apoB-100 has a pentapartite structure composed of three amphipathic alpha-helical domains alternating with two amphipathic beta-strand domains. Detection by the computer program LOCATE

The Soret coefficient of human low-density lipoprotein in solution: a thermophilic behavior

Thermodiffusion, or Soret effect, is the physical phenomenon of matter gradients originated by the migration of chemical species induced by thermal gradients. Thermodiffusion has been widely applied in the study of colloidal suspensions. In this study, we investigate the termodiffusion behavior of low-density lipoprotein (LDL) particles, by the Soret coefficient measurement. It is a new approach to studies of plasma lipoproteins. The experimental work was based on thermal- and Soret-lens effects. These effects were induced by laser irradiation of the samples, at two different time scales, in a Z-scan setup. LDL samples were analyzed under physiological conditions, notedly, ionic strength and pH, and at different temperatures. Temperature dependence of Soret coefficient showed a slight decrease in the absolute value of this coefficient, as a function of temperature increasing. However, its sign does not change at the temperatures investigated (15, 22.5 and 37.5 °C). The results show that LDL particles exhibit thermophilic behavior. The origin of this thermophilic behavior is not yet completely understood. We discuss some aspects that can be related with the Soret effect in LDL samples.

Targeting structural flexibility in low density lipoprotein by integrating cryo-electron microscopy and high-speed atomic force microscopy

Low-density lipoprotein (LDL) plays a crucial role in cholesterol metabolism. Responsible for cholesterol transport from the liver to the organs, LDL accumulation in the arteries is a primary cause of cardiovascular diseases, such as atherosclerosis. This work focuses on the fundamental question of the LDL molecular structure, as well as the topology and molecular motions of apolipoprotein B-100 (apo B-100), which is addressed by single-particle cryo-electron microscopy (cryo-EM) and high-speed atomic force microscopy (HS-AFM). Our results suggest a revised model of the LDL core organization with respect to the cholesterol ester (CE) arrangement. In addition, a high-density region close to the flattened poles could be identified, likely enriched in free cholesterol. The most remarkable new details are two protrusions on the LDL surface, attributed to the protein apo B-100. HS-AFM adds the dimension of time and reveals for the first time a highly dynamic direct description of LDL, where we could follow large domain fluctuations of the protrusions in real time. To tackle the inherent flexibility and heterogeneity of LDL, the cryo-EM maps are further assessed by 3D variability analysis. Our study gives a detailed explanation how to approach the intrinsic flexibility of a complex system comprising lipids and protein.

Novel Biomarkers for Atherosclerotic Disease: Advances in Cardiovascular Risk Assessment

Atherosclerosis is a significant health concern with a growing incidence worldwide. It is directly linked to an increased cardiovascular risk and to major adverse cardiovascular events, such as acute coronary syndromes. In this review, we try to assess the potential diagnostic role of biomarkers in the early identification of patients susceptible to the development of atherosclerosis and other adverse cardiovascular events. We have collected publications concerning already established parameters, such as low-density lipoprotein cholesterol (LDL-C), as well as newer markers, e.g., apolipoprotein B (apoB) and the ratio between apoB and apoA. Additionally, given the inflammatory nature of the development of atherosclerosis, high-sensitivity c-reactive protein (hs-CRP) or interleukin-6 (IL-6) are also discussed. Additionally, newer publications on other emerging components linked to atherosclerosis were considered in the context of patient evaluation. Apart from the already in-use markers (e.g., LDL-C), emerging research highlights the potential of newer molecules in optimizing the diagnosis of atherosclerotic disease in earlier stages. After further studies, they might be fully implemented in the screening protocols.

