid avid. Recent studies have shown that the Q360H polymorphism is associated with increased postprandial hypertriglyceridemia, a reduced low-density lipoprotein response to dietary cholesterol in the setting of a moderate fat intake, an increased high-density lipoprotein response to changes in total dietary fat content, and lower body mass and adiposity; the T347S polymorphism appears to confer the opposite effects. Studies on the diet-gene interactions of other apolipoprotein A-IV alleles are needed, as are studies on the interactions between apolipoprotein A-IV alleles and other apolipoprotein polymorphisms.

Structural models of human apolipoprotein A-I: a critical analysis and review

Human apolipoprotein (apo) A-I has been the subject of intense investigation because of its well-documented anti-atherogenic properties. About 70% of the protein found in high density lipoprotein complexes is apo A-I, a molecule that contains a series of highly homologous amphipathic alpha-helices. A number of significant experimental observations have allowed increasing sophisticated structural models for both the lipid-bound and the lipid-free forms of the apo A-I molecule to be tested critically. It seems clear, for example, that interactions between amphipathic domains in apo A-I may be crucial to understanding the dynamic nature of the molecule and the pathways by which the lipid-free molecule binds to lipid, both in a discoidal and a spherical particle. The state of the art of these structural studies is discussed and placed in context with current models and concepts of the physiological role of apo A-I and high-density lipoprotein in atherosclerosis and lipid metabolism.

Apolipoprotein specificity for lipid efflux by the human ABCAI transporter

ABCAI, a member of the ATP binding cassette family, mediates the efflux of excess cellular lipid to HDL and is defective in Tangier disease. The apolipoprotein acceptor specificity for lipid efflux by ABCAI was examined in stably transfected Hela cells, expressing a human ABCAI-GFP fusion protein. ApoA-I and all of the other exchangeable apolipoproteins tested (apoA-II, apoA-IV, apoC-I, apoC-II, apoC-III, apoE) showed greater than a threefold increase in cholesterol and phospholipid efflux from ABCAI-GFP transfected cells compared to control cells. Expression of ABCAI in Hela cells also resulted in a marked increase in specific binding of both apoA-I (Kd = 0.60 microg/mL) and apoA-II (Kd = 0.58 microg/mL) to a common binding site. In summary, ABCAI-mediated cellular binding of apolipoproteins and lipid efflux is not specific for only apoA-I but can also occur with other apolipoproteins that contain multiple amphipathic helical domains.

Atheroprotective mechanisms of HDL

The aim of this review was to bring together results obtained from studies on different aspects of HDL as related to CHD and atherosclerosis. As atherosclerosis is a multistep process, the various components of HDL can intervene at different stages, such as induction of monocyte adhesion molecules, prevention of LDL modification and removal of excess cholesterol by reverse cholesterol transport. Transgenic technology has provided a model for atherosclerosis, and permitted evaluation of the contributions of different HDL components towards the global effect. The availability of apo AIV transgenic mice amplified the results obtained from apo AI overexpressors with respect to prevention of atherosclerosis. Prevention of atherosclerosis in apo E deficient mice by relatively small amounts of macrophage derived apo E may open new possibilities for therapeutic intervention. Contrary to early notions, increased plasma levels of CETP, even in the presence of low but functionally normal HDL, were atheroprotective. The extent to which paraoxonase and apo J participate in prevention of human atherosclerosis needs further evaluation. The findings that LCAT overexpression in rabbits was atheroprotective in contrast to increase in atherosclerosis in h LCAT tg mice, which was only partially corrected by CETP expression, call for some caution in the extrapolation of results from transgenic animals to humans. The important discovery of SR-BI as the receptor for selective uptake of CE from HDL revived interest in the clearance of CE from plasma. This pathway supplies also the vital precursor for steroidogenesis in adrenals and gonads and was shown to be dependent on apo AI.

