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

Structural diversity of ex vivo amyloid fibrils studied by cryo-electron microscopy

Cryo-electron microscopy studies are presented on amyloid fibrils isolated from amyloidotic organs of two patients with different forms of hereditary non-neuropathic systemic amyloidosis, caused, respectively, by Leu60Arg apolipoprotein AI and Asp67His lysozyme. Although ex vivo amyloid fibrils were thought to be more uniform in structure than those assembled in vitro, our findings show that these fibrils are also quite variable in structure. Structural disorder and variability of the fibrils have precluded three-dimensional reconstruction, but averaged cryo-electron microscopy images suggest models for protofilament packing in the lysozyme fibrils. We conclude that ex vivo amyloid fibrils, although variable, assemble as characteristic structures according to the identity of the precursor protein.

Apolipoprotein A-I promotes the formation of phosphatidylcholine core aldehydes that are hydrolyzed by paraoxonase (PON-1) during high density lipoprotein oxidation with a peroxynitrite donor

High density lipoprotein (HDL) is rich in polyunsaturated phospholipids that are sensitive to oxidation. However, the effect of apolipoprotein A-I and paraoxonase-1 (PON-1) on phosphatidylcholine oxidation products has not been identified. We subjected native HDL, trypsinized HDL, and HDL lipid suspensions to oxidation by the peroxynitrite donor, 3-morpholinosydnonimine. HDL had a basal level of phosphatidylcholine mono- and di-hydroperoxides that increased to a greater extent in HDL, compared with either trypsinized HDL or HDL lipid alone. Phosphatidylcholine core aldehydes, which were present in small amounts, increased 10-fold during oxidation of native HDL, compared with trypsinized HDL (p = 0.004), and 4-fold compared with HDL lipid suspensions (p = 0.0021). In addition, the content of lysophosphatidylcholine increased 300% during oxidation of native HDL, but only 80 and 25%, respectively, during oxidation of trypsinized HDL and HDL lipid suspensions. Phosphatidylcholine isoprostanes accumulated in comparable amounts during the oxidation of all three preparations. Incubation of apolipoprotein A-I with 1-palmitoyl-2-linoleoyl glycerophosphocholine proteoliposomes in the presence of 3-morpholinosydnonimine or apoAI with phosphatidylcholine hydroperoxides resulted in a significant increase in phosphatidylcholine core aldehydes with no formation of lysophosphatidylcholine. We propose that apolipoprotein A-I catalyzes a one-electron oxidation of alkoxyl radicals. Purified PON-1 hydrolyzed phosphatidylcholine core aldehydes to lysophosphatidylcholine. We conclude that, upon HDL oxidation with peroxynitrite, apolipoprotein AI increases the formation of phosphatidylcholine core aldehydes that are subsequently hydrolyzed by PON1.

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.

Functional similarities of human and chicken apolipoprotein A-I: dependence on secondary and tertiary rather than primary structure

To investigate the sequence requirements for apolipoprotein (apo) AI functions, comparisons of human and chicken apoAI were performed. In lipid binding assays, chicken apoAI was capable of transforming phospholipid vesicles into discoidal bilayer structures, similar in both size and apolipoprotein content to those produced with human apoAI under the same conditions. Human and chicken apoAI were indistinguishable in their relative abilities to prevent phospholipase C-induced aggregation of human low density lipoprotein. This activity, which is dependent upon formation of a stable interaction with the modified lipoprotein, represents a sensitive measure of apolipoprotein association with spherical lipoprotein particles. The ability of chicken versus human apoAI to mobilize the regulatory pool of cholesterol available for esterification by acyl-CoA:cholesterol acyltransferase by human fibroblasts was also assessed. Lipid-free chicken and human apoAI were equivalent in their ability to deplete cholesterol from this pool, as were intact chicken high density lipoprotein (HDL) and human HDL(3). Based on the overall sequence identity of chicken and human apoAI (48%), and comparison of regions thought to be responsible for key apoAI functions, these data indicate that amphipathic alpha-helical structure, rather than specific amino acid sequence, is the major determinant of apoAI lipid binding and ability to mobilize the regulatory pool of cellular cholesterol.

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.

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.

