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

rHDL modeling and the anchoring mechanism of LCAT activation

Lecithin:cholesterol-acyl transferase (LCAT) plays a major role in cholesterol metabolism as it is the only extracellular enzyme able to esterify cholesterol. LCAT activity is required for lipoprotein remodeling and, most specifically, for the growth and maturation of HDLs. In fact, genetic alterations affecting LCAT functionality may cause a severe reduction in plasma levels of HDL-cholesterol with important clinical consequences. Although several hypotheses were formulated, the exact molecular recognition mechanism between LCAT and HDLs is still unknown. We employed a combination of structural bioinformatics procedures to deepen the insights into the HDL-LCAT interplay that promotes LCAT activation and cholesterol esterification. We have generated a data-driven model of reconstituted HDL (rHDL) and studied the dynamics of an assembled rHDL::LCAT supramolecular complex, pinpointing the conformational changes originating from the interaction between LCAT and apolipoprotein A-I (apoA-I) that are necessary for LCAT activation. Specifically, we propose a mechanism in which the anchoring of LCAT lid to apoA-I helices allows the formation of a hydrophobic hood that expands the LCAT active site and shields it from the solvent, allowing the enzyme to process large hydrophobic substrates.

Immunochemical Approach for Monitoring of Structural Transition of ApoA-I upon HDL Formation Using Novel Monoclonal Antibodies

Apolipoprotein A-I (apoA-I) undergoes a large conformational reorganization during remodeling of high-density lipoprotein (HDL) particles. To detect structural transition of apoA-I upon HDL formation, we developed novel monoclonal antibodies (mAbs). Splenocytes from BALB/c mice immunized with a recombinant human apoA-I, with or without conjugation with keyhole limpet hemocyanin, were fused with P3/NS1/1-Ag4-1 myeloma cells. After the HAT-selection and cloning, we established nine hybridoma clones secreting anti-apoA-I mAbs in which four mAbs recognize epitopes on the N-terminal half of apoA-I while the other five mAbs recognize the central region. ELISA and bio-layer interferometry measurements demonstrated that mAbs whose epitopes are within residues 1–43 or 44–65 obviously discriminate discoidal and spherical reconstituted HDL particles despite their great reactivities to lipid-free apoA-I and plasma HDL, suggesting the possibility of these mAbs to detect structural transition of apoA-I on HDL. Importantly, a helix-disrupting mutation of W50R into residues 44–65 restored the immunoreactivity of mAbs whose epitope being within residues 44–65 against reconstituted HDL particles, indicating that these mAbs specifically recognize the epitope region in a random coil state. These results encourage us to develop mAbs targeting epitopes in the N-terminal residues of apoA-I as useful probes for monitoring formation and remodeling of HDL particles.

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.

Crystal structure of Δ(185-243)ApoA-I suggests a mechanistic framework for the protein adaptation to the changing lipid load in good cholesterol: from flatland to sphereland via double belt, belt buckle, double hairpin and trefoil/tetrafoil

Apolipoprotein A-I (apoA-I) is the major protein of plasma high-density lipoproteins (HDLs), macromolecular assemblies of proteins and lipids that remove cell cholesterol and protect against atherosclerosis. HDL heterogeneity, large size (7.7-12 nm), and ability to exchange proteins have prevented high-resolution structural analysis. Low-resolution studies showed that two apoA-I molecules form an antiparallel α-helical "double belt" around an HDL particle. The atomic-resolution structure of the C-terminal truncated lipid-free Δ(185-243)apoA-I, determined recently by Mei and Atkinson, provides unprecedented new insights into HDL structure-function. It allows us to propose a molecular mechanism for the adaptation of the full-length protein to increasing lipid load during cholesterol transport. ApoA-I conformations on small, midsize, and large HDLs are proposed based on the tandem α-helical repeats and the crystal structure of Δ(185-243)apoA-I and are validated by comparison with extensive biophysical data reported by many groups. In our models, the central half of the double belt ("constant" segment 66-184) is structurally conserved while the N- and C-terminal half ("variable" segments 1-65 and 185-243) rearranges upon HDL growth. This includes incremental unhinging of the N-terminal bundle around two flexible regions containing G39 and G65 to elongate the belt, along with concerted swing motion of the double belt around G65-P66 and G185-G186 hinges that are aligned on various-size particles, to confer two-dimensional surface curvature to spherical HDLs. The proposed conformational ensemble integrates and improves several existing HDL models. It helps provide a structural framework necessary to understand functional interactions with over 60 other HDL-associated proteins and, ultimately, improve the cardioprotective function of HDL.

Apolipoprotein A-I helical structure and stability in discoidal high-density lipoprotein (HDL) particles by hydrogen exchange and mass spectrometry

To understand high-density lipoprotein (HDL) structure at the molecular level, the location and stability of α-helical segments in human apolipoprotein (apo) A-I in large (9.6 nm) and small (7.8 nm) discoidal HDL particles were determined by hydrogen-deuterium exchange (HX) and mass spectrometry methods. The measured HX kinetics of some 100 apoA-I peptides specify, at close to amino acid resolution, the structural condition of segments throughout the protein sequence and changes in structure and stability that occur on incorporation into lipoprotein particles. When incorporated into the large HDL particle, the nonhelical regions in lipid-free apoA-I (residues 45-53, 66-69, 116-146, and 179-236) change conformation from random coil to α-helix so that nearly the entire apoA-I molecule adopts helical structure (except for the terminal residues 1-6 and 237-243). The amphipathic α-helices have relatively low stability, in the range 3-5 kcal/mol, indicating high flexibility and dynamic unfolding and refolding in seconds or less. A segment encompassed by residues 125-158 exhibits bimodal HX labeling indicating co-existing helical and disordered loop conformations that interchange on a time scale of minutes. When incorporated around the edge of the smaller HDL particle, the increase in packing density of the two apoA-I molecules forces about 20% more residues out of direct contact with the phospholipid molecules to form disordered loops, and these are the same segments that form loops in the lipid-free state. The region of disc-associated apoA-I that binds the lecithin-cholesterol acyltransferase enzyme is well structured and not a protruding unstructured loop as reported by others.