Identification and Functional Analysis of APOB Variants in a Cohort of Hypercholesterolemic Patients

Mutations in APOB are the second most frequent cause of familial hypercholesterolemia (FH). APOB is highly polymorphic, and many variants are benign or of uncertain significance, so functional analysis is necessary to ascertain their pathogenicity. Our aim was to identify and characterize APOB variants in patients with hypercholesterolemia. Index patients (n = 825) with clinically suspected FH were analyzed using next-generation sequencing. In total, 40% of the patients presented a variant in LDLR, APOB, PCSK9 or LDLRAP1, with 12% of the variants in APOB. These variants showed frequencies in the general population lower than 0.5% and were classified as damaging and/or probably damaging by 3 or more predictors of pathogenicity. The variants c.10030A>G;p.(Lys3344Glu) and c.11401T>A;p.(Ser3801Thr) were characterized. The p.(Lys3344Glu) variant co-segregated with high low-density lipoprotein (LDL)-cholesterol in 2 families studied. LDL isolated from apoB p.(Lys3344Glu) heterozygous patients showed reduced ability to compete with fluorescently-labelled LDL for cellular binding and uptake compared with control LDL and was markedly deficient in supporting U937 cell proliferation. LDL that was carrying apoB p.(Ser3801Thr) was not defective in competing with control LDL for cellular binding and uptake. We conclude that the apoB p.(Lys3344Glu) variant is defective in the interaction with the LDL receptor and is causative of FH, whereas the apoB p.(Ser3801Thr) variant is benign.

Novel protein-truncating variant in the APOB gene may protect from coronary artery disease and adverse cardiovascular events

Background and aims

Genetic testing is still rarely used for the diagnosis of dyslipidemia, even though gene variants determining plasma lipids levels are not uncommon.

Methods

Starting from a a pilot-analysis of targeted Next Generation Sequencing (NGS) of 5 genes related to familial hypercholesterolemia (LDLR, APOB, PCSK9, HMGCR, APOE) within a cardiovascular cohort in subjects with extreme plasma concentrations of low-density lipoprotein (LDL) cholesterol, we discovered and characterized a novel point mutation in the APOB gene, which was associated with very low levels of apolipoprotein B (ApoB) and LDL cholesterol.

Results

APOB c.6943 G > T induces a premature stop codon at the level of exon 26 in the APOB gene and generates a protein which has the 51% of the mass of the wild type ApoB-100 (ApoB-51), with a truncation at the level of residue 2315. The premature stop codon occurs after the one needed for the synthesis of ApoB-48, allowing chylomicron production at intestinal level and thus avoiding potential nutritional impairments. The heterozygous carrier of APOB c.6943G > T, despite a very high-risk profile encompassing all the traditional risk factors except for dyslipidemia, had normal coronary arteries by angiography and did not report any major adverse cardiovascular event during a 20-years follow-up, thereby obtaining advantage from the gene variant as regards protection against atherosclerosis, apparently without any metabolic retaliation.

Conclusions

Our data support the use of targeted NGS in well-characterized clinical settings, as well as they indicate that.a partial block of ApoB production may be well tolerated and improve cardiovascular outcomes.

Apolipoprotein B and Cardiovascular Disease: Biomarker and Potential Therapeutic Target

Apolipoprotein (apo) B, the critical structural protein of the atherogenic lipoproteins, has two major isoforms: apoB48 and apoB100. ApoB48 is found in chylomicrons and chylomicron remnants with one apoB48 molecule per chylomicron particle. Similarly, a single apoB100 molecule is contained per particle of very-low-density lipoprotein (VLDL), intermediate density lipoprotein, LDL and lipoprotein(a). This unique one apoB per particle ratio makes plasma apoB concentration a direct measure of the number of circulating atherogenic lipoproteins. ApoB levels indicate the atherogenic particle concentration independent of the particle cholesterol content, which is variable. While LDL, the major cholesterol-carrying serum lipoprotein, is the primary therapeutic target for management and prevention of atherosclerotic cardiovascular disease, there is strong evidence that apoB is a more accurate indicator of cardiovascular risk than either total cholesterol or LDL cholesterol. This review examines multiple aspects of apoB structure and function, with a focus on the controversy over use of apoB as a therapeutic target in clinical practice. Ongoing coronary artery disease residual risk, despite lipid-lowering treatment, has left patients and clinicians with unsatisfactory options for monitoring cardiovascular health. At the present time, the substitution of apoB for LDL-C in cardiovascular disease prevention guidelines has been deemed unjustified, but discussions continue.