Folding stability of the kinetoplastid membrane protein-11 (KMP-11) from Leishmania infantum

Kinetoplastid membrane protein-11 (KMP-11) is a major component of the cell surface of kinetoplastids, and acts as a potent B- and T-cell immunogen during Leishmania infection. Here we report that the Leishmania infantum KMP-11 secondary structure adopts mainly an alpha-helical conformation at pH 7.5 and that its urea- and thermally-induced unfolding constitute a fully reversible two-step process. This allows estimation of a half-denaturation temperature of approximately 65 degrees C, a delta GDH2O at 20 degrees C of approximately 14.63 kJ.mol-1, and an increment of the reaction heat of approximately 183.92 kJ.mol-1 and an entropy of approximately 543.4 J.mol-1.deg-1, respectively, for the native-denatured equilibrium of the KMP-11 in solution. We also report that the KPM-11 protein is induced to adopt a molten globule state at a pH range between pH 4 and pH 6. As a whole, the stability and the specific features of the denaturing effect induced by changes in pH are similar in KMP-11 to various other lipoproteins.

Insight into lipid surface recognition and reversible conformational adaptations of an exchangeable apolipoprotein by multidimensional heteronuclear NMR techniques

Apolipophorin III (apoLp-III) from the insect Manduca sexta is a 166-residue (Mr 18,340) member of the exchangeable apolipoprotein class that functions to stabilize lipid-enriched plasma lipoproteins. In the present study, we present the secondary structure and global fold of recombinant apoLp-III derived from three-dimensional heteronuclear NMR spectroscopy experiments. Five discrete alpha-helical segments (21-30 residues in length) with well defined boundaries were characterized by four NMR parameters: medium range nuclear Overhauser enhancement contacts between proton pairs, chemical shift index, coupling constants, and amide proton exchange rates. An antiparallel arrangement of helical segments has been obtained based on the long range interhelical nuclear Overhauser enhancement contacts. The NMR solution structure reveals a globular, up and down helix bundle organization similar to that of Locusta migratoria apoLp-III (Breiter, D. R., Kanost, M. R., Benning, M. M., Wesenberg, G., Law, J. H., Wells, M. A., Rayment, I., and Holden, H. M. (1991) Biochemistry 30, 603-608). However, a short helix (comprised of 5 amino acids) has been identified in the region between helix 3 and helix 4. This helix is postulated to play a role in lipid surface recognition and/or initiation of binding. Our results also indicate the existence of buried polar and charged residues in the helix bundle, providing a structural basis for the relatively low stability of apoLp-III in its lipid-free state. It is suggested that the intrinsic low stability of lipid-free apoLp-III may be important in terms of its ability to undergo a reversible, lipid binding-induced, conformational change. This study underscores the striking resemblance in molecular architecture between insect apoLp-III and the N-terminal domain of human apolipoprotein E. The potential for application of NMR techniques to studies of the exchangeable apolipoproteins, possibly in their biologically active, lipid-associated state, has broad implications in terms of our understanding of the molecular basis of their physiological functions.

Disulfide bond engineering to monitor conformational opening of apolipophorin III during lipid binding

Apolipophorin III (apoLp-III) from the Sphinx moth, Manduca sexta, is an exchangeable, amphipathic apolipoprotein that alternately exists in water-soluble and lipid-bound forms. It is organized as a five-helix bundle in solution, which has been postulated to open at putative hinge domains to expose the hydrophobic interior, thereby facilitating interaction with the lipoprotein surface (Breiter, D. R. , Kanost, M. R., Benning, M. M., Wesenberg, G., Law, J. H., Wells, M. A., Rayment, I., and Holden, H. M. (1991) Biochemistry 30, 603-608). To test this hypothesis, we engineered two cysteine residues in apoLp-III, which otherwise lacks cysteine, by site-directed mutagenesis at Asn-40 and Leu-90. Under oxidizing conditions the two cysteines spontaneously form a disulfide bond, which should tether the helix bundle and thereby prevent opening and concomitant lipid interaction. N40C/L90C apoLp-III was overexpressed in Escherichia coli and characterized for disulfide bond formation, secondary structure content, and stability, under both oxidizing and reducing conditions. Functional characterization was carried out by comparing the abilities of the oxidized and reduced protein to associate with modified lipoproteins in vitro. While the reduced form behaved like wild type apoLp-III, the oxidized form was unable to associate with lipoproteins. These results suggest that opening of the helix bundle is required for interaction with lipoproteins and provide a molecular basis for the dual existence of water-soluble and lipid-bound forms of apoLp-III. However, in phospholipid bilayer association assays, wild type, reduced, and oxidized N40C/L90C apoLp-III exhibited similar abilities to transform dimyristoylphosphatidylcholine multilamellar vesicles to disc-like complexes, as judged by electron microscopy. These data emphasize that underlying differences exist in initiating or maintaining a stable interaction of apoLp-III with phospholipid disc complexes versus spherical lipoprotein surfaces.

Activation of human plasma cholesteryl ester transfer protein by human apolipoprotein A-IV

Function of apolipoprotein (apo) A-IV was studied for its role in cholesteryl ester transfer protein (CETP; lipid transfer protein, LTP) reaction between lipid microemulsions having the diameter of low density lipoprotein, being compared to apoA-I. CETP hardly catalyzed lipid transfer without apolipoproteins. ApoA-IV bound to the surface of the microemulsion in equilibrium with a similar affinity to that of other helical apolipoproteins, and activated the transfer reaction by CETP of cholesteryl ester, triacylglycerol and phosphatidylcholine between the emulsions. The rate of the transfer reaction of cholesteryl ester and triacylglycerol was directly proportional to the amount of the bound apoA-IV to the surface of the emulsion. For phosphatidylcholine, activation was less effective until 40% of total binding capacity of lipid emulsion was occupied by the apolipoprotein. Cholesteryl ester was highly preferred by CETP over triacylglycerol when equal amount of these lipids was present in the core of the apoA-IV-activated emulsion, resulting in almost no triacylglycerol transfer. However, when the emulsion has the core exclusively of triacylglycerol, triacylglycerol was transferred by CETP with the rate in the same order as that of cholesteryl ester transfer. These findings were all comparable to the results with apoA-I, and also consistent with our previous observation for other amphiphilic helical apolipoproteins such as apoA-II, E and C-III.

Lipoproteins containing apolipoprotein A-IV but not apolipoprotein A-I take up and esterify cell-derived cholesterol in plasma

Two-dimensional nondenaturing polyacrylamide gradient gel electrophoresis (2D-PAGGE) identifies distinct apoA-I-or apoE-containing subclasses of high-density lipoproteins (HDLs), each of which plays a different role in reverse cholesterol transport. In this study we used 2D-PAGGE to investigate the role of apoA-IV-containing lipoproteins in reverse cholesterol transport in native plasma. Incubation of 2D electrophoretograms with anti-apoA-IV antibodies identified up to three subclasses of particles. The smaller particle subclasses, LpA-IV-1 and LpA-IV-2, were found in every plasma sample. The largest particle subclass, LpA-IV-3, was observed in fewer than 10% of the plasmas analyzed. 2D-PAGGE of apoA-I-deficient plasma and apoA-I-depleted plasma and anti-apoA-I immunosubtracting 2D-PAGGE of normal plasma revealed that LpA-IV-1 and LpA-IV-2 do not contain apoA-I. The importance of LpA-IV-1 and LpA-IV-2 for uptake and esterification of cell-derived cholesterol was investigated using pulse-chase incubations of plasma with [3H]cholesterol-labeled fibroblasts followed by anti-apoA-I immunosubtracting 2D-PAGGE. During 1-minute pulse incubation with cells, [3H]cholesterol was taken up by gamma-LpE > LpA-IV-1 > pre-beta 1-LpA-I > LpA-IV-2 (">" denotes "more than"). During subsequent chase incubation without cells, proportionately less radioactivity disappeared from LpA-IV-1 and LpA-IV-2 than from pre-beta 1-LpA-I and gamma-LpE. During 5-minute pulse incubations, radioactive cholesteryl esters were formed in pre-beta 3-LpA-I > alpha-LpA-I > LpA-IV-1 > LpA-IV-2. The fractional estertification rate was highest in pre-beta 2-LpA-I and lowest in alpha-LpA-I.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of apoA-IV-containing lipoprotein particles isolated from human plasma and interstitial fluid

Apolipoprotein (apo) A-IV has been proposed to play a role in reverse cholesterol transport. ApoA-IV-containing lipoprotein particles (A-IVLp) were isolated from human plasma and interstitial fluid and characterized by immunoaffinity chromatography. Two major A-IVLp subpopulations, lipoprotein particles containing apoA-IV with apoA-I (LpA-I:A-IV) and lipoprotein particles containing apoA-IV without apoA-I (LpA-IV), were identified. The larger subpopulation of A-IVLp is the LpA-IV that represents 70% (protein mass) of the initial particles. Only 5.8% of apoA-IV was recovered in the retained fraction after affinity chromatography with an anti-apoA-I immunosorbent. ApoA-I, apoA-II, apoA-IV, apoB, apoC-III, apoD, apoE, apoH, lecithin: cholesterol acyltransferase (LCAT), cholesteryl ester transfer (CET) protein, proline-rich protein, and a protein of Mr 59,000 were detected in the A-IVLp. These particles contain more than 20% triglycerides (lipid mass). ApoA-IV-containing particles that were isolated from plasma are heterogeneous in size, consisting of two major populations with Stokes' diameters of 10.3 nm and 9.3 nm. Both subpopulations of A-IVLp contain LCAT and CET activities and promote cholesterol efflux from cholesterol-preloaded adipose cells. These data support the hypothesis that A-IVLp particles may be involved in reverse cholesterol transport.

Functional characterization of human recombinant apolipoprotein AIV produced in Escherichia coli

Apolipoprotein AIV (apoAIV), a protein which is known to activate the enzyme lecithin: cholesterol acyltransferase, to bind to apoAI/AII receptor sites and also to promote cholesterol efflux from adipose cells, may play an important role in reverse cholesterol transport. In this report, the high-level production of soluble recombinant mature human apoAIV (isoform 1) in Escherichia coli is described. The recombinant protein was purified by avoiding lipid extraction or denaturation. The apoAIV preparation was analysed by its reactivity with antibodies raised against human apoAIV, SDS-gel electrophoresis, isoelectric focusing and N-terminal sequencing. The purified recombinant protein retains an extra methionine at the N-terminus. Purified recombinant and natural apoAIV proteins were indistinguishable with regard to their denaturation properties, thermo-stability or their fluorescence emission properties in the presence of various quantities of a quenching agent. Complexes of ApoAIV with L-alpha-dimyristoyl-glycerophosphocholine (Myr2GroPCho), glycerophosphocholine (GroPCho), or L-alpha-1-palmitoyl-2-oleoylglycerophosphocholine (PamOleGroPCho) prepared from plasmatic and from recombinant apoAIV proteins have similar densities as revealed by analytical centrifugation. They also share the same cofactor properties for the lecithin:cholesterol acyltransferase reaction. Recombinant apoAIV complex with Myr2GroPCho was also able to bind to the same apoAI/AII receptor sites and to promote cholesterol efflux to an equal extent from adipose cells. It is concluded that the recombinant protein is functionally identical to the plasmatic apoAIV and may therefore be very useful in helping to elucidate the physiological role of apoAIV.

Binding of human apolipoprotein A-IV to human hepatocellular plasma membranes

We have investigated the binding of human apolipoprotein A-IV (apo A-IV) to human hepatocellular plasma membranes. Addition of increasing concentrations of radiolabeled apo A-IV to hepatic plasma membranes, in the presence and absence of a 25-fold excess of unlabeled apo A-IV, revealed saturation binding to the membranes with a KD of 154 nM and a binding maximum of 1.6 ng/microgram of membrane protein. The binding was temperature-insensitive, partially calcium-dependent, abolished when apo A-IV was denatured by guanidine hydrochloride or when the membranes were treated with Pronase and decreased when apo A-IV was incorporated into phospholipid/cholesterol proteoliposomes. In displacement studies using purified apolipoproteins and isolated lipoproteins, only unlabeled apo A-IV, apo A-I and high-density lipoproteins effectively competed with radiolabeled apo A-IV for membrane binding sites. We conclude that human apo A-IV exhibits high-affinity binding to isolated human hepatocellular plasma membranes which is saturable, reversible and specific.

A repeating amino acid motif in CDC23 defines a family of proteins and a new relationship among genes required for mitosis and RNA synthesis

We have identified and characterized a novel, repeating 34 amino acid motif (the TPR motif) that is reiterated several times within the CDC23 gene product of S. cerevisiae. Multiple copies of this motif were discovered in five other proteins, three encoded by cell division cycle genes required to complete mitosis and two involved in RNA synthesis. Quantitative sequence analyses suggest the existence of a common underlying structure in each TPR unit that consists of amphipathic alpha-helical regions punctuated by proline-induced turns. The TPR motif defines a new family of genes and an important structural unit common to several proteins whose functions are required for mitosis and RNA synthesis.