Role of apolipoprotein A-II in the structure and remodeling of human high-density lipoprotein (HDL): protein conformational ensemble on HDL

High-density lipoproteins (HDL, or "good cholesterol") are heterogeneous nanoparticles that remove excess cell cholesterol and protect against atherosclerosis. The cardioprotective action of HDL and its major protein, apolipoprotein A-I (apoA-I), is well-established, yet the function of the second major protein, apolipoprotein A-II (apoA-II), is less clear. In this review, we postulate an ensemble of apolipoprotein conformations on various HDL. This ensemble is based on the crystal structure of Δ(185-243)apoA-I determined by Mei and Atkinson combined with the "double-hairpin" conformation of apoA-II(dimer) proposed in the cross-linking studies by Silva's team, and is supported by the wide array of low-resolution structural, biophysical, and biochemical data obtained by many teams over decades. The proposed conformational ensemble helps integrate and improve several existing HDL models, including the "buckle-belt" conformation of apoA-I on the midsize disks and the "trefoil/tetrafoil" arrangement on spherical HDL. This ensemble prompts us to hypothesize that endogenous apoA-II (i) helps confer lipid surface curvature during conversion of nascent discoidal HDL(A-I) and HDL(A-II) containing either apoA-I or apoA-II to mature spherical HDL(A-I/A-II) containing both proteins, and (ii) hinders remodeling of HDL(A-I/A-II) by hindering the expansion of the apoA-I conformation. Also, we report that, although endogenous apoA-II circulates mainly on the midsize spherical HDL(A-I/A-II), exogenous apoA-II can bind to HDL of any size, thereby slightly increasing this size and stabilizing the HDL assembly. This suggests distinctly different effects of the endogenous and exogenous apoA-II on HDL. Taken together, the existing results and models prompt us to postulate a new structural and functional role of apoA-II on human HDL.

Nanotechnology for synthetic high-density lipoproteins

Atherosclerosis is the disease mechanism responsible for coronary heart disease (CHD), the leading cause of death worldwide. One strategy to combat atherosclerosis is to increase the amount of circulating high-density lipoproteins (HDL), which transport cholesterol from peripheral tissues to the liver for excretion. The process, known as reverse cholesterol transport, is thought to be one of the main reasons for the significant inverse correlation observed between HDL blood levels and the development of CHD. This article highlights the most common strategies for treating atherosclerosis using HDL. We further detail potential treatment opportunities that utilize nanotechnology to increase the amount of HDL in circulation. The synthesis of biomimetic HDL nanostructures that replicate the chemical and physical properties of natural HDL provides novel materials for investigating the structure-function relationships of HDL and for potential new therapeutics to combat CHD.

Small discoidal pre-beta1 HDL particles are efficient acceptors of cell cholesterol via ABCA1 and ABCG1

The aim of this study was to correlate the lipid content and size of discoidal reconstituted HDL particles with their ability to promote cellular cholesterol efflux. Homogeneous discoidal rHDL particles containing apoA-I and POPC, with diameters of 7.8, 9.6, 10.8, 12.5, and 17.0 nm, were prepared by the cholate dialysis technique. Cholesterol efflux to rHDL was evaluated in pathway-specific cell models for ABCA1-, ABCG1-, and SR-BI-mediated efflux. ABCA1-mediated efflux was efficiently promoted by the 7.8 nm rHDL containing 82 POPC molecules per particle. This rHDL also promoted ABCG1, but not SR-BI, cholesterol efflux. All large and lipid-rich rHDLs, with a diameter of >or=9.6 nm and a phospholipid content of >/=202 molecules per particle, promoted both SR-BI- and ABCG1-mediated efflux. Our results indicated that the ABCA1-mediated cell cholesterol efflux can be efficiently driven not only by monomolecular lipid free/poor apoA-I but also by a small discoidal phospholipid-containing particle resembling plasma pre-beta1 HDL. This same particle also promotes ABCG1- but not SR-BI-mediated efflux. These results help to clarify the role of plasma pre-beta1 HDL in reverse cholesterol transport.

Structural features and dynamics properties of human apolipoprotein A-I in a model of synthetic HDL

High-density lipoproteins (HDL) play a major role in the reverse transport of cholesterol and have antiatherogenic activities. Their major protein component is apolipoprotein (apo) A-I. While apoA-I amphipathic alpha-helix based secondary structure has been extensively investigated, for its lipid-bound tertiary structure only theoretical models have been proposed. In the past years, experimental approaches aimed at a direct visualization of HDL structure have been exploited, but data obtained through different microscopy techniques are conflicting and do not settle the issue. Here we present a 50 ns molecular dynamics simulation of a synthetic HDL containing two molecules of apoA-I and 101 of l-alpha-palmitoyl-oleoyl-phosphatidylcholine. Essential dynamics and structural property investigations suggest that the stabilization of the system is obtained through specific motions, whose driving forces are protein-phospholipid interactions. The most important are: the relative sliding of the two apoA-I molecules along their major axes, the relative rotation of the protein chains, and the out-of-plane deformation around proline hinges. The sliding and the out-of-plane deformation allow apoA-I to optimize its interactions with phospholipids, while the rotation is useful to maximize protein-protein salt bridges. The correspondence between computed parameters and their experimental counterparts contributes to validate our model and its dynamic behaviors. Our findings help in defining a molecular model for apoA-I contained in HDL and suggest a possible mechanism through which apoA-I can vary its diameter and accommodate different numbers of phospholipids during the metabolism of HDL.

Reference distributions for apolipoproteins AI and B and B/AI ratios: comparison of a large cohort to the world's literature

Limiting the clinical utility of apolipoproteins AI (apo AI) and B (apo B) and the apo B/AI ratios until the last decade has been the lack of satisfactory methods for quantifying serum levels and credible reference materials. Great technological strides have been made in the last few years. The remaining barrier to more relevant and cost-effective use of serum protein data for diagnosis and prognosis has been the availability of widely recognized reliable reference intervals from birth to old age for both males and females. A total of 82 publications reporting reference intervals have been identified that meet most of the same inclusion criteria used in our prior six studies. These have been analyzed statistically and compared to similar studies, i.e., sufficient number, listed subject criteria, method, and reference material, in general terms. Published smaller studies with constrained age ranges, agree on average with our large series of life-long reference intervals that range from less than one year to over 80 years. This study was performed to assess the degree of agreement between smaller reference interval studies to our large population analysis. This meta-analysis provides support and reassurance that many of the smaller reference intervals published previously fall within reasonable limits of out large population.

Structure, function and amyloidogenic propensity of apolipoprotein A-I

Apolipoprotein A-I, the major structural apolipoprotein of high-density lipoproteins, efficiently protects humans from cholesterol accumulation in tissues; however, it can cause systemic amyloidosis in the presence of peculiar amino acid replacements. The wild-type molecule also has an intrinsic tendency to generate amyloid fibrils that localise within the atherosclerotic plaques. The structure, folding and metabolism of normal apolipoprotein A-I are extremely complex and as yet not completely clarified, but their understanding appears essential for the elucidation of the amyloid transition. We reviewed present knowledge on the structure, function and amyloidogenic propensity of apolipoprotein A-I with the aim of highlighting the possible molecular mechanisms that might contribute to the pathogenesis of this disease. Important clues on apolipoprotein A-I amyloidogenesis may be obtained from classical comparative studies of the properties of the wild-type versus the amyloidogenic counterpart. Additionally, in the case of apoA-I, further insights on the molecular mechanisms underlying its amyloidogenic propensity may derive from comparative studies between amyloidogenic variants and other mutations associated with hypoalphalipoproteinemia without amyloidosis.