The Targeting of Native Proteins to the Endoplasmic Reticulum-Associated Degradation (ERAD) Pathway: An Expanding Repertoire of Regulated Substrates

All proteins are subject to quality control processes during or soon after their synthesis, and these cellular quality control pathways play critical roles in maintaining homeostasis in the cell and in organism health. Protein quality control is particularly vital for those polypeptides that enter the endoplasmic reticulum (ER). Approximately one-quarter to one-third of all proteins synthesized in eukaryotic cells access the ER because they are destined for transport to the extracellular space, because they represent integral membrane proteins, or because they reside within one of the many compartments of the secretory pathway. However, proteins that mature inefficiently are subject to ER-associated degradation (ERAD), a multi-step pathway involving the chaperone-mediated selection, ubiquitination, and extraction (or “retrotranslocation”) of protein substrates from the ER. Ultimately, these substrates are degraded by the cytosolic proteasome. Interestingly, there is an increasing number of native enzymes and metabolite and solute transporters that are also targeted for ERAD. While some of these proteins may transiently misfold, the ERAD pathway also provides a route to rapidly and quantitatively downregulate the levels and thus the activities of a variety of proteins that mature or reside in the ER.

Interactions of different lipoproteins with supported phospholipid raft membrane (SPRM) patterns to understand similar in-vivo processes

To better understand how lipoproteins interact and enter endothelium and participate in cellular processes, we investigated preferential lipid partitioning of triglyceride rich lipoproteins (TGRL), chylomicrons (CM), low density lipoproteins (LDL), very low density lipoproteins (VLDL) and their lipolysis products using supported phospholipid raft membrane (SPRM) patterns. We prepared SPRM patterns with Texas red labeled phospholipid patterns and Marina blue labeled raft patterns and added Atto-520 labeled lipoproteins (TGRL, CM, VLDL, LDL) and their lipolysis products in separate experiments and characterized these interactions using fluorescence microscopy. We observed that VLDL and LDL preferentially interacted with raft patterns. In contrast the TGRL and lipolysed products of TGRL interacted with both the patterns, slightly elevated preference for raft patterns and CM and its lipolysis products showed greater affinity to phospholipid patterns. The clear preference of VLDL and LDL for raft patterns suggests that these lipoproteins associate with cholesterol and sphingomyelin rich lipid micro-domains during their early interactions with endothelial cells, leading to atherosclerosis.

Analysis of the acrolein-modified sites of apolipoprotein B-100 in LDL

We have reported that acrolein-conjugated low-density lipoprotein (Acro-LDL) uptake by scavenger receptor class A type 1 (SR-A1) can mediate macrophage foam cell formation. The purpose of this study was to determine which amino acid residues of apoB protein in LDL are conjugated with acrolein. Acro-apoB was prepared by incubation of LDL with acrolein (10 to 60 μM) at 37 °C for 7 days. Identification of acrolein-conjugated amino acid residues in apoB was performed using LC-MS/MS. The levels of acrolein-conjugated amino acid residues of apoB as well as crosslinking apoB increased in proportion to acrolein concentration. The level of LDL uptake by macrophages was parallel with the acrolein-conjugated monomer apoB. Acrolein-conjugated amino acid residues in apoB were C212, K327, K742, K949, K1087, H1923, K2634, K3237 and K3846. The NH₂-teriminal four amino acid residues (C212, K327, K742 and K949) were located at the scavenger receptor SR-A1 recognition site, suggesting that these four acrolein-conjugated amino acids are involved in the rapid uptake of Acro-LDL by macrophages. It is proposed that the rapid uptake of LDL by macrophages is dependent on acrolein conjugation of four amino acids residues at the scavenger receptor recognition site of apoB in LDL.