Amyloid fibrils derived from the apolipoprotein A1 Leu174Ser variant contain elements of ordered helical structure

We recently described a new apolipoprotein A1 variant presenting a Leu174Ser replacement mutation that is associated with a familial form of systemic amyloidosis displaying predominant heart involvement. We have now identified a second unrelated patient with very similar clinical presentation and carrying the identical apolipoprotein A1 mutation. In this new patient the main protein constituent of the amyloid fibrils is the polypeptide derived from the first 93 residues of the protein, the identical fragment to that found in the patient previously described to carry this mutation. The X-ray fiber diffraction pattern obtained from preparations of partially aligned fibrils displays the cross-beta reflections characteristic of all amyloid fibrils. In addition to these cross-beta reflections, other reflections suggest the presence of well-defined coiled-coil helical structure arranged with a defined orientation within the fibrils. In both cases the fibrils contain a trace amount of full-length apolipoprotein A1 with an apparent prevalence of the wild-type species over the variant protein. We have found a ratio of full-length wild-type to mutant protein in plasma HDL of three to one. The polypeptide 1--93 purified from natural fibrils can be solubilized in aqueous solutions containing denaturants, and after removal of denaturants it acquires a monomeric state that, based on CD and NMR studies, has a predominantly random coil structure. The addition of phospholipids to the monomeric form induces the formation of some helical structure, thought most likely to occur at the C-terminal end of the polypeptide.

Structural determination of lipid-bound ApoA-I using fluorescence resonance energy transfer

Based on the x-ray crystal structure of lipid-free Delta43 apoA-I, two monomers of apoA-I were suggested to bind to a phospholipid bilayer in an antiparallel paired dimer, or "belt orientation." This hypothesis challenges the currently held model in which each of the two apoA-I monomers fold as antiparallel alpha-helices or "picket fence orientation." When apoA-I is bound to a phospholipid disc, the first model predicts that the glutamine at position 132 on one apoA-I molecule lies within 16 A of glutamine 132 in the second monomer, whereas, the second model predicts glutamines at position 132 to be 104 A apart. To distinguish between these models, glutamine at position 132 was mutated to cysteine in wild-type apoA-I to produce Q132C apoA-I, which were labeled with thiol-reactive fluorescent probes. Q132C apoA-I was labeled with either fluorescein (donor probe) or tetramethylrhodamine (acceptor probe) and then used to make recombinant phospholipid discs (recombinant high density lipoprotein (rHDL)). The rHDL containing donor- and acceptor-labeled Q132C apoA-I were of similar size, composition, and lecithin:cholesterol acyltransferase reactivity when compared to rHDL-containing human plasma apoA-I. Analysis of donor probe fluorescence showed highly efficient quenching in rHDL containing one donor- and one acceptor-labeled Q132C apoA-I. rHDL containing only acceptor probe-labeled Q132C apoA-I showed rhodamine self-quenching. Both of these observations demonstrate that position 132 in two lipid-bound apoA-I monomers were in close proximity, supporting the "belt conformation" hypothesis for apoA-I on rHDL.

Structure of low density lipoprotein (LDL) particles: basis for understanding molecular changes in modified LDL

Low density lipoprotein (LDL) particles are the major cholesterol carriers in circulation and their physiological function is to carry cholesterol to the cells. In the process of atherogenesis these particles are modified and they accumulate in the arterial wall. Although the composition and overall structure of the LDL particles is well known, the fundamental molecular interactions and their impact on the structure of LDL particles are not well understood. Here, the existing pieces of structural information on LDL particles are combined with computer models of the individual molecular components to give a detailed structural model and visualization of the particles. Strong evidence is presented in favor of interactions between LDL lipid constituents that lead to specific domain formation in the particles. A new three-layer model, which divides the LDL particle into outer surface, interfacial layer, and core, and which is capable of explaining some seemingly contradictory interpretations of molecular interactions in LDL particles, is also presented. A new molecular interaction model for the beta-sheet structure and phosphatidylcholine headgroups is introduced and an overall view of the tertiary structure of apolipoprotein B-100 in the LDL particles is presented. This structural information is also utilized to understand and explain the molecular characteristics and interactions of modified, atherogenic LDL particles.

Molecular belt models for the apolipoprotein A-I Paris and Milano mutations

Models for the binding of the 200-residue carboxy-terminal domain of two mutants of apolipoprotein A-I (apo A-I), apo A-I(R173C)(Milano) and apo A-I(R151C)(Paris), to lipid in discoidal high-density lipoprotein (HDL) particles are presented. In both models, two monomers of the mutant apo A-I molecule bind to lipid in an antiparallel manner, with the long axes of their helical repeats running perpendicular to the normal of the lipid bilayer to form a single disulfide-linked homodimer. The overall structures of the models of these two mutants are very similar, differing only in helix-helix registration. Thus these models are consistent with experimental observations that reconstituted HDL particles containing apo A-I(Milano) and apo A-I(Paris) are very similar in diameter to reconstituted HDL particles containing wild-type apo A-I, and they support the belief that apo A-I binds to lipid in discoidal HDL particles via the belt conformation.

Detailed molecular model of apolipoprotein A-I on the surface of high-density lipoproteins and its functional implications

The major apolipoprotein (apo) A-I containing lipoprotein, high- density lipoprotein, is a negative risk factor for cardiovascular disease. An atomic resolution molecular model for lipid-associated apo A-I was recently proposed in which two apo A-I molecules are wrapped beltwise around a small discoidal patch of phospholipid bilayer. Because of its detailed predictions of lipid-associated apo A-I structure, this molecular belt model, if confirmed, provides a blueprint for understanding the molecular mechanisms of reverse cholesterol transport, and thus for the rational design of new classes of drugs for reversal of atherosclerosis and cardiovascular disease. The details and implications of the model are currently being explored by site-directed mutagenesis.

The protofilament substructure of amyloid fibrils

Tissue deposition of normally soluble proteins, or their fragments, as insoluble amyloid fibrils causes the usually fatal, acquired and hereditary systemic amyloidoses and is associated with the pathology of Alzheimer's disease, type 2 diabetes and the transmissible spongiform encephalopathies. Although each type of amyloidosis is characterised by a specific amyloid fibril protein, the deposits share pathognomonic histochemical properties and the structural morphology of all amyloid fibrils is very similar. We have previously demonstrated that transthyretin amyloid fibrils contain four constituent protofilaments packed in a square array. Here, we have used cross-correlation techniques to average electron microscopy images of multiple cross-sections in order to reconstruct the sub-structure of ex vivo amyloid fibrils composed of amyloid A protein, monoclonal immunoglobulin lambda light chain, Leu60Arg variant apolipoprotein AI, and Asp67His variant lysozyme, as well as synthetic fibrils derived from a ten-residue peptide corresponding to the A-strand of transthyretin. All the fibrils had an electron-lucent core but the packing arrangement comprised five or six protofilaments rather than four. The structural similarity that defines amyloid fibres thus exists principally at the level of beta-sheet folding of the polypeptides within the protofilament, while the different types vary in the supramolecular assembly of their protofilaments.

Binding of high density lipoprotein (HDL) and discoidal reconstituted HDL to the HDL receptor scavenger receptor class B type I. Effect of lipid association and APOA-I mutations on receptor binding

The binding of apoA-I-containing ligands to the HDL receptor scavenger receptor class B type I (SR-BI) was characterized using two different assays. The first employed conventional binding or competition assays with (125)I-labeled ligands. The second is a new nonradioactive ligand binding assay, in which the receptor-associated ligand is detected by quantitative immunoblotting ("immunoreceptor assay"). Using both methods, we observed that the K(d) value for spherical HDL (density = 1.1-1.13 g/ml) was approximately 16 microgram of protein/ml, while the values for discoidal reconstituted HDL (rHDL) containing proapoA-I or plasma apoA-I were substantially lower (approximately 0.4-5 microgram of protein/ml). We also observed reduced affinity and/or competition for spherical (125)I-HDL cell association by higher relative to lower density HDL and very poor competition by lipid-free apoA-I and pre-beta-1 HDL. Deletion of either 58 carboxyl-terminal or 59 amino-terminal residues from apoA-I, relative to full-length control apoA-I, resulted in little or no change in the affinity of corresponding rHDL particles. However, rHDL particles containing a double mutant lacking both terminal domains competed poorly with spherical (125)I-HDL for binding to SR-BI. These findings suggest an important role for apoA-I and its conformation/organization within particles in mediating HDL binding to SR-BI and indicate that the NH(2) and COOH termini of apoA-I directly or indirectly contribute independently to binding to SR-BI.

Oxidation of methionine residues to methionine sulfoxides does not decrease potential antiatherogenic properties of apolipoprotein A-I

The initial stage of oxidation of high density lipoproteins (HDL) is accompanied by the lipid hydroperoxide-dependent, selective oxidation of two of the three Met residues of apolipoprotein A-I (apoA-I) to Met sulfoxides (Met(O)). Formation of such selectively oxidized apoA-I (i.e. apoA-I(+32)) may affect the antiatherogenic properties of HDL, because it has been suggested that Met(86) and Met(112) are important for cholesterol efflux and Met(148) is involved in the activation of lecithin:cholesterol acyl transferase (LCAT). We therefore determined which Met residues were oxidized in apoA-I(+32) and how such oxidation of apoA-I affects its secondary structure, the affinity for lipids, and its ability to remove lipids from human macrophages. We also assessed the capacity of discoidal reconstituted HDL containing apoA-I(+32) to act as substrate for LCAT, and the dissociation of apoA-I and apoA-I(+32) from reconstituted HDL. Met(86) and Met(112) were present as Met(O), as determined by amino acid sequencing and mass spectrometry of isolated peptides derived from apoA-I(+32). Selective oxidation did not alter the alpha-helicity of lipid-free and lipid-associated apoA-I as assessed by circular dichroism, and the affinity for LCAT was comparable for reconstituted HDL containing apoA-I or apoA-I(+32). Cholesteryl ester transfer protein mediated the dissociation of apoA-I more readily from reconstituted HDL containing apoA-I(+32) than unoxidized apoA-I. Also, compared with native apoA-I, apoA-I(+32) had a 2- to 3-fold greater affinity for lipid (as determined by the rate of clearance of multilamellar phospholipid vesicles) and its ability to cause efflux of [(3)H]cholesterol, [(3)H]phospholipid, and [(14)C]alpha-tocopherol from lipid-laden human monocyte-derived macrophages was significantly enhanced. By contrast, no difference was observed for cholesterol and alpha-tocopherol efflux to lipid-associated apolipoproteins. Together, these results suggest that selective oxidation of Met residues enhances rather than diminishes known antiatherogenic activities of apoA-I, consistent with the overall hypothesis that detoxification of lipid hydroperoxides by HDL is potentially antiatherogenic.

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.

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.

Structure and function of apolipoprotein A-I and high-density lipoprotein

Structural biology and molecular modeling have provided intriguing insights into the atomic details of the lipid-associated structure of the major protein component of HDL, apo A-I. For the first time, an atomic resolution map is available for future studies of the molecular interactions of HDL in such biological processes as ABC1-regulated HDL assembly, LCAT activation, receptor binding, reverse lipid transport and HDL heterogeneity. Within the context of this paradigm, the current review summarizes the state of HDL research.

Crystal structure of the vinculin tail suggests a pathway for activation

Vinculin plays a dynamic role in the assembly of the actin cytoskeleton. A strong interaction between its head and tail domains that regulates binding to other cytoskeletal components is disrupted by acidic phospholipids. Here, we present the crystal structure of the vinculin tail, residues 879-1066. Five amphipathic helices form an antiparallel bundle that resembles exchangeable apolipoproteins. A C-terminal arm wraps across the base of the bundle and emerges as a hydrophobic hairpin surrounded by a collar of basic residues, adjacent to the N terminus. We show that the C-terminal arm is required for binding to acidic phospholipids but not to actin, and that binding either ligand induces conformational changes that may represent the first step in activation.

A detailed molecular belt model for apolipoprotein A-I in discoidal high density lipoprotein

Apolipoprotein A-I (apoA-I) is the principal protein of high density lipoprotein particles (HDL). ApoA-I contains a globular N-terminal domain (residues 1-43) and a lipid-binding C-terminal domain (residues 44-243). Here we propose a detailed model for the smallest discoidal HDL, consisting of two apoA-I molecules wrapped beltwise around a small patch of bilayer containing 160 lipid molecules. The C-terminal domain of each monomer is ringlike, a curved, planar amphipathic alpha helix with an average of 3.67 residues per turn, and with the hydrophobic surface curved toward the lipids. We have explored all possible geometries for forming the dimer of stacked rings, subject to the hypothesis that the optimal geometry will maximize intermolecular salt bridge interactions. The resulting model is an antiparallel arrangement with an alignment matching that of the (nonplanar) crystal structure of lipid-free apoA-I.

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.

Lipoprotein A-I structure

High density lipoproteins are produced by the liver as protein-lipid complexes with a characteristic discoidal shape. A crystal structure is available for the chief protein component of these complexes, apolipoprotein A-I, but controversy about how this protein is situated with respect to the lipid components has flourished for lack of experimental techniques that can characterize protein structure in a lipid environment. New spectroscopic techniques developed to address this problem now indicate that apolipoprotein A-I is arranged as a helical belt around a bilayer of phospholipids. This is an important step towards understanding how these lipoproteins regulate cholesterol transport.