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.

Crystal structure of human apolipoprotein A-I: insights into its protective effect against cardiovascular diseases

Despite three decades of extensive studies on human apolipoprotein A-I (apoA-I), the major protein component in high-density lipoproteins, the molecular basis for its antiatherogenic function is elusive, in part because of lack of a structure of the full-length protein. We describe here the crystal structure of lipid-free apoA-I at 2.4 A. The structure shows that apoA-I is comprised of an N-terminal four-helix bundle and two C-terminal helices. The N-terminal domain plays a prominent role in maintaining its lipid-free conformation, indicating that mutants with truncations in this region form inadequate models for explaining functional properties of apoA-I. A model for transformation of the lipid-free conformation to the high-density lipoprotein-bound form follows from an analysis of solvent-accessible hydrophobic patches on the surface of the structure and their proximity to the hydrophobic core of the four-helix bundle. The crystal structure of human apoA-I displays a hitherto-unobserved array of positively and negatively charged areas on the surface. Positioning of the charged surface patches relative to hydrophobic regions near the C terminus of the protein offers insights into its interaction with cell-surface components of the reverse cholesterol transport pathway and antiatherogenic properties of this protein. This structure provides a much-needed structural template for exploration of molecular mechanisms by which human apoA-I ameliorates atherosclerosis and inflammatory diseases.

ApoA-I Lipidation in Primary Mouse Hepatocytes

The liver is the major site of both apolipoprotein A-I (apoA-I) synthesis and ATP-binding cassette transporter A1 (ABCA1) expression. Here, we compare the lipidation with cholesterol and phospholipid of newly synthesized human apoA-I (hapoA-I) using adenoviral vector-mediated endogenous expression or exogenously added hapoA-I in wild type and ABCA1-null hepatocytes. Hepatocytes were labeled with [3H]cholesterol (delivered with LDL or methyl-beta-cyclodextrin), [3H]mevalonate, or [3H]choline. ABCA1 deficiency decreased apoA-I phospholipidation by 80%, but acquisition of de novo synthesized and exogenous cholesterol only decreased by 40-60%. The transfer of de novo synthesized cholesterol to apoA-I was decreased at all time points, but that of exogenously delivered cholesterol was independent of ABCA1 activity at the early time points. Progesterone does not affect apoA-I synthesis or its lipidation but inhibited the early phase of apoA-I cholesterol lipidation in both wild type and ABCA1-null hepatocytes. Fast protein liquid chromatography analysis of medium lipoproteins confirmed that with ABCA1 deficiency, the proportion of secreted high density lipoprotein-associated apoA-I and cholesterol decreased by about 50%. The very low density lipoprotein (VLDL)/LDL size fraction also contained a significant level of cholesterol in ABCA1 deficiency, consistent with the result of immunoprecipitations showing the presence of lipoproteins with both apoA-I and murine apoB. ApoA-I lipidation with newly synthesized cholesterol in ABCA1-null hepatocytes was significantly decreased by brefeldin A and monensin. In conclusion, we demonstrate that: (i) whereas most hepatic phospholipidation of apoA-I is mediated by ABCA1, acquisition of cholesterol depends on active transfer from intracellular compartments by ABCA1-dependent and -independent pathways, both sensitive to progesterone and (ii) there is separate regulation of phospholipid and cholesterol lipidation of apoA-I in hepatocytes.

Evidence that endothelial lipase remodels high density lipoproteins without mediating the dissociation of apolipoprotein A-I

Endothelial lipase (EL) is a triglyceride lipase gene family member that has high phospholipase and low triglyceride lipase activity. The aim of this study was to determine whether the phospholipase activity of EL is sufficient to remodel HDLs into small particles and mediate the dissociation of apolipoprotein A-I (apoA-I). Spherical, reconstituted HDLs (rHDLs) containing apoA-I only [(A-I)rHDLs], apoA-II only [(A-II)rHDLs], or both apoA-I and apoA-II [(A-I/A-II) rHDLs] were prepared. The rHDLs, which contained only cholesteryl esters in their core and POPC on the surface, were incubated with EL. As the rHDLs did not contain triacylglycerol, only the POPC was hydrolyzed. Hydrolysis was greater in the (A-I/A-II)rHDLs than in the (A-I)rHDLs. The (A-II)rHDL phospholipids were not hydrolyzed by EL. EL remodeled the (A-I)rHDLs and (A-I/A-II)rHDLs, but not the (A-II)rHDLs, into smaller particles. The reduction in particle size was related to the amount of phospholipid hydrolysis, with the diameter of the (A-I/A-II)rHDLs decreasing more than that of the (A-I)rHDLs. These changes did not affect the conformation of apoA-I, and neither apoA-I nor apoA-II dissociated from the rHDLs. Comparable results were obtained when human plasma HDLs were incubated with EL. These results establish that the phospholipase activity of EL remodels plasma HDLs and rHDLs into smaller particles without mediating the dissociation of apolipoproteins.

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.

The lipid composition of high-density lipoprotein affects its re-absorption in the kidney by proximal tubule epithelial cells

The kidney is believed to play a major role in the clearance of apoA-I (apolipoprotein A-I) and HDL (high-density lipoprotein) particles from the bloodstream. Proximal tubule epithelial cells of the kidney appear to prevent the loss of these proteins in the urine by re-absorbing them from the urinary filtrate. Experiments were undertaken to investigate the factors that regulate the renal re-absorption of apoA-I and small HDL in a transformed human proximal tubule epithelial (HKC-8) cell line. Fluorescent microscopic studies show that HKC-8 cells can readily bind and take up HDL particles. Intracellular localization of fluorescently labelled native HDL shows its accumulation in endocytotic vesicles, in a perinuclear region after 1 h. Binding studies reveal a saturable cell association of (125)I-HDL with the HKC-8 cell surface after 2 h. HKC-8 cells do not degrade apoA-I or other HDL-apoproteins. The specific cell association of lipid-free apoA-I is approx. 2-fold less than that observed for native HDL. Similarly, reconstituted HDL prepared from HDL-apoproteins and pure phospholipids also exhibits a low cell association with the HKC-8 cells. In contrast, reconstituted HDL prepared with the extracted lipids of HDL and pure apoA-I exhibits an even higher cell association than that observed with the native lipoprotein. A detailed characterization of the major lipid classes in reconstituted HDL shows that only cholesteryl ester increases the cell association of the recombinant particles. These results show that the cholesteryl ester content of HDL may play an important role in the re-absorptive salvage of HDL by the proximal tubule cells of the kidney.