Diabetes Impairs Cellular Cholesterol Efflux From ABCA1 to Small HDL Particles

RATIONALE

HDL (high-density lipoprotein) may be cardioprotective because it accepts cholesterol from macrophages via the cholesterol transport proteins ABCA1 (ATP-binding cassette transporter A1) and ABCG1 (ATP-binding cassette transporter G1). The ABCA1-specific cellular cholesterol efflux capacity (ABCA1 CEC) of HDL strongly and negatively associates with cardiovascular disease risk, but how diabetes mellitus impacts that step is unclear.

OBJECTIVE

To test the hypothesis that HDL's cholesterol efflux capacity is impaired in subjects with type 2 diabetes mellitus.

METHODS AND RESULTS

We performed a case-control study with 19 subjects with type 2 diabetes mellitus and 20 control subjects. Three sizes of HDL particles, small HDL, medium HDL, and large HDL, were isolated by high-resolution size exclusion chromatography from study subjects. Then we assessed the ABCA1 CEC of equimolar concentrations of particles. Small HDL accounted for almost all of ABCA1 CEC activity of HDL. ABCA1 CEC-but not ABCG1 CEC-of small HDL was lower in the subjects with type 2 diabetes mellitus than the control subjects. Isotope dilution tandem mass spectrometry demonstrated that the concentration of SERPINA1 (serpin family A member 1) in small HDL was also lower in subjects with diabetes mellitus. Enriching small HDL with SERPINA1 enhanced ABCA1 CEC. Structural analysis of SERPINA1 identified 3 amphipathic α-helices clustered in the N-terminal domain of the protein; biochemical analyses demonstrated that SERPINA1 binds phospholipid vesicles.

CONCLUSIONS

The ABCA1 CEC of small HDL is selectively impaired in type 2 diabetes mellitus, likely because of lower levels of SERPINA1. SERPINA1 contains a cluster of amphipathic α-helices that enable apolipoproteins to bind phospholipid and promote ABCA1 activity. Thus, impaired ABCA1 activity of small HDL particles deficient in SERPINA1 could increase cardiovascular disease risk in subjects with diabetes mellitus.

Lipid transfer proteins in the assembly of apoB-containing lipoproteins

A better understanding of intracellular lipoprotein assembly may help identify proteins with important roles in lipid disorders. apoB-containing lipoproteins (B-lps) are macromolecular lipid and protein micelles that act as specialized transport vehicles for hydrophobic lipids. They are assembled predominantly in enterocytes and hepatocytes to transport dietary and endogenous fat, respectively, to different tissues. Assembly occurs in the endoplasmic reticulum (ER) and is dependent on lipid resynthesis in the ER and on a chaperone, namely, microsomal triglyceride transfer protein (MTTP). Precursors for lipid synthesis are obtained from extracellular sources and from cytoplasmic lipid droplets. MTTP is the major and essential lipid transfer protein that transfers phospholipids and triacylglycerols to nascent apoB for the assembly of lipoproteins. Assembly is aided by cell death-inducing DFF45-like effector B and by phospholipid transfer protein, which may facilitate additional deposition of triacylglycerols and phospholipids, respectively, to apoB. Here, we summarize the current understanding of the different steps in the assembly of B-lps and discuss the role of lipid transfer proteins in these steps to help identify new clinical targets for lipid-associated disorders, such as heart disease.

Can modulators of apolipoproteinB biogenesis serve as an alternate target for cholesterol-lowering drugs?

Understanding the molecular defects underlying cardiovascular disease is necessary for the development of therapeutics. The most common method to lower circulating lipids, which reduces the incidence of cardiovascular disease, is statins, but other drugs are now entering the clinic, some of which have been approved. Nevertheless, patients cannot tolerate some of these therapeutics, the drugs are costly, and/or the treatments are approved for only rare forms of disease. Efforts to find alternative treatments have focused on other factors, such as apolipoproteinB (apoB), which transports cholesterol in the blood stream. The levels of apoB are regulated by endoplasmic reticulum (ER) associated degradation as well as by a post ER degradation pathway in model systems, and we suggest that these events provide novel therapeutic targets. We discuss first how cardiovascular disease arises and how cholesterol is regulated, and then summarize the mechanisms of action of existing treatments for cardiovascular disease. We then review the apoB biosynthetic pathway, focusing on steps that might be amenable to therapeutic interventions.

Relationship between Amphipathic β Structures in the β1 Domain of Apolipoprotein B and the Properties of the Secreted Lipoprotein Particles in McA-RH7777 Cells

Our previous studies demonstrated that the first 1000 amino acid residues (the βα₁ domain) of human apolipoprotein (apo) B-100, termed apoB:1000, are required for the initiation of lipoprotein assembly and the formation of a monodisperse stable phospholipid (PL)-rich particle. The objectives of this study were (a) to assess the effects on the properties of apoB truncates undergoing sequential inclusion of the amphipathic β strands in the 700 N-terminal residues of the β₁ domain of apoB-100 and (b) to identify the subdomain in the β₁ domain that is required for the formation of a microsomal triglyceride transfer protein (MTP)-dependent triacylglycerol (TAG)-rich apoB-containing particle. Characterization of particles secreted by stable transformants of McA-RH7777 cells demonstrated the following. (1) The presence of amphipathic β strands in the 200 N-terminal residues of the β₁ domain resulted in the secretion of apoB truncates (apoB:1050 to apoB:1200) as both lipidated and lipid-poor particles. (2) Inclusion of residues 300-700 of the β₁ domain led to the secretion of apoB:1300, apoB:1400, apoB:1500, and apoB:1700 predominantly as lipidated particles. (3) Particles containing residues 1050-1500 were all rich in PL. (4) There was a marked increase in the lipid loading capacity and TAG content of apoB:1700-containing particles. (5) Only the level of secretion of apoB:1700 was markedly diminished by MTP inhibitor BMS-197636. These results suggest that apoB:1700 marks the threshold for the formation of a TAG-rich particle and support the concept that MTP participates in apoB assembly and secretion at the stage where particles undergo a transition from PL-rich to TAG-rich.

Identification of a novel lipid binding motif in apolipoprotein B by the analysis of hydrophobic cluster domains

Apolipoprotein B (apoB) is a large amphipathic protein that is the structural scaffold for the formation of several classes of lipoproteins involved in lipid transport throughout the body. The goal of the present study was to identify specific domains in the apoB sequence that contribute to its lipid binding properties. A sequence analysis algorithm was developed to identify stretches of hydrophobic amino acids devoid of charged amino acids, which are referred to as hydrophobic cluster domains (HCDs). This analysis identified 78 HCDs in apoB with hydrophobic stretches ranging from 6 to 26 residues. Each HCD was analyzed in silico for secondary structure and lipid binding properties, and a subset was synthesized for experimental evaluation. One HCD peptide, B38, showed high affinity binding to both isolated HDL and LDL, and could exchange between lipoproteins. All-atom molecular dynamics simulations indicate that B38 inserts 3.7Å below the phosphate plane of the bilayer. B38 forms an unusual α-helix with a broad hydrophobic face and polar serine and threonine residues on the opposite face. Based on this structure, we hypothesized that B38 could efflux cholesterol from cells. B38 showed a 12-fold greater activity than the 5A peptide, a bihelical Class A amphipathic helix (EC₅₀ of 0.2658 vs. 3.188μM; p<0.0001), in promoting cholesterol efflux from ABCA1 expressing BHK-1 cells. In conclusion, we have identified novel domains within apoB that contribute to its lipid biding properties. Additionally, we have discovered a unique amphipathic helix design for efficient ABCA1-specific cholesterol efflux.

Evaluation of Methods for the Measurement of Low-Density Lipoprotein Cholesterol

A high concentration of low-density lipoprotein cholesterol (LDL-C) in plasma is one of the strongest risk factors for atherosclerotic cardiovascular disease and mortality. The most common approach to determining LDL-C in the clinical laboratory is the Friedewald calculation. There is an increased interest to improve the accuracy of LDL-C estimated by this equation. The expert panel convened by National Cholesterol Education Program has recommended the development of accurate direct methods to measure LDL-C. Several homogeneous and fