The epitopes of apolipoprotein A-I define distinct structural domains including a mobile middle region

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.

Lipid-free apoA-I structure - Origins of model diversity

Apolipoprotein A-I (apoA-I) is a prominent member of the exchangeable apolipoprotein class of proteins, capable of transitioning between lipid-bound and lipid-free states. It is the primary structural and functional protein of high density lipoprotein (HDL). Lipid-free apoA-I is critical to de novo HDL formation as it is the preferred substrate of the lipid transporter, ATP Binding Cassette Transporter A1 (ABCA1) Remaley et al. (2001) [1]. Lipid-free apoA-I is an important element in reverse cholesterol transport and comprehension of its structure is a core issue in our understanding of cholesterol metabolism. However, lipid-free apoA-I is highly conformationally dynamic making it a challenging subject for structural analysis. Over the past 20years there have been significant advances in overcoming the dynamic nature of lipid-free apoA-I, which have resulted in a multitude of proposed conformational models.

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.

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.

Binding characteristics of a panel of monoclonal antibodies against the ligand binding domain of the human LDLr

To obtain a panel of monoclonal antibodies (MAbs) to study the folding and conformation of the low density lipoprotein receptor (LDLr), we have generated hybridomas from LDLr-deficient mice that had been immunized with the extracellular domain of the human LDLr. The 12 MAbs were specific for the ligand binding domain of the LDLr, with individual MAbs recognizing epitopes in ligand binding repeats 1, 2, 3, 5, and 7. A subset of the MAbs failed to react with the LDLr when disulfide bonds were reduced, and one MAb, specific for an epitope that spans ligand binding repeats 1 and 2, recognized two conformational forms of the LDLr with different affinities. Antibodies specific for ligand binding repeats 3, 5, and 7 completely blocked the binding of LDL particles to the LDLr on cultured human fibroblasts, whereas MAbs with epitopes in ligand binding repeats 1 and 2 partially blocked the binding of LDL to the LDLr. These anti-LDLr MAbs will serve as useful probes for further analysis of LDLr conformation and LDLr-mediated lipoprotein binding.

Intracellular Lipidation of Newly Synthesized Apolipoprotein A-I in Primary Murine Hepatocytes

Hepatocytes, which are the main site of apolipoprotein (apo)A-I and ATP-binding cassette transporter A1 (ABCA1) expression, are also the main source of circulating high density lipoprotein. Here we have characterized the intracellular lipidation of newly synthesized apoA-I, in primary hepatocytes cultured with [3H]choline to label choline-phospholipids, low density lipoprotein-[3H]cholesterol to label the cell surface, or [3H]mevalonate to label de novo synthesized cholesterol. Phospholipidation of apoA-I is significant and most evident in endoplasmic reticulum (ER) and medial Golgi, both in the lumen and on the membrane fractions of the ER and medial Golgi. In the presence of cycloheximide, endogenous apoA-I is substantially phospholipidated intracellularly but acquires some additional lipid after export out of the cell. In cells labeled with low density lipoprotein-[3H]cholesterol, intracellular cholesterol lipidation of apoA-I is entirely absent, but the secreted apoA-I rapidly accumulates cholesterol after secretion from the cell in the media. On the other hand, de novo synthesized cholesterol can lipidate apoA-I intracellularly. We also showed the interaction between apoA-I and ABCA1 in ER and Golgi fractions. In hepatocytes lacking ABCA1, lipidation by low density lipoprotein-cholesterol was significantly reduced at the plasma membrane, phospholipidation and lipidation by de novo synthesized sterols were both reduced in Golgi compartments, whereas ER lipidation remained mostly unchanged. Therefore, the early lipidation in ER is ABCA1 independent, but in contrast, the lipidation of apoA-I in Golgi and at the plasma membrane requires ABCA1. Thus, we demonstrated that apoA-I phospholipidation starts early in the ER and is partially dependent on ABCA1, with the bulk of lipidation by phospholipids and cholesterol occurring in the Golgi and at the plasma membrane, respectively. Finally, we showed that the previously reported association of newly synthesized apoA-I and apoB (Zheng, H., Kiss, R. S., Franklin, V., Wang, M. D., Haidar, B., and Marcel, Y. L. (2005) J. Biol. Chem. 280, 21612-21621) occurs after secretion at the cell surface.

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.

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

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

The antiphospholipid syndrome and atherosclerosis: clue to pathogenesis

Regulation of blood vessels is intrinsically tied to inflammatory signaling. Recent research suggests that chronic inflammation is associated with atherosclerosis risk. The antiphospholipid syndrome is a prototypic autoimmune disease. Disturbance of blood vessel homeostasis in this disorder may increase risk for atherosclerosis by mechanisms that are direct (through antibody targeting of blood vessel-regulating proteins) or indirect (via inflammatory mechanisms that have recently been implicated in autoantibody-mediated thrombosis).

The lipidation by hepatocytes of human apolipoprotein A-I occurs by both ABCA1-dependent and -independent pathways

The pathways of hepatic intra- and peri-cellular lipidation of apolipoprotein A-I (apoA-I) were studied by infecting primary mouse hepatocytes from either apoA-I-deficient or ABCA1-deficient mice with a recombinant adenovirus expressing the human apoA-I (hapoA-I) cDNA (endo apoA-I) or incubating the hepatocytes with exogenously added hapoA-I (exo apoA-I) and examining the hapoA-I-containing lipoproteins formed. The cells, maintained in serum-free medium, were labeled with [(3)H]choline, and the cell medium was separated by fast protein liquid chromatography or immunoprecipitated to quantify labeled choline phospholipids specifically associated with hapoA-I. With the apoA-I-deficient hepatocytes, the high density lipoprotein fraction formed with endo apoA-I contained proportionally more phospholipids than that formed with exo apoA-I. However, the lipoprotein size and electrophoretic mobility and phospholipid profiles were similar for exo apoA-I and endo apoA-I. Taken together, these data demonstrate that a significant proportion of hapoA-I is secreted from hepatocytes in a phospholipidated state but that hapoA-I is also phospholipidated peri-cellularly. With primary hepatocytes from ABCA1-deficient mice, the expression and net secretion of adenoviral-generated endogenous apoA-I was unchanged compared with control mice, but (3)H-phospholipids associated with endo apoA-I and exo apoA-I decreased by 63 and 25%, respectively. The lipoprotein size and electrophoretic migration and their phospholipid profiles remained unchanged. In conclusion, we demonstrated that intracellular and peri-cellular lipidation of apoA-I represent distinct and additive pathways that may be regulated independently. Hepatocyte expression of ABCA1 is central to the lipidation of newly synthesized apoA-I but also contributes to the lipidation of exogenous apoA-I. However, a significant basal level of phospholipidation occurs in the absence of ABCA1.

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.

Structural and functional properties of apolipoprotein A-I mutants containing disulfide-linked cysteines at positions 124 or 232

Recombinant Cys mutants of apolipoprotein A-I (apoA-I) (A124C and A232C) have been prepared in disulfide-linked forms in order to assess the effects of unnatural covalent constraints on the folding of apoA-I in solution, its ability to bind lipids, form HDL-like particles, activate LCAT, and undergo structural adaptations to changing lipid contents. Both mutants, in dimer form, were shown to fold similarly to plasma apoA-I in solution, but had a slightly decreased alpha-helix content and no evidence of intermonomer interactions. All forms of the mutants bound to and disrupted dimyristoylphosphatidylcholine (DMPC) liposomes with similar kinetics and efficiency to plasma apoA-I, and formed reconstituted HDL (rHDL) particles with palmitoyloleoylphosphatidylcholine (POPC) in high yields at three different ratios of lipid/protein. While the monomeric mutants produced identical rHDL to plasma apoA-I, the disulfide-linked dimers had distinct particle distributions from each other and from native apoA-I. The A124C-dimer formed rHDL with diameters of 86 and 78 A, while the A232C-dimer predominantly formed 96 A rHDL. These particles, and particles containing plasma apoA-I (96 and 78 A), were purified prior to structural and functional analyses. The structural properties of particles with similar diameters were comparable, as were their reactivities with LCAT; however, their ability to undergo structural rearrangements differed. The larger rHDL particles (96 and 86 A) containing native apoA-I or A124C-dimer, rearranged into smaller 78 A particles, while the 96 A particles containing A232C-dimer were resistant to rearrangement and did not form 78 A particles. From the results, it is concluded that synthetic, random disulfide-linked dimers of apoA-I have many properties analogous to those of the naturally occurring Cys mutants, apoA-I-Milano and apoA-I-Paris, which are thought to have antiatherogenic effects in vivo. Also, the results have implications for current models of rHDL structure.

What causes the antiphospholipid syndrome?

The antiphospholipid syndrome (APS) is characterized by unpredictable, sporadic, thrombotic events. The cause of the thrombosis is probably multifactorial and may involve disparate effects of the autoantibodies associated with the syndrome, which are known to interfere with various protein regulators of hemostasis. An integrated theory of pathogenesis that accounts for the diversity of autoantibodies and their effects suggests that cellular inflammation or apoptosis within the vasculature may lead to oxidation or turnover in phospholipid membranes. Thus, normally cryptic, functionally important epitopes of phospholipid-binding proteins are subjected to increased exposure to immune surveillance.

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

The folding and organization of apolipoprotein A-I (apoA-I) in discoidal, high-density lipoprotein (HDL) complexes with phospholipids are not yet completely resolved. For about 20 years, it was generally accepted that the amphipathic helices of apoA-I lie parallel to the acyl chains of the phospholipids ("picket fence" model). However, based on the X-ray crystal structure of a large, lipid-free fragment of apoA-I, a "belt model" was recently proposed. In this model, the helices of two antiparallel apoA-I molecules are extended in a circular arrangement and lie perpendicular to the phospholipid acyl chains. To obtain conclusive information on the spatial organization of apoA-I in discoidal HDL, we engineered three separate cysteine mutants of apoA-I (D9C, A124C, A232C) for specific labeling with the fluorescence probes ALEXA-488 or ALEXA-546 (fluorescein and rhodamine derivatives). The labeled apoA-I was reconstituted into well-defined HDL complexes containing two molecules of protein and dipalmitoylphosphatidylcholine, and the complexes were used in three quantitative fluorescence resonance energy transfer (FRET) experiments to determine the distances between two specific sites in an HDL particle. Comparison of the distances measured by FRET (4.7-7.8 nm) with those predicted from the existing models indicated that neither the picket fence nor the belt model can account for the experimental results; rather, a hairpin folding of each apoA-I monomer with most helices perpendicular to the phospholipid acyl chains and a random head-to-tail and head-to-head arrangement of the two apoA-I molecules in the HDL particles are strongly suggested by the distance and lifetime data.

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.

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.

Apolipoprotein AI and transthyretin as components of amyloid fibrils in a kindred with apoAI Leu178His amyloidosis

We found a new C-terminal amyloidogenic variant of apolipoprotein AI (apoAI), Leu178His in a French kindred, associated with cardiac and larynx amyloidosis and skin lesions with onset during the fourth decade. This single-point mutation in exon 4 of the apoAI gene was detected by DNA sequencing of polymerase chain reaction amplified material and restriction fragment length polymorphism analysis in two siblings. Blood, larynx, and skin biopsies were available from one sibling. Anti-apoAI immunoblotting of isoelectric focusing of plasma showed a +1 alteration in the charge of the protein. Extraction of fibrils from the skin biopsy revealed both full-length and N-terminal fragments of apoAI and transthyretin (TTR). ApoAI and TTR co-localized in amyloid deposits as demonstrated by immunohistochemistry. The present report, together with the first recently described C-terminal amyloidogenic variant of apoAI, Arg173Pro, shows that amyloidogenicity of apoAI is not a feature exclusive to N-terminal variants. The most striking characteristic of amyloid fibrils in Leu178His is that wild-type TTR is co-localized with apoAI in the fibrils. We have previously determined that a fraction of plasma TTR circulates in plasma bound to high-density lipoprotein and that this interaction occurs through binding to apoAI. Therefore we hypothesize that nonmutated TTR might influence deposition of apoAI as amyloid.

Role of the kidney in regulating the metabolism of HDL in rabbits: evidence that iodination alters the catabolism of apolipoprotein A-I by the kidney

To evaluate the factors that regulate HDL catabolism in vivo, we have measured the clearance of human apoA-I from rabbit plasma by following the isotopic decay of (125)I-apoA-I and the clearance of unlabeled apoA-I using a radioimmunometric assay (RIA). We show that the clearance of unlabeled apoA-I is 3-fold slower than that of (125)I-apoA-I. The mass clearance of iodinated apoA-I, as determined by RIA, is superimposable with the isotopic clearance of (125)I-apoA-I. The data demonstrate that iodination of tyrosine residues alters the apoA-I molecule in a manner that promotes an accelerated catabolism. The clearance from rabbit plasma of unmodified apoA-I on HDL(3) and a reconstituted HDL particle (LpA-I) were very similar and about 3-4-fold slower than that for (125)I-apoA-I on the lipoproteins. Therefore, HDL turnover in the rabbit is much slower than that estimated from tracer kinetic studies. To determine the role of the kidney in HDL metabolism, the kinetics of unmodified apoA-I and LpA-I were reevaluated in animals after a unilateral nephrectomy. Removal of one kidney was associated with a 40-50% reduction in creatinine clearance rates and a 34% decrease in the clearance rate of unlabeled apoA-I and LpA-I particles. In contrast, the clearance of (125)I-labeled molecules was much less affected by the removal of a kidney; FCR for (125)I-LpA-I was reduced by <10%. The data show that the kidneys are responsible for most (70%) of the catabolism of apoA-I and HDL in vivo, while (125)I-labeled apoA-I and HDL are rapidly catabolized by different tissues. Thus, the kidney is the major site for HDL catabolism in vivo. Modification of tyrosine residues on apoA-I may increase its plasma clearance rate by enhancing extra-renal degradation pathways.

Cell cholesterol efflux to reconstituted high-density lipoproteins containing the apolipoprotein A-IMilano dimer

The apolipoprotein A-IMilano (apoA-IM) is a molecular variant of apoA-I characterized by the Arg(173)-->Cys substitution, resulting in the formation of homodimers A-IM/A-IM. The introduction of the interchain disulfide bridge in the A-IM dimer limits the apolipoprotein conformational flexibility and restricts HDL particle size heterogeneity, thus possibly affecting HDL function in lipid metabolism and atherosclerosis protection. To investigate whether the structural changes in A-IM/A-IM affect apoA-I capacity for cell cholesterol uptake, we tested the ability of four reconstituted HDL (rHDL), that contained either apoA-I or A-IM/A-IM, to remove cholesterol from Fu5AH hepatoma cells and cholesterol-loaded murine primary macrophages (MPM). As the HDL particle size is known to affect the rHDL capacity for cell cholesterol uptake, the reconstitution conditions were carefully selected to produce two sets of rHDL particles of small and large size (7.8 and 12.5 nm in diameter). The small A-IM/A-IM rHDL were more efficient than the corresponding apoA-I particles as acceptors of membrane cholesterol from Fu5AH cells and MPM, and as inhibitors of cholesterol esterification in MPM. The large rHDL and the lipid-free apolipoproteins displayed instead similar capacities for cell cholesterol efflux. These results suggest that cell cholesterol efflux to rHDL particles of different size occurs through different mechanisms. Large HDL accommodate and retain the cholesterol molecules that have desorbed from the cell membrane into the extracellular fluid, in a process that is less sensitive to protein conformation. Small HDL accelerate the desorption of cholesterol from the cell membrane, in a process that is influenced by the conformation of the proteins on the surface of the acceptor particle. The enhanced efficiency of small A-IM/A-IM rHDL seems related to the peculiar structure of the protein on the rHDL surface, with a hydrophobic C-terminal domain extending out of the rHDL particle, available for anchoring the acceptor to the plasma membrane.

Deletion of the C-terminal domain of apolipoprotein A-I impairs cell surface binding and lipid efflux in macrophage

The contribution of the amphipathic alpha-helices of apoA-I toward lipid efflux from human skin fibroblasts and macrophage was examined. Four apoA-I mutants were designed, each by deletion of a pair of predicted adjacent helices. Three mutants lacked two consecutive central alpha-helices [Delta(100-143), Delta(122-165), and Delta(144-186)], whereas the final mutant lacked the C-terminal domain [Delta(187-243)]. When compared to recombinant wild-type apoA-I and mutants with central domain deletions, Delta(187-243) exhibited a marked reduction in its ability to promote either cholesterol or phospholipid efflux from THP-1 macrophages. This mutant also demonstrated a decreased ability to bind lipids and to form lipoprotein complexes. In contrast, the four mutants and apoA-I equally supported cholesterol efflux from fibroblasts, albeit with a reduced capacity when compared to macrophages. Delta(187-243) bound poorly to the macrophage cell surface when compared to apoA-I, and competitive binding studies with the central domain and C-terminal deletions mutants showed that only Delta(187-243) did not compete effectively with [(125)I]apoA-I. Omission of PMA during cholesterol loading enhanced cholesterol efflux to both apoA-I (1.5-fold) and the C-terminal deletion mutant (2.5-fold). Inclusion of the Sandoz ACAT inhibitor (58-035) during loading and, in the absence of PMA, increased and equalized cholesterol efflux to apoA-I and Delta(187-243). Surprisingly, omission of PMA during cholesterol loading had minimal effects on the binding of apoA-I or Delta(187-243) to the THP-1 cell surface. Overall, these results show that cholesterol efflux from cells such as fibroblasts does not require any specific sequence between residues 100 and 243 of apoA-I. In contrast, optimal cholesterol efflux in macrophages requires binding of the C-terminal domain of apoA-I to a cell surface-binding site and the subsequent translocation of intracellular cholesterol to an efflux-competent pool.

Conformation of human apolipoprotein C-I in a lipid-mimetic environment determined by CD and NMR spectroscopy

The high-resolution conformation of human apoC-I in complexes with sodium dodecyl sulfate (SDS) is presented. As estimated from CD data, apoC-I adopts 54% helical secondary structure when bound to SDS, which is similar to the helical content previously found with phospholipids. The NMR-derived conformation of apoC-I is composed of two amphipathic helices, residues 7-29 and 38-52, separated by a flexible linker. The N-terminal helix contains a mobile hinge involving residues 12-15. The hydrophobic side chains cluster on the nonpolar face of both helices, thus forming two discrete lipid-binding sites in the N-terminal helix and one in the C-terminal helix. As suggested by amide proton resonance line widths and deuterium exchange rates, the N-terminal helix is more flexible and may bind less tightly to the detergent than the C-terminal helix. The different mobility of both helices appears to be related to side-chain composition, rather than length of the amphipathic helix, and may play a role in the function of apoC-I as an activator of lecithin:cholesterol acyltransferase (LCAT). A model is suggested in which the C-terminal helix serves as a lipid anchor while the N-terminal helix may hinge off the lipid surface to make specific contacts with LCAT.

Structural organization of the N-terminal domain of apolipoprotein A-I: studies of tryptophan mutants

Site-directed mutagenesis and detailed fluorescence studies were used to study the structure and dynamics of recombinant human proapolipoprotein (proapo) A-I in the lipid free state and in reconstituted high-density lipoprotein (rHDL) particles. Five different mutants of proapoA-I, each containing a single tryptophan residue, were produced in bacteria corresponding to each of the naturally occurring Trp residues (position -3 in the pro-segment, 8, 50, 72, and 108) in the N-terminal half of the protein. Structural analyses indicated that the conservative Phe-Trp substitutions did not perturb the conformation of the mutants with respect to the wild-type protein. Steady-state fluorescence studies indicated that all of the Trp residues exist in nonpolar environments that are highly protected from solvent in both the lipid-free and lipid-bound forms. Time-resolved lifetime and anisotropy studies indicated that the shape of the monomeric form of proapoA-I is a prolate ellipsoid with an axial ratio of about 6:1. In addition, the region surrounding Trp 108 appears to be more mobile than the rest of the protein in the lipid-free state. However, in rHDL particles, no significant domain motion was detected for any of the Trp residues. The results presented in this work are consistent with a model for monomeric lipid-free proapoA-I in which the N-terminal half of the molecule is organized into a bundle of helices.

Hepatitis C virus and other flaviviridae viruses enter cells via low density lipoprotein receptor

Endocytosis of the Flaviviridae viruses, hepatitis C virus, GB virus C/hepatitis G virus, and bovine viral diarrheal virus (BVDV) was shown to be mediated by low density lipoprotein (LDL) receptors on cultured cells by several lines of evidence: by the demonstration that endocytosis of these virus correlated with LDL receptor activity, by complete inhibition of detectable endocytosis by anti-LDL receptor antibody, by inhibition with anti-apolipoprotein E and -apolipoprotein B antibodies, by chemical methods abrogating lipoprotein/LDL receptor interactions, and by inhibition with the endocytosis inhibitor phenylarsine oxide. Confirmatory evidence was provided by the lack of detectable LDL receptor on cells known to be resistant to BVDV infection. Endocytosis via the LDL receptor was shown to be mediated by complexing of the virus to very low density lipoprotein or LDL but not high density lipoprotein. Studies using LDL receptor-deficient cells or a cytolytic BVDV system indicated that the LDL receptor may be the main but not exclusive means of cell entry of these viruses. Studies on other types of viruses indicated that this mechanism may not be exclusive to Flaviviridae but may be used by viruses that associate with lipoprotein in the blood. These findings provide evidence that the family of LDL receptors may serve as viral receptors.

The new apolipoprotein A-I variant leu(174) --> Ser causes hereditary cardiac amyloidosis, and the amyloid fibrils are constituted by the 93-residue N-terminal polypeptide

We identified a novel missense mutation in the apolipoprotein A-I gene, T2069C Leu(174) --> Ser, in a patient affected by familial systemic nonneuropathic amyloidosis. The amyloid deposits mostly affected the heart of the proband, who underwent transplantation for end-stage congestive heart failure. Amyloid fibrils of myocardial and periumbilical fat samples immunoreacted exclusively with anti-ApoA-I antibodies. Amyloid fibrils extracted from the heart were constituted, according to amino acid sequencing and mass spectrometry analysis, by an amino-terminal polypeptide ending at Val(93) of apolipoprotein A-I (apoA-I); no other significant fragments were detected. The mutation segregates with the disease; it was demonstrated in the proband and in an affected uncle and excluded in three healthy siblings. The plasma levels of high-density lipoprotein and apoA-I were significantly lower in the patient than in unaffected individuals. This represents the first case of familial apoA-I amyloidosis in which the mutation is outside the polypeptide fragment deposited as fibrils. Visualization of the mutation in the three-dimensional structure of lipid-free apoA-I, composed of four identical polypeptide chains, indicates that position 174 of one chain is located near position 93 of an adjacent chain and suggests that the amino acid replacement in position 174 is permissive for a proteolytic split at the C-terminal of Val(93).

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.

Frequency of antibodies to the cholesterol transport protein apolipoprotein A1 in patients with SLE

In examining reasons for premature atherosclerosis in systemic lupus erythematosus (SLE), we previously reported low levels of the cholesterol transport protein apolipoprotein A1 (apoA1) in these patients, and specific antibodies to purified apoA1 were identified in the sera of 5 out of 30 lupus patients. The current study was initiated to determine whether these antibodies are common in lupus patients. 520 serum samples from 175 patients with SLE or primary antiphospholipid syndrome (PAPS) were tested for antibodies to purified apoA1. Positive sera were retested for binding to apolipoprotein incorporated into reconstructed nascent or mature high-density lipoprotein (HDL). Autoantibodies to apoA1 were found in 32.5% of patients with SLE and 22.9% of patients with PAPS, associated with the presence of aPL (anti-beta2 glycoprotein-1, anti-beta2 GP1) antibodies. When reconstructed, nascent and mature HDL molecules were compared as antigen-containing environments, positive sera reacted best to apoA1 embedded in mature HDL molecules. This report confirms the high prevalence of antibodies to apoA1 in patients with systemic lupus and suggests a high affinity of these antibodies for mature HDL.

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

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

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.

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.

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

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

In vivo glucosylated LpA-I subfraction. Evidence for structural and functional alterations

This study compared the structural and functional properties of glucosylated and non-glucosylated LpA-I particle subfractions (GLpA-I and NGLpA-I, respectively) isolated from patients with poorly controlled type 1 (insulin-dependent) diabetes. Compared with NGLpA-I, GLpA-I showed an enrichment in triglycerides (P < .05) and a depletion in phospholipid (P < .05) content. Moreover, the triglycerides-to-cholesteryl esters ratio was increased (P < .05), suggesting an increased cholesteryl ester transfer protein activity and a possible transport defect that accelerates atherogenesis. The surface-to-core constituents ratio, an indirect estimate of particles size, is lower in GLpA-I (P < .01) than in NGLpA-I, correlating well with a larger median size (P < .05) as seen by electron microscopy. The apolipoprotein (apo) A-I conformation was evaluated through determination of the immunological accessibility of three different domains defining specific epitopes for anti-apo A-I monoclonal antibodies. We observed a marked decreased accessibility for two of these regions, which interestingly have already been implicated in the interaction with cells. Cell culture data suggest that nonenzymatic glycosylation occurring on apo A-I can modify lipoprotein function, since it results in a decreased binding of GLpA-I to HeLa cells and impaired cholesterol efflux from Fu5AH rat hepatoma cells.

Reconstituted high-density lipoproteins with a disulfide-linked apolipoprotein A-I dimer: evidence for restricted particle size heterogeneity

The apolipoprotein A-IMilano (apoA-IM) is a molecular variant of apoA-I characterized by the Arg173-->Cys substitution, resulting in the formation of homodimers (A-IM/A-IM) and heterodimers with apoA-II. In order to examine the effects of the introduction of an interchain disulfide bridge on the lipid-binding properties of apoA-I, the present studies compare the kinetics of association of A-IM/A-IM and apoA-I with dimyristoylphosphatidylcholine (DMPC), and the structure and properties of reconstituted HDL containing palmitoyloleoylphosphatidylcholine (POPC) and either A-IM/A-IM or apoA-I. The results show that apoA-I dimerization does not affect the rate of association with DMPC. Apolipoprotein-POPC complexes instead, when analyzed by nondenaturing gradient gel electrophoresis, demonstrate that, differently from apoA-I, A-IM/A-IM forms only two species of rHDL particles despite a wide range of initial lipid to protein ratios. These two rHDL species contain one or two A-IM/A-IM molecules and have a diameter of 7.8 nm and 12.5 nm. Investigations of the A-IM/A-IM structure in these two rHDL, by circular dichroism, fluorescence, and second-derivative UV spectroscopy, suggest that the secondary and tertiary structures of A-IM/A-IM are remarkably similar in both small and large particles. These results suggest that the introduction of an interchain disulfide bridge does not affect the association of apoA-I with lipids but restricts HDL particle size heterogeneity, thus possibly affecting HDL function in lipid metabolism and atherosclerosis protection.

Deletion of central alpha-helices in human apolipoprotein A-I: effect on phospholipid association

In order to better understand the structure-function properties of apolipoprotein (apo) A-I, we have constructed and expressed three apoA-I mutants using a system previously described for the expression of human apolipoprotein A-I (Rec.-apoA-I). These mutants (corresponding to deletion of apoA-I residues 100-143, 122-165, 144-186) have been studied for their ability to form reconstituted apoA-I-containing lipoproteins (LpA-I) with POPC and DMPC, and for their structural and physical properties. Rec.- and native apoA-I can form homogeneous discoidal Lp2A-I over a wide range of POPC/apoA-I ratios [(20-130)/1] and exhibit sizes ranging from 9.5 to 10.5 nm. When recombined with varying POPC content [(20-130)/1, POPC/A-I)], the three mutants produce homogeneous discoidal Lp2A-I that contain a low POPC/A-I molar ratio [(20-40)/l for all mutants] and exhibit a nearly constant size [7.5-7.6 nm for delta (100-143) and 7.9-8.0 nm for the other two mutants]. Kinetics of association of these proteins with DMPC are similar for delta (100-143) and Rec.-apoA-I (t 1/2 of 4.0 and 4.4 min, respectively) but appear significantly reduced for delta (122-165) and delta (144-186) (t 1/2 of 7.5 and 6.9 min, respectively). While in the lipid-free form, all proteins have a similar thermodynamic stability with a very comparable free energy of unfolding (delta GD degree) for the alpha-helical structure, as determined by isothermal denaturation studies. delta-(100-143) has a significantly lower alpha-helical content (33%) as compared to the other proteins [40, 41, and 45% for Rec.-apoA-I. delta (122-165), and delta (144-186), respectively]. When associated to POPC, delta (122-165) and delta (144-186) have a higher alpha-helicity (63 and 63%) and an enhanced stability (2.5 and 2.3 kcal/mol, respectively) as compared to delta (100-143) (49% and 1.8 kcal/mol) and Rec.-apoA-I (52% and 1.9 kcal/mol). These results suggest that the amphipathic alpha-helices within residues 100-186 are directly involved in interactions with phospholipids. The helical region 100-121 appears to be more important to the stabilization of the lipid-apoprotein complex formed whereas helices within residues 122-186 appear to be critical to the initial rates of association of the apoprotein with DMPC. These data suggest that an important role of the central domain 100-186 may be to maintain the plasticity of apoA-I and its ability to form different classes of HDL particles. Therefore, it is likely that this region may also play an important role in the functional properties of this apoprotein.

Truncation of the amino terminus of human apolipoprotein A-I substantially alters only the lipid-free conformation

An amino-terminal deletion mutant (residues 1-43) of human apolipoprotein A-I (apo hA-I) has been produced from a bacterial expression system to explore the structural and functional role of these amino acids, encoded by exon 3, in apo hA-I. Lipid binding of apo delta (1-43)A-I and lipid binding of apo hA-I are very similar as assessed by surface activity, lipid association with palmitoyloleoylphosphatidylcholine (POPC) vesicles, and lipid association with plasma lipoproteins. Preliminary kinetic measurements appear to show that the reactivity of lecithin:cholesterol acyltransferase (LCAT) with the mutant is slightly decreased compared to wild-type apo hA-I. Collectively, these results indicate that the N-terminal region is not necessary for lipid binding or activation of LCAT. In contrast, there are significant structural differences between lipid-free apo delta (1-43)A-I and apo hA-I, as judged by denaturant-induced unfolding, binding of the fluorescent probe 1-anilinonaphthalene-8-sulfonate, surface balance measurements, and far- and near-ultraviolet circular dichroic spectroscopy. All spectral and physical measurements indicate apo delta (1-43)A-I has a folded, tertiary structure, although it is significantly less stable than that of apo hA-I. It is concluded that the N-terminal 43 residues are an important structural element of the lipid-free conformational state of apo hA-I, the absence of which induces a fundamentally different fold for the remaining carboxy-terminal residues, compared to those in native apo hA-I.

The role of apolipoprotein AI domains in lipid binding

Apolipoprotein AI (apoAI) is the principal protein constituent of high density lipoproteins and it plays a key role in human cholesterol homeostasis; however, the structure of apoAI is not clearly understood. To test the hypothesis that apoAI is organized into domains, three deletion mutants of human apo AI expressed in Escherichia coli were studied in solution and in reconstituted high density lipoprotein particles. Each mutant lacked one of three specific regions that together encompass almost the entire 243 aa sequence of native apoAI (apoAI delta 44-126, apoAI delta 139-170, and apoAI delta 190-243). Circular dichroism spectroscopy showed that the alpha-helical content of lipid-free apoAI delta 44-126 was 27% while the other mutants and native apoAI averaged 55 +/- 2%, suggesting that the missing N-terminal portion contains most of the alpha-helical structure of lipid-free apoAI. ApoAI delta 44-126 exhibited the largest increase in alpha-helix upon lipid binding (125% increase versus an average of 25% for the others), confirming the importance of the C-terminal half of apoAI in lipid binding. Denaturation studies showed that the N-terminal half of apoAI is primarily responsible for alpha-helix stability in the lipid-free state, whereas the C terminus is required for alpha-helix stability when lipid-bound. We conclude that the N-terminal half (aa 44-126) of apoAI is responsible for most of the alpha-helical structure and the marginal stability of lipid-free apoAI while the C terminus (aa 139-243) is less organized. The increase in alpha-helical content observed when native apoAI binds lipid results from the formation of alpha-helix primarily in the C-terminal half of the molecule.

Conformation of human serum apolipoprotein A-I(166-185) in the presence of sodium dodecyl sulfate or dodecylphosphocholine by 1H-NMR and CD. Evidence for specific peptide-SDS interactions

The segment, YSDELRQRLAARLEALKENG, corresponding to residues 166 to 185 of human serum apolipoprotein A-I, was studied by circular dichroism and NMR spectroscopy in sodium dodecyl sulfate and dodecylphosphocholine micelles. 2-Dimensional NOESY, TOCSY and DQF-COSY spectra of apoA-I(166-185) in perdeuterated sodium dodecyl sulfate (SDS-d25) and dodecylphosphocholine (DPC-d38) micelles were collected at a peptide/SDS (DPC) ratio of 1:40. Similar CD spectra and NOE connectivity patterns were observed for apoA-I(166-185) in SDS and DPC, indicating a similar helical conformation in both. Conformations of apoA-I(166-185) in DPC-d38 micelles, and in SDS-d25 micelles at two pH values, 6.6 and 3.7, were determined using distance geometry calculations. Backbone superposition (N,C alpha,C = O) for an ensemble of twenty-nine structures in DPC at pH 6.0 gave a RMSD of 0.45 +/- 0.09 A for the region D168 to K182, while for all atoms it was 1.60 +/- 0.17 A. In SDS, the ensemble of nineteen structures each at pH 6.6 and 3.7 gave RMSDs of 0.28 +/- 0.07 A and 0.35 +/- 0.10 A, respectively, for the region D168 to K182. RMSD for superposition of all atoms was 1.36 +/- 0.10 A and 1.38 +/- 0.21 A at the respective pH values. In all cases a highly defined class A amphipathic helical structure was found for the region R171 to K182. Since the same structure occurs in micelles with either negatively charged or zwitterionic head groups it strongly suggests a dominant role for hydrophobic interactions in stabilizing the complex. The Y166 aromatic ring is bent back upon the helix axis at the lower pH. NMR determination of pKa values for D168, E169, E179 and E183 in the presence of SDS or DPC indicated a micro-pH at the micellar surface approximately one pH unit higher than the normal residue pKa. SDS interactions with the peptide were examined by collecting 1H NOESY spectra in the presence of protiated SDS. Residues R171, R173, R177, as well as the aromatic ring of Y166, were shown by intermolecular NOE measurements to interact with SDS, hence a key interaction in stabilizing the complex appears to be between interfacial basic side-chains and SDS alkyl chains.

Serum albumin is a significant intermediate in cholesterol transfer between cells and lipoproteins

The function of albumin in the movement of cholesterol into and out of non-cholesterol-loaded fibroblasts has been investigated. Cholesterol efflux from cholesterol labeled normal human skin fibroblasts to fatty acid-free human serum albumin (HSA) is biphasic with a rapid first phase that plateaus at about 15 min followed by a nearly linear phase up to 90 min, the longest incubation in this study. Saturation of efflux is observed at about 10 mg of albumin/mL. Efflux is specific to albumin since other molecules, such as ovalbumin or gelatin, do not induce efflux. The ability of HSA to induce cellular cholesterol efflux is low compared to reconstituted discoidal lipoprotein A-I (LpA-I). HSA at 2 mg/mL produces a rate of cholesterol efflux similar to that of LpA-I at 45 micrograms of protein/mL; however, these concentrations are within the physiological range for both HSA and apolipoprotein A-I (apoA-I). The efflux to the medium containing both LpA-I and HSA is greater than that to each of them alone but does not show complete additivity, indicating a competition between HSA and LpA-I. The HSA-mediated cholesterol movement is bidirectional as demonstrated by the transfer of cholesterol from HSA-(3H)- cholesterol complexes to fibroblasts; moreover, the HSA-mediated transfer is much faster than that from cholesterol-containing LpA-I (0.8 versus 0.2 pmol (micrograms of cell protein)-1 (90 min)-1. However, the presence of either low-density lipoprotein (LDL) or LpA-I in the incubation medium significantly inhibits the transfer of cholesterol from HSA-(3H)-cholesterol complexes to fibroblasts, thus allowing the bidirectional transfer of cholesterol between HSA and cells to possibly operate as a net efflux. In conclusion, albumin plays a significant role in cholesterol transfer between cells and lipoproteins.