Structure-function relationships of apolipoprotein A-I: a flexible protein with dynamic lipid associations

PURPOSE OF REVIEW

Apolipoprotein A-I is the major structural protein of HDL. Its physicochemical properties maintain a delicate balance between maintenance of stable lipoproteins and the ability to associate with and dissociate from the lipid transported. Here we review the progress made in the last 2-3 years on the structure-function relationships of apolipoprotein A-I, including elements related to the ATP binding cassette transporter A1.

RECENT FINDINGS

Current evidence now supports the so-called 'belt' or 'hairpin' models for apolipoprotein A-I conformation when bound to discoidal lipoproteins. In-vivo expression of apolipoprotein A-I mutant proteins has shown that both the N- and C-terminal domains are important for lipid association as well as for the esterification reaction, particularly binding of cholesteryl esters and formation of mature alpha-migrating lipoproteins. This property is apparently quite distinct from the activation of the enzyme lecithin cholesterol acyl transferase, which requires interaction with the central helix 6. The interaction of apolipoprotein A-I with the ATP binding cassette transporter A1 has been shown to require the C-terminal domain, which is proposed to mediate the opening of the helix bundle formed by lipid-free or lipid-poor apolipoprotein A-I and allow its association with hydrophobic binding sites.

SUMMARY

Significant progress has been made in the understanding of the molecular mechanisms controlling the folding of apolipoprotein A-I and its interaction with lipids and various other protein factors involved in HDL metabolism.

Apolipoprotein composition and particle size affect HDL degradation by chymase: effect on cellular cholesterol efflux

Mast cell chymase, a chymotrypsin-like neutral protease, can proteolyze HDL3. Here we studied the ability of rat and human chymase to proteolyze discoidal pre beta-migrating reconstituted HDL particles (rHDLs) containing either apolipoprotein A-I (apoA-I) or apoA-II. Both chymases cleaved apoA-I in rHDL at identical sites, either at the N-terminus (Tyr18 or Phe33) or at the C-terminus (Phe225), so generating three major truncated polypeptides that remained bound to the rHDL. The cleavage sites were independent of the size of the rHDL particles, but small particles were more susceptible to degradation than bigger ones. Chymase-induced truncation of apoA-I yielded functionally compromised rHDL with reduced ability to promote cellular cholesterol efflux. In sharp contrast to apoA-I, apoA-II was resistant to degradation. However, when apoA-II was present in rHDL that also contained apoA-I, it was degraded by chymase. We conclude that chymase reduces the ability of apoA-I in discoidal rHDL particles to induce cholesterol efflux by cleaving off either its amino- or carboxy-terminal portion. This observation supports the concept that limited extracellular proteolysis of apoA-I is one pathophysiologic mechanism leading to the generation and maintenance of foam cells in atherosclerotic lesions.

NMR studies of lipoprotein structure

Early NMR structural studies of serum lipoproteins were based on (1)H, (13)C, (31)P, and (2)H studies of lipid components. From the early studies information on composition, lipid chain dynamics and order parameters, and monolayer organization resulted. More recently, selective or complete isotopic labeling techniques, combined with multidimensional NMR spectroscopy, have resulted in structural information of apoprotein fragments. Finally, use of heteronuclear three- and four-dimensional experiments have yielded solution structures and protein-lipid interactions of intact apolipoproteins C-I, C-II, and A-I.

Heteronuclear NMR studies of human serum apolipoprotein A-I. Part I. Secondary structure in lipid-mimetic solution

The apolipoprotein A-I (apoA-I) solution structure in the presence of sodium dodecyl sulfate (SDS) was determined by combination of chemical shift index and torsion angle likelihood obtained from shift and sequence similarity methods. ApoA-I in lipid-mimetic solution is composed of alpha-helices (residues 8-32, 45-64, 67-77, 82-86, 90-97, 100-118, 122-140, 146-162, 167-205, 210-216 and 221-239), with 2-5 residue irregular segments between helical repeats, and the irregular segment 78-81 within helical repeat 2. ApoA-I is a monomer in the SDS complex and no evidence of interhelical interactions is found. Comparison of the apoA-I and apoA-I(1-186) [Okon et al., FEBS Lett. 487 (2001) 390-396] solution structures revealed that apoA-I undergoes a conformational change around Pro121.

Proteolytic degradation and impaired secretion of an apolipoprotein A-I mutant associated with dominantly inherited hypoalphalipoproteinemia

We have devised a combined in vivo, ex vivo, and in vitro approach to elucidate the mechanism(s) responsible for the hypoalphalipoproteinemia in heterozygous carriers of a naturally occurring apolipoprotein A-I (apoA-I) variant (Leu(159) to Arg) known as apoA-I Finland (apoA-I(FIN)). Adenovirus-mediated expression of apoA-I(FIN) decreased apoA-I and high density lipoprotein cholesterol concentrations in both wild-type C57BL/6J mice and in apoA-I-deficient mice expressing native human apoA-I (hapoA-I). Interestingly, apoA-I(FIN) was degraded in the plasma, and the extent of proteolysis correlated with the most significant reductions in murine apoA-I concentrations. ApoA-I(FIN) had impaired activation of lecithin:cholesterol acyltransferase in vitro compared with hapoA-I, but in a mixed lipoprotein preparation consisting of both hapoA-I and apoA-I(FIN) there was only a moderate reduction in the activation of this enzyme. Importantly, secretion of apoA-I was also decreased from primary apoA-I-deficient hepatocytes when hapoA-I was co-expressed with apoA-I(FIN) following infection with recombinant adenoviruses, a condition that mimics secretion in heterozygotes. Thus, this is the first demonstration of an apoA-I point mutation that decreases LCAT activation, impairs hepatocyte secretion of apoA-I, and makes apoA-I susceptible to proteolysis leading to dominantly inherited hypoalphalipoproteinemia.

Evidence for a central apolipoprotein A-I domain loosely bound to lipids in discoidal lipoproteins that is capable of penetrating the bilayer of phospholipid vesicles

Previous evidence indicated that discoidal reconstituted high density lipoproteins (rHDL) of apolipoprotein A-I (apoA-I) can interact with lipid membranes (Tricerri, M. A., Córsico, B., Toledo, J. D., Garda, H. A., and Brenner, R. R. (1998) Biochim. Biophys. Acta 1391, 67-78). With the aim of studying this interaction, photoactivable reagents and protein cleavage with CNBr and hydroxylamine were used. The generic hydrophobic reagent 3-(trifluoromethyl)-3-(m-[125I]iodophenyl)diazirine gave information on the apoA-I regions in contact with the lipid phase in the rHDL discs. Two protein regions loosely bound to lipids were detected: a C-terminal domain and a central one located between residues 87 and 112. They consist of class Y amphipathic alpha-helices that have a different distribution of the charged residues in their polar faces by comparison with class A helices, which predominate in the rest of the apoA-I molecule. The phospholipid analog 1-O-hexadecanoyl-2-O-[9-[[[2-[125I]iodo-4-(trifluoro-methyl-3-H-diazirin-3-yl)benzyl]oxy]carbonyl]nonanoyl]-sn-glycero-3-phosphocholine, which does not undergo significant exchange between membranes and lipoproteins, was used to identify the apoA-I domain directly involved in the interaction of rHDL discs with membranes. By incubating either rHDL or lipid-free apoA-I with lipid vesicles containing 125I-TID-PC, only the 87-112 apoA-I segment becomes labeled after photoactivation. These results indicate that the central domain formed by two type Y helices swings away from lipid contact in the discoidal lipoproteins and is able to insert into membrane bilayers, a process that may be of great importance for the mechanism of cholesterol exchange between high density lipoproteins and cell membranes.

Secondary structure of human apolipoprotein A-I(1-186) in lipid-mimetic solution

The solution structure of an apoA-I deletion mutant, apoA-I(1-186) was determined by the chemical shift index (CSI) method and the torsion angle likelihood obtained from shift and sequence similarity (TALOS) method, using heteronuclear multidimensional NMR spectra of [u-(13)C, u-(15)N, u-50% (2)H]apoA-I(1-186) in the presence of sodium dodecyl sulfate (SDS). The backbone resonances were assigned from a combination of triple-resonance data (HNCO, HNCA, HN(CO)CA, HN(CA)CO and HN(COCA)HA), and intraresidue and sequential NOEs (three-dimensional (3D) and four-dimensional (4D) 13C- and 15N-edited NOESY). Analysis of the NOEs, H(alpha), C(alpha) and C' chemical shifts shows that apoA-I(1-186) in lipid-mimetic solution is composed of alpha-helices (which include the residues 8-32, 45-64, 67-77, 83-87, 90-97, 100-140, 146-162, and 166-181), interrupted by short irregular segments. There is one relatively long, irregular and mostly flexible region (residues 33-44), that separates the N-terminal domain (residues 1-32) from the main body of protein. In addition, we report, for the first time, the structure of the N-terminal domain of apoA-I in a lipid-mimetic environment. Its structure (alpha-helix 8-32 and flexible linker 33-44) would suggest that this domain is structurally, and possibly functionally, separated from the other part of the molecule.

Monoclonal antibodies to human apolipoproteins: application to the study of high density lipoprotein subpopulations

We produced, selected and cloned hybridomas that secrete monoclonal antibodies against human apolipoprotein (apo) A-I. All of the antibodies corresponded to the IgG(1) subclass and were named 1C11, 2B4, 2C10, 7C5, 8A4 and 8A5. The antibodies were characterized by their reactivity with whole lipoproteins, apolipoproteins, synthetic peptides and fragments generated by cleavage of the apo A-I. Three of the monoclonal antibodies studied (2B4, 2C10 and 7C5) were similarly inhibited by an amino-terminal peptide (amino acid sequence 1-20) of apo A-I, whereas antibodies 1C11, 8A4 and 8A5 had no reaction. Other results show that monoclonal antibody 1C11 recognizes an epitope located between amino acids 135-148. We evaluated the monoclonal antibody 8A4 against different HDL subpopulations by competitive displacement analysis and it showed a similar reactivity with the HDL particles: LpA-I and LpA-I:A-II. This antibody was used to standardize a sandwich ELISA to quantitate LpA-I in plasma. We conclude that these monoclonal antibodies are relevant for the study of apo A-I epitope expression and for quantitating apo A-I containing lipoparticles.

The orientation of helix 4 in apolipoprotein A-I-containing reconstituted high density lipoproteins

The three-dimensional structure of the high density lipoprotein (HDL) component apolipoprotein (apo) A-I and the molecular basis for its protection against coronary artery disease are unknown. In terms of discoidal HDL particles, there has been a debate as to the orientation of the apoA-I alpha-helices around the disc edge. The "picket fence" model states that the alpha-helical repeats, separated by turns, are arranged parallel to the phospholipid acyl chains of the enclosed lipid bilayer. On the other hand, the "belt" model states that the helical segments run perpendicular to the acyl chains. To distinguish between these models, we used nitroxide spin labels present at various depths in the bilayer of reconstituted HDL (rHDL) to measure the position of Trp residues in single Trp mutants of human proapoA-I. Two mutants were studied; the first contained a Trp at position 108, which was located near the center of helix 4. The second contained a Trp at position 115, two turns along the same helix. The picket fence model predicts that these Trp residues should be at different depths in the bilayer, whereas the belt model predicts that they should be at similar depths. Different sized rHDL particles were produced that contained 2, 3, and >4 molecules of proapoA-I per complex. In each case, parallax analysis indicated that Trp-108 and Trp-115 were present at similar depths of about 6 A from the center of the bilayer, consistent with helix 4 being oriented perpendicular to the acyl chains (in agreement with the belt model). Similar experiments showed that control transmembrane peptides were oriented parallel to the acyl chains in vesicles, demonstrating that the method was capable of distinguishing between the two models. This study provides one of the first experimental measurements of the location of an apoA-I helix with respect to the bilayer edge.

Distinct central amphipathic alpha-helices in apolipoprotein A-I contribute to the in vivo maturation of high density lipoprotein by either activating lecithin-cholesterol acyltransferase or binding lipids

Recombinant adenoviruses with cDNAs for human apolipoprotein A-I (wild type (wt) apoA-I) and three mutants, referred to as Delta4-5A-I, Delta5-6A-I, and Delta6-7A-I, that have deletions removing regions coding for amino acids 100-143, 122-165, and 144-186, respectively, were created to study structure/function relationships of apoA-I in vivo. All mutants were expressed at lower concentrations than wt apoA-I in plasma of fasting apoA-I-deficient mice. The Delta5-6A-I mutant was found primarily in the lipid-poor high density lipoprotein (HDL) pool and at lower concentrations than Delta4-5A-I and Delta6-7A-I that formed more buoyant HDL(2/3) particles. At an elevated adenovirus dose and earlier blood sampling from fed mice, both Delta5-6A-I and Delta6-7A-I increased HDL-free cholesterol and phospholipid but not cholesteryl ester. In contrast, wt apoA-I and Delta4-5A-I produced significant increases in HDL cholesteryl ester. Further analysis showed that Delta6-7A-I and native apoA-I could bind similar amounts of phospholipid and cholesterol that were reduced slightly for Delta5-6A-I and greatly for Delta4-5A-I. We conclude from these findings that amino acids (aa) 100-143, specifically helix 4 (aa 100-121), contributes to the maturation of HDL through a role in lipid binding and that the downstream sequence (aa 144-186) centered around helix 6 (aa 144-165) is responsible for the activation of lecithin-cholesterol acyltransferase.