Probing the conformation of a human apolipoprotein C-1 by amino acid substitutions and trimethylamine-N-oxide

To test, at the level of individual amino acids, the conformation of an exchangeable apolipoprotein in aqueous solution and in the presence of an osmolyte trimethylamine-N-oxide (TMAO), six synthetic peptide analogues of human apolipoprotein C-1 (apoC-1, 57 residues) containing point mutations in the predicted alpha-helical regions were analyzed by circular dichroism (CD). The CD spectra and the melting curves of the monomeric wild-type and plasma apoC-1 in neutral low-salt solutions superimpose, indicating 31 +/- 4% alpha-helical structure at 22 degrees C that melts reversibly with T(m,WT) = 50 +/- 2 degrees C and van't Hoff enthalpy deltaH(v,WT)(Tm) = 18 +/- 2 kcal/mol. G15A substitution leads to an increased alpha-helical content of 42 +/- 4% and an increased T(m,G15A) = 57 +/- 2 degrees C, which corresponds to stabilization by delta deltaG(app) = +0.4 +/- 1.5 kcal/mol. G15P mutant has approximately 20% alpha-helical content at 22 degrees C and unfolds with low cooperativity upon heating to 90 degrees C. R23P and T45P mutants are fully unfolded at 0-90 degrees C. In contrast, Q31P mutation leads to no destabilization or unfolding. Consequently, the R23 and T45 locations are essential for the stability of the cooperative alpha-helical unit in apoC-1 monomer, G15 is peripheral to it, and Q31 is located in a nonhelical linker region. Our results suggest that Pro mutagenesis coupled with CD provides a tool for assigning the secondary structure to protein groups, which should be useful for other self-associating proteins that are not amenable to NMR structural analysis in aqueous solution. TMAO induces a reversible cooperative coil-to-helix transition in apoC-1, with the maximal alpha-helical content reaching 74%. Comparison with the maximal alpha-helical content of 73% observed in lipid-bound apoC-1 suggests that the TMAO-stabilized secondary structure resembles the functional lipid-bound apolipoprotein conformation.

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).

Interaction of an exchangeable apolipoprotein with phospholipid vesicles and lipoprotein particles. Role of leucines 32, 34, and 95 in Locusta migratoria apolipophorin III

Apolipophorin III (apoLp-III) from Locusta migratoria is an exchangeable apolipoprotein that binds reversibly to lipid surfaces. In the lipid-free state this 164-residue protein exists as a bundle of five elongated amphipathic alpha-helices. Upon lipid binding, apoLp-III undergoes a significant conformational change, resulting in exposure of its hydrophobic interior to the lipid environment. On the basis of x-ray crystallographic data (Breiter, D. R., Kanost, M. R., Benning, M. M., Wesenberg, G., Law, J. H., Wells, M. A., Rayment, I., and Holden, H. M. (1991) Biochemistry 30, 603-608), it was proposed that hydrophobic residues, present in loops that connect helices 1 and 2 (Leu-32 and Leu-34) and helices 3 and 4 (Leu-95), may function in initiation of lipid binding. To examine this hypothesis, mutant apoLp-IIIs were designed wherein the three Leu residues were replaced by Arg, individually or together. Circular dichroism spectroscopy and temperature and guanidine hydrochloride denaturation studies showed that the mutations did not cause major changes in secondary structure content or stability. In lipid binding assays, addition of apoLp-III to phospholipid vesicles caused a rapid clearance of vesicle turbidity due to transformation to discoidal complexes. L34R and L32R/L34R/L95R apoLp-IIIs displayed a much stronger interaction with lipid vesicles than wild-type apoLp-III. Furthermore, it was demonstrated that the mutant apoLp-IIIs retained their ability to bind to lipoprotein particles. However, in lipoprotein competition binding assays, the mutants displayed an impaired ability to initiate a binding interaction when compared with wild-type apoLp-III. The data indicate that the loops connecting helices 1 and 2 and helices 3 and 4 are critical regions in the protein, contributing to recognition of hydrophobic defects on lipoprotein surfaces by apoLp-III.

An amphipathic alpha-helix at a membrane interface: a structural study using a novel X-ray diffraction method

The amphipathic alpha-helix is a recurrent feature of membrane-active proteins, peptides, and toxins. Despite extensive biophysical studies, the structural details of its affinity for membrane interfaces remain rather vague. We report here the first results of an effort to obtain detailed structural information about alpha-helices in membranes by means of a novel X-ray diffraction method. Specifically, we determined the transbilayer position and orientation of an archetypal class A amphipathic helical peptide in oriented fluid-state dioleoylphosphatidylcholine (DOPC) bilayers. The peptide, Ac-18A-NH2(Ac-DWLKAFYDKVAEKLKEAF-NH2), is a model for class A amphipathic helices of apolipoprotein A-I and other exchangeable lipoproteins. The diffraction method relies upon experimental determinations of absolute scattering-length density profiles along the bilayer normal and the transbilayer distribution of the DOPC double bonds by means of specific bromination, and molecular modeling of the perturbed lipid bilayer (derived using the transbilayer distribution of the double bonds) and the peptide. The diffraction results showed that Ac-18A-NH2was located in the bilayer interface and that its transbilayer distribution could be described by a Gaussian function with a 1/e-halfwidth of 4.5(+/-0.3) A located 17.1(+/-0.3) A from the bilayer center, close to the glycerol moiety. Molecular modeling suggested that Ac-18A-NH2is helical and oriented generally parallel with the bilayer plane. The helicity and orientation were confirmed by oriented circular dichroism measurements. The width of the Gaussian distribution, a measure of the diameter of the helix, indicated that the Ac-18A-NH2helix penetrated the hydrocarbon core to about the level of the DOPC double bonds. Bilayer perturbations caused by Ac-18A-NH2were surprisingly modest, consisting of a slight decrease in bilayer thickness with a concomitant shift of the double-bond distribution toward the bilayer center, as expected from a small increase in lipid-specific area caused by the peptide.

Semliki Forest virus mRNA capping enzyme requires association with anionic membrane phospholipids for activity

The replication complexes of all positive strand RNA viruses of eukaryotes are associated with membranes. In the case of Semliki Forest virus (SFV), the main determinant of membrane attachment seems to be the virus-encoded non-structural protein NSP1, the capping enzyme of the viral mRNAs, which has guanine-7-methyltransferase and guanylyltransferase activities. We show here that both enzymatic activities of SFV NSP1 are inactivated by detergents and reactivated by anionic phospholipids, especially phosphatidylserine. The region of NSP1 responsible for binding to membranes as well as to liposomes was mapped to a short segment, which is conserved in the large alphavirus-like superfamily of viruses. A synthetic peptide of 20 amino acids from the putative binding site competed with in vitro synthesized NSP1 for binding to liposomes containing phosphatidylserine. These findings suggest a molecular mechanism by which RNA virus replicases attach to intracellular membranes and why they depend on the membranous environment.

The structure of human lipoprotein A-I. Evidence for the "belt" model

The two main competing models for the structure of discoidal lipoprotein A-I complexes both presume that the protein component is helical and situated around the perimeter of a lipid bilayer disc. However, the more popular "picket fence" model orients the protein helices perpendicular to the surface of the lipid bilayer, while the alternative "belt" model orients them parallel to the bilayer surface. To distinguish between these models, we have investigated the structure of human lipoprotein A-I using a novel form of polarized internal reflection infrared spectroscopy that can characterize the relative orientation of protein and lipid components in the lipoprotein complexes under native conditions. Our results verify lipid bilayer structure in the complexes and point unambiguously to the belt model.

A novel apolipoprotein A-1 variant, Arg173Pro, associated with cardiac and cutaneous amyloidosis

An American kindred was found to have hereditary amyloidosis with cutaneous and cardiac involvement. Characterization of fibrils isolated from skin identified the amyloid protein as the N-terminal 90 to 100 residues of apolipoprotein A-1. Sequence of the apolipoprotein A-1 gene was normal except for a G/C transversion at position 1638 which predicts an Arg to Pro substitution at residue 173. This mutation, unlike previously described amyloidogenic mutations is not in the N-terminal fragment which is incorporated into the fibril. The mutation is at the same residue as in apolipoprotein A-1 Milano (Arg173Cys) which does not result in amyloid formation. Decreased plasma HDL cholesterol levels in carriers of the Arg173Pro mutation suggest an increased rate of catabolism as has been shown for the amyloidogenic Gly26Arg mutation. This suggests that altered metabolism caused by the mutation may be a significant factor in apolipoprotein A-1 fibrillogenesis.

Molecular dynamics on a model for nascent high-density lipoprotein: role of salt bridges

The results of an all-atom molecular dynamics simulation on a discoidal complex made of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and a synthetic alpha-helical 18-mer peptide with an apolipoprotein-like charge distribution are presented. The system consists of 12 acetyl-18A-amide (Ac-18A-NH2) (. J. Biol. Chem. 260:10248-10255) molecules and 20 molecules of POPC in a bilayer, 10 in each leaflet, solvated in a sphere of water for a total of 28,522 atoms. The peptide molecules are oriented with their long axes normal to the bilayer (the "picket fence" orientation). This system is analogous to complexes formed in nascent high-density lipoprotein and to Ac-18A-NH2/phospholipid complexes observed experimentally. The simulation extended over 700 ps, with the last 493 ps used for analysis. The symmetry of this system allows for averaging over different helices to improve sampling, while maintaining explicit all-atom representation of all peptides. The complex is stable on the simulated time scale. Several possible salt bridges between and within helices were studied. A few salt bridge formations and disruptions were observed. Salt bridges provide specificity in interhelical interactions.

Evolution of the diverse antifreeze proteins

Different types of ice-growth-inhibiting antifreeze proteins, first recognized in fish, have now been isolated from insects and plants, and the list continues to expand. Their structures are amazingly diverse; how they attain the same function are subjects of intense research. Evolutionary precursors of several members have been identified - divergent proteins of apparently unrelated function. The hybridization of information from structural and molecular evolution studies of these molecules provides a forum in which issues of selection, gene genealogy, adaptive evolution, and invention of a novel function can be coherently addressed.

The low density lipoprotein receptor active conformation of apolipoprotein E. Helix organization in n-terminal domain-phospholipid disc particles

Lipid association is a prerequisite for receptor interactions of apolipoprotein E (apoE). Disc complexes of the N-terminal 22-kDa apoE3 receptor binding domain and dimyristoylphosphatidylcholine display full receptor binding activity. Studies have been performed to characterize conformational adaptations of the globular, lipid-free four-helix bundle structure that culminate in stable association of its amphipathic alpha-helices with a lipid surface. Helix-lipid interactions in bilayer disc complexes can conceivably adopt two orientations: parallel or perpendicular to the phospholipid acyl chains. Evidence based on infrared dichroism, geometrical arguments, and x-ray crystallography support the view that defined helical segments in the four-helix bundle realign upon lipid association, orienting perpendicular to the phospholipid fatty acyl chains, circumscribing the bilayer disc. Thus, it is likely that paired helical segments align in tandem, presenting a convex receptor-active surface.

Enzymatic repair of oxidative damage to human apolipoprotein A-I

Oxidative damage to apolipoprotein A-I that occurs in vivo commonly involves methionine oxidation, and is accompanied by alterations in structure, lipid association, and cholesterol efflux function. We have used the enzyme peptide methionine sulfoxide reductase (PMSR) to reverse this damage, and shown by a variety of criteria that enzyme treatment restores the primary, secondary, and tertiary structure and lipid association characteristic of the native unoxidized protein. Lipid-associated as well as lipid-free apolipoprotein A-I reacts with PMSR, suggesting that enzymatic reduction of oxidized apolipoprotein A-I in high density lipoproteins can result in restoration of biological activity and the ability to promote cholesterol efflux from cells.

Enhanced cholesterol efflux by tyrosyl radical-oxidized high density lipoprotein is mediated by apolipoprotein AI-AII heterodimers

Myeloperoxidase secreted by phagocytes in the artery wall may be a catalyst for lipoprotein oxidation. High density lipoprotein (HDL) oxidized by peroxidase-generated tyrosyl radical has a markedly enhanced ability to deplete cultured cells of cholesterol. We have investigated the structural modifications in tyrosylated HDL responsible for this effect. Spherical reconstituted HDL (rHDL) containing the whole apolipoprotein (apo) fraction of tyrosylated HDL reproduced the ability of intact tyrosylated HDL to enhance cholesterol efflux from cholesterol-loaded human fibroblasts when reconstituted with the whole lipid fraction of either HDL or tyrosylated HDL. Free apoAI or apoAII showed no increased capacity to induce cholesterol efflux from cholesterol-loaded fibroblasts following oxidation by tyrosyl radical, either in their lipid-free forms or in rHDL. The product of oxidation of a mixture of apoAI and apoAII (1:1 molar ratio) by tyrosyl radical, however, reproduced the enhanced ability of tyrosylated HDL to induce cholesterol efflux when reconstituted with the whole lipid fraction of HDL. HDL containing only apoAI or apoAII showed no enhanced ability to promote cholesterol efflux following oxidation by tyrosyl radical, whereas HDL containing both apoAI and apoAII did. rHDL containing apoAI-apoAIImonomer and apoAI-(apoAII)2 heterodimers showed a markedly increased ability to prevent the accumulation of LDL-derived cholesterol mass by sterol-depleted fibroblasts compared with other apolipoprotein species of tyrosylated HDL. These results indicate a novel product of HDL oxidation, apoAI-apoAII heterodimers, with a markedly enhanced capacity to deplete cells of the regulatory pool of free cholesterol and total cholesterol mass. The recent observation of tyrosyl radical-oxidized LDL in vivo suggests that a similar modification of HDL would significantly enhance its ability to deplete peripheral cells of cholesterol in the first step of reverse cholesterol transport.

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.

The hydrophobic face orientation of apolipoprotein A-I amphipathic helix domain 143-164 regulates lecithin:cholesterol acyltransferase activation

Apolipoprotein A-I (apoA-I) activates the plasma enzyme lecithin:cholesterol acyltransferase (LCAT), catalyzing the rapid conversion of lipoprotein cholesterol to cholesterol ester. Structural mutants of apoA-I have been used to study the details of apoA-I-LCAT-catalyzed cholesterol ester formation. Several studies have shown that the alpha-helical segments corresponding to amino acids 143-164 and 165-186 (repeats 6 and 7) are essential for LCAT activation. In the present studies, we examined how the orientation of the hydrophobic face, independent of an increase in overall hydrophobicity, affects LCAT activation. We designed, expressed, and characterized a mutant, reverse of 6 apoA-I (RO6 apoA-I), in which the primary amino acid sequence of repeat 6 (amino acids 143-164) was reversed from its normal orientation. This mutation rotates the hydrophobic face of repeat 6 approximately 80 degrees. Lipid-free RO6 apoA-I showed a marked stabilization when denatured by guanidine hydrochloride, but showed significant destabilization to guanidine hydrochloride denaturation in the lipid-bound state compared with wild-type apoA-I. Recombinant high density lipoprotein discs (rHDL) formed from RO6 apoA-I, sn-1-palmitoyl-sn-2-oleoyl phosphati-dylcholine, and cholesterol were approximately 12 A smaller than wild-type apoA-I rHDL. The reduced size suggests that one of the repeats did not effectively participate in phospholipid binding and organization. The sn-1-palmitoyl-sn-2-oleoyl phosphatidylcholine RO6 rHDL were a less effective substrate for LCAT. Mapping the entire lipid-free and lipid-bound RO6 apoA-I with a series of monoclonal antibodies revealed that both the lipid-free and lipid-bound RO6 apoA-I displayed altered or absent epitopes in domains within and adjacent to repeat 6. Together, these results suggest that the proper alignment and orientation of the hydrophobic face of repeat 6 is an important determinant for maintaining and stabilizing helix-bilayer and helix-helix interactions.

Stabilization of alpha-synuclein secondary structure upon binding to synthetic membranes

alpha-Synuclein is a highly conserved presynaptic protein of unknown function. A mutation in the protein has been causally linked to Parkinson's disease in humans, and the normal protein is an abundant component of the intraneuronal inclusions (Lewy bodies) characteristic of the disease. alpha-Synuclein is also the precursor to an intrinsic component of extracellular plaques in Alzheimer's disease. The alpha-synuclein sequence is largely composed of degenerate 11-residue repeats reminiscent of the amphipathic alpha-helical domains of the exchangeable apolipoproteins. We hypothesized that alpha-synuclein should associate with phospholipid bilayers and that this lipid association should stabilize an alpha-helical secondary structure in the protein. We report that alpha-synuclein binds to small unilamellar phospholipid vesicles containing acidic phospholipids, but not to vesicles with a net neutral charge. We further show that the protein associates preferentially with vesicles of smaller diameter (20-25 nm) as opposed to larger (approximately 125 nm) vesicles. Lipid binding is accompanied by an increase in alpha-helicity from 3% to approximately 80%. These observations are consistent with a role in vesicle function at the presynaptic terminal.