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

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

Recombinant apolipoproteins for the treatment of vascular diseases

The protein components of human lipoproteins, apolipoproteins, allow the redistribution of cholesterol from the arterial wall to other tissues and exert beneficial effects on systems involved in the development of arterial lesions, like inflammation and hemostasis. Because of these properties, the antiatherogenic apolipoproteins, particularly apo A-I and apo E, may provide an innovative approach to the management of vascular diseases. The recent availability of extractive or biosynthetic molecules is allowing a detailed overview of their therapeutic potential in a number of animal models of arterial disease. Infusions of apo E, or more dramatically, of apo A-I, both recombinant or extractive, cause a direct reduction of the atherosclerotic burden in experimental animals. Naturally, as the apo A-I(Milano) (apo A-I(M)) dimer, or engineered recombinant apolipoproteins with prolonged permanence in plasma and improved function may offer an even better approach to the therapeutic handling of arterial disease. This progress will go on in parallel with innovations in the technologies for direct, non invasive assessments of human atherosclerosis, thus allowing closer monitoring of this potential new approach to therapy.

Branched synthetic peptide constructs mimic cellular binding and efflux of apolipoprotein AI in reconstituted high density lipoproteins

This study investigates the suitability of the trimeric apolipoprotein (apo)AI(145-183) peptide that we recently described, to serve as a model to probe the relationship between apoAI structure and function. Three copies of the apoAI(145-183) unit, composed each of two amphipathic alpha-helical segments, were branched onto a covalent core matrix and the construct was recombined with phospholipids. A similar construct was made with the apoAI(102-140) peptide and used as a comparison with dimyristoylglycerophosphocholine (DMPC)-apoAI complexes. The DMPC-trimeric-apoAI(145-183) complexes had similar immunological reactivity with monoclonal antibodies directed against the 149-186 apoAI sequence (A44), suggesting that the A44 epitope is exposed similarly in both the synthetic peptide and the native apoAI complexes. The complexes generated with the trimeric-apoAI(145-183) bind specifically to HeLa cells with comparable affinity to the DMPC apoAI complexes; they are a good competitor for binding of apoAI to both HeLa cells and Fu5AH rat hepatoma cells; finally, these complexes promote cholesterol efflux from Fu5AH cells with an efficiency comparable with the apo AI/lipid complexes. To study LCAT activation by the trimeric apo AI(145-183) construct, complexes were prepared with dipalmitoylphosphatidylcholine (DPPC), cholesterol (C) and either the trimeric construct or apoAI. LCAT activation by the trimeric construct was much lower than by apo AI, possibly because the conformation of the trimeric 145-183 peptide in DPPC/C/peptide complexes does not mimic that of apoAI in the corresponding complexes. In comparison, the complexes generated with the multimeric apoAI(102-140) construct had a poor capacity to mimic the physico-chemical and biological properties of apoAI. The apoAI(102-140) construct had low affinity for lipid compared with the (145-183) construct. After association with lipids, it was a poor competitor of DMPC-apoAI complexes for cellular binding and had only limited capacity to promote cholesterol efflux. These results suggest trimeric constructs can serve as an appropriate models for apoAI, enabling further investigations and new experimental approaches to determine the structure-function relationship of apoAI.

Apolipoprotein A-I induced amyloidosis

Amyloidosis is characterized by extracellular deposits of protein fibrils with a high content of beta-sheets in secondary structure. The protein forms together with proteoglycans amyloid fibrils causing organ damage and serious morbidity. Intact apolipoprotein A-I (apoA-I) is an important protein in lipid metabolism regulating the synthesis and catabolism of high density lipoproteins (HDL). Usually, apoA-I is not associated with amyloidosis. However, four naturally occurring mutant forms of apoA-I are known so far resulting in amyloidosis. The most important feature of all variants is the very similar formation of N-terminal fragments which were found in the amyloid deposits (residues 1-83 to 1-94). The new insights in the understanding of the association of apoA-I with HDL, its metabolism, and its hypothesized structural findings may explain a common mechanism for the genesis of apoA-I induced amyloidosis. Here we summarized the specific features of all known amyloidogenic variants of apoA-I and speculate about its metabolic pathway, which may have general implications for the metabolism of apoA-I.

Conformation of apolipoprotein AI in reconstituted lipoprotein particles and particle-membrane interaction: effect of cholesterol

Discoidal recombinant high density lipoproteins (rHDL) of apolipoprotein AI (apoAI) and palmitoyloleoylphosphatidylcholine (POPC), with or without cholesterol, were prepared by cholate dialysis. By gel filtration, rHDL containing 2-4 (Lp2, Lp3 and Lp4) apoAI molecules/particle were obtained. The ApoAI conformation in these rHDL was investigated by tryptophan fluorescence, denaturation with guanidine HCl, and immunoreactivity with two monoclonal antibodies recognizing epitopes in the N-terminal and central domains. Data show that apoAI conformation is highly dependent on particle size as well as on cholesterol. The ability of rHDL to interact with lipid bilayer was studied by measuring leakage induction on POPC and POPC/cholesterol vesicles loaded with terbium/dipicolinic acid. Among the cholesterol-free rHDL, the most efficient ones were the smallest Lp2. Leakage induction on POPC vesicles is dramatically decreased by the presence of cholesterol in Lp2 and Lp3. All the rHDL, but specially those containing cholesterol, induced more leakage on the POPC/cholesterol than on the POPC vesicles. These results suggest that in small cholesterol-poor particles, apoAI could have a conformation determining a high affinity for membranes, which could facilitate cholesterol efflux. After cholesterol enrichment, a conformational change in apoAI could decrease the affinity for membranes allowing the lipoprotein release.

Fluorescence studies of exchangeable apolipoprotein-lipid interactions. Superficial association of apolipophorin III with lipoprotein surfaces

Apolipophorin III (apoLp-III) from the Sphinx moth, Manduca sexta, is an 18-kDa exchangeable apolipoprotein that reversibly associates with lipoprotein particles. In the absence of lipid, apoLp-III exists as an elongated bundle of five amphipathic alpha-helices. Upon lipid association, the protein is postulated to undergo a major conformational change, wherein the bundle opens around hinge loop regions, resulting in exposure of its hydrophobic interior. Fluorescence quenching techniques have been employed to study apoLp-III helix topography and spatial arrangement in phospholipid disc complexes and intact lipoprotein particles. Intrinsic fluorescence of the single tyrosine in apoLp-III was exploited to monitor the location of helix 5 in model disc complexes. To investigate other regions of the protein, site-directed mutagenesis was performed to introduce cysteine residues, replacing Asn-40 (helix 2, N40C) or Leu-90 (helix 3, L90C), thereby providing two mutant apoLp-IIIs, each with a single site for covalent attachment of the extrinsic fluorescent probe, N-(1-pyrene) maleimide. In the lipid-free state, pyrene-N40C- and pyrene-L90C-apoLp-III were highly accessible to the negatively charged aqueous quencher KI, yielding Ksv values of 27.1 and 19.8 M-1, respectively. Upon binding to the surface of a spherical lipoprotein particle, Ksv values for KI decreased by about 90% for both pyrene-labeled apoLp-IIIs, indicating a significant change in the local microenvironment of the fluorophores. A lesser decrease in Ksv was observed when the pyrene-labeled apoLp-IIIs were bound to phospholipid disc complexes. When spin-labeled fatty acids 5-doxylstearic acid and 12-doxylstearic acid were used as lipophilic quenchers, tyrosine and pyrene fluorescence were more effectively quenched by 5-doxylstearic acid in both phospholipid bilayer disc complexes and spherical lipoprotein particles. These data provide insight into the spatial topography of apoLp-III alpha-helices in phospholipid disc complexes and support the concept that interaction with spherical lipoprotein particles results in superficial contact of apoL-III helical segments with the monolayer surface, providing a basis for its reversible binding ability.

The effect of apolipoprotein A-II on the structure and function of apolipoprotein A-I in a homogeneous reconstituted high density lipoprotein particle

In this study we examined the effects of apoA-II on the structure and function of apoA-I in homogeneous reconstituted HDL (rHDL). First, we measured the binding of apoA-II to apoA-I-rHDL, containing dipalmitoylphosphatidylcholine or palmitoyloleoylphosphatidylcholine, and the degree of apoA-I displacement at various ratios of apolipoproteins. Using fluorescence methods, we determined that apoA-II binding is rapid, irreversible, and associated with apoA-I displacement only when the molar ratio of apoA-II/apoA-I is greater than 1:2. Next, we used the stable apoA-II/apoA-I-rHDL complex at the apoA-II/apoA-I ratio of 1:2 to examine its physical properties, apoA-I structure, and reactivity with lecithin:cholesterol acyltransferase (LCAT). Using chemical cross-linking in conjunction with fluorescence and electrophoretic methods, we demonstrated that the conformation of apoA-I must be flexible to allow apoA-II binding to the apoA-I-rHDL particles and showed that the hybrid particles have an unchanged Stokes diameter. Fluorescence and circular dichroism measurements revealed little or no change in the secondary structure or in the N-terminal domain of apoA-I, but showed a marked destabilization of apoA-I to denaturation by guanidine hydrochloride. Limited tryptic digestion indicated that the central region of apoA-I becomes accessible to proteolysis in the hybrid particles. Together, these results suggest that amphipathic alpha-helices of apoA-II replace four central helices of one apoA-I molecule (residues approximately 99-187) in the complex and in the process destabilize apoA-I. Thus, apoA-II binding at physiologic ratios may not completely displace apoA-I from HDL, but may provide a reservoir of easily exchangeable apoA-I. Finally, we showed that the reaction of the hybrid HDL with LCAT was inhibited 2-5-fold, relative to apoA-I-rHDL, due to a corresponding increase in the apparent Km value. This suggests that LCAT binding to the hybrid particles is sterically hindered by the excess protein (portions of apoA-I and apoA-II not bound to lipid). Therefore, apoA-II can modulate the reaction of HDL with LCAT by decreasing LCAT binding to hybrid particles and making the enzyme available for reaction with other substrates.

Activation of lecithin cholesterol acyltransferase by a disulfide-linked apolipoprotein A-I dimer

Apolipoprotein A-IMilano is a molecular variant of apoA-I, containing the Arg173-->Cys substitution that forms a disulfide linked homodimer (A-IM/A-IM). To assess the effect of this structural modification on a major function of the apolipoprotein, its activation of lecithin cholesterol acyltransferase (LCAT), we prepared well-defined complexes of A-IM/A-IM and apoA-I with phospholipids and cholesterol and compared their reactivities with LCAT. Particles with A-IM/A-IM had very similar diameters to apoA-I particles (7.8 and 12.5 nm) but had distinct apolipoprotein and phospholipid contents and protein secondary structures; they bound LCAT with comparable affinities, but were less efficient substrates for the enzyme (40 to 70% less reactive). We conclude that the local structural constraints in A-IM/A-IM do not prevent the formation of well-defined complexes with phospholipids and do not influence the binding of the enzyme to the particles, but have an inhibitory effect on LCAT activation.

Characterization of human apolipoprotein A-I expressed in Escherichia coli

Human apolipoprotein A-I (apoA-I), with an additional N-terminal extension (Met-Arg-Gly-Ser-(His)6-Met) (His-apoA-I), has been produced in Escherichia coli with a final yield after purification of 10 mg protein/1 of culture medium. We have characterized the conformation and structural properties of His-apoA-I in lipid-free form, and in reconstituted lipoproteins containing two apoA-I per particle (Lp2A-I) by both immunochemical and physicochemical techniques. The lipid-free forms of the two proteins present very similar secondary structure and stability, and have also very similar kinetics of association with dimyristoyl phosphatidylcholine. His-apoA-I and native apoA-I can be complexed with 1-palmitoyl-2-oleoyl phosphatidylcholine (POPC) to form similar, stable, either discoidal or spherical (sonicated) Lp2A-I particles. Lipid-bound native apoA-I and His-apoA-I showed very similar alpha-helical content (69% and 66%, respectively in discoidal Lp2A-I and 54% and 51%, respectively in spherical Lp2A-I). The conformation of His-apoA-I in lipid-free form and in discoidal or spherical Lp2A-I has also been shown to be similar to native apoA-I by immunochemical measurements using 13 monoclonal antibodies recognizing distinct apoA-I epitopes. In the free protein and in reconstituted Lp2A-I, the N-terminal has no effect on the affinity of any of the monoclonal antibodies and minimal effect on immunoreactivity values. Small differences in the exposure of some apoA-I epitopes are evident on discoidal particles, while no difference is apparent in the expression of any epitope of apoA-I on spherical Lp2A-I. The presence of the N-terminal extension also has no effect on the reaction of LCAT with the discoidal Lp2A-I or on the ability of complexes to promote cholesterol efflux from fibroblasts in culture. In conclusion, we show that His-apoA-I expressed in E. coli exhibits similar physicochemical properties to native apoA-I and is also identical to the native protein in its ability to interact with phospholipids and to promote cholesterol esterification and cellular cholesterol efflux.

Human apolipoprotein A-II inhibits the formation of pre-beta high density lipoproteins

The role of human apolipoprotein A-II (apoA-II) in the remodeling of human high density lipoproteins (HDL) was investigated during incubation of native and reduced-carboxamidomethylated (RCM) HDL3 with a lipoprotein-depleted plasma fraction (LPDP) in the presence of triglyceride-rich particles (TGRP) isolated from Intralipid. Reduction-carboxamidomethylation of HDL3 entirely converts the disulfide-linked apoA-II dimers into monomers, without affecting the structure, composition and particle size distribution of HDL3. Following incubation with LPDP and TGRP, unmodified HDL3 are mainly converted into large, HDL2 particles (diameter: 9.90 +/- 0.07 nm), enriched in triglycerides and depleted of cholesteryl esters. RCM-HDL3 are converted into both large HDL2 (9.86 +/- 0.07 nm) and small (7.53 +/- 0.06 nm) HDL3. The small products are protein-rich and cholesterol-poor, and consist of two different particles: a component with pre-beta mobility, containing only apoA-I, and a component with alpha mobility, containing both apoA-I and apoA-II. Kinetic studies suggest that a two-step process is involved in the formation of small, pre beta-HDL3, by which changes in lipid composition cause alterations in lipoprotein structure/stability, favoring the dissociation of apolipoproteins and reduction of particle size. These findings indicate that apolipoprotein structure is a major determinant of HDL remodeling, apoA-II potentially counteracting the anti-atherogenic properties of apoA-I by inhibiting the formation of small, pre-beta-migrating HDL.

Only the two end helixes of eight tandem amphipathic helical domains of human apo A-I have significant lipid affinity. Implications for HDL assembly

Human apolipoprotein A-I (apo A-I) possesses multiple tandem repeating 22-mer amphipathic alpha-helixes. Computer analysis and studies of model synthetic peptides and recombinant protein-lipid complexes of phospholipids have suggested that apo A-I interacts with HDL surface lipids through cooperation among its individual amphipathic helical domains. To delineate the overall lipid-associating properties of apo A-I, the first step is to understand the lipid-associating properties of individual amphipathic helical domains. To this end, we synthesized and studied each of the eight tandem repeating 22-mer domains of apo A-I: residues 44-65, 66-87, 99-120, 121-142, 143-164, 165-186, 187-208, and 220-241. Among the 22-mers, only the N- and C-terminal peptides (44-65 and 220-241) were effective in clarifying multilamellar vesicles (MLVs) of dimyristoylphosphatidylcholine (DMPC). These two peptides also exhibited the highest partition coefficient into 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylcholine liposomes, the highest exclusion pressure for penetration into an egg yolk phosphatidylcholine monolayer, and the greatest reduction in the enthalpy of the gel-to-liquid crystalline phase transition of DMPC MLVs. These results suggest that the strong, lipid-associating properties of apo A-I are localized to the N- and C-terminal amphipathic domains. Although each of the eight peptides studied has an amphipathic structure, models based on changes in residual effective amino acid hydrophobicity resulting from differing depths of helix penetration into the lipid are best able to explain the high lipid affinity possessed by the two terminal domains. Differential scanning calorimetry (DSC) studies showed that on a molar basis, apo A-I is about 10 times more effective than the most effective peptide analyzed in reducing the enthalpy of the gel-to-liquid crystalline phase transition of DMPC MLVs. Because previous proteolysis experiments coupled with the present DSC results suggest that the lipid-associating domains of apo A-I are distributed throughout the length of the 243 amino acid residues, we propose that the terminal amphipathic helical domains are involved in the initial binding of apo A-I to the lipid surface to form HDL particles, followed by cooperative binding of the middle six amphipathic helical domains, perhaps aided by salt-bridge formation between adjacent helixes arranged in an antiparallel orientation.

The molecular structure of apolipoprotein A-II modulates the capacity of HDL to promote cell cholesterol efflux

The influence of apolipoprotein A-II (apoA-II) molecular structure on the capacity of high density lipoproteins (HDL) to promote cellular cholesterol efflux was investigated in cultured mouse peritoneal macrophages (MPM). Conversion by reduction and carboxamidomethylation of the naturally occurring dimeric apoA-II to its monomeric form in both native or reconstituted HDL did not change apolipoprotein secondary structure and lipoprotein size/composition. All particles containing monomeric apoA-II, i.e., native HDL3 or reconstituted HDL with or without apoA-I, showed a higher ability to promote cholesterol efflux originating from plasma membrane and intracellular stores, compared to particles containing dimeric apoA-II. These findings indicate that apolipoprotein molecular structure is a major determinant of HDL capacity to promote cholesterol efflux from cells.

Apolipoprotein A-I conformation in reconstituted discoidal lipoproteins varying in phospholipid and cholesterol content

The effects of the size and cholesterol content on the conformation of apolipoprotein A-I (apoA-I) have been studied in reconstituted discoidal lipoproteins containing two apoA-I per particle (Lp2A-I). The immunoreactivity of a series of 13 epitopes distributed along the apoA-I sequence has been evaluated in Lp2A-I with a phospholipid/apoA-I molar ratio ranging from 31 to 156 and in Lp2A-I with constant phospholipids but varying in cholesterol content from 0 to 22 molecules. The results are compatible with a three domain structure in apoA-I in which the central domain is located between residues 99 and 143 and postulated to be a hinged domain that responds differentially to changes in phospholipid and cholesterol contents. Increasing the phospholipid content results in significant changes of epitope immunoreactivity throughout the N-terminal and central domains of apoA-I with fewer modifications in the C-terminal domain. In contrast, increasing Lp2A-I of two central epitopes, A11 (residues 99-132) and 5F6 (residues 118-148), and an extreme N-terminal epitope, 4H1 (residues 2-8). Interestingly, the effects of increasing cholesterol or phospholipids on these epitopes are opposite. This suggests a specific effect of cholesterol on the central domain tertiary structure between residues 99 and 143. Competition binding assays among pairs of antibodies binding to apoA-I on Lp2A-I are best explained by invoking inter- as well as intramolecular competitions. The specificity of the intermolecular competitions suggests an N to C termini arrangement of the two apoA-I molecules around the disc. Increasing the phospholipid content of Lp2A-I mainly increases the competitions between 3G10 and antibodies binding to most adjacent epitopes. Simultaneously as Lp2A-I enlarges, several of these antibodies also enhance the binding of 3G10. This has been interpreted as evidence of a structural rearrangement of apoA-I as a result of the size increase where the alpha-helix (residues 99-121) that contains the 3G10 epitope is increasingly interacting with lipids resulting in the enhanced expression of this epitope. The increasing interactions of apoA-I helices with lipids in the enlarging disc are compatible with previous reports of a greater apoA-I stability in the large discs. By contrast, cholesterol has limited but specific effects on antibody competitions and decreases the interaction of the N-terminal domain with the domain containing 3G10, either by direct cholesterol protein interaction or by modification of the lipid phase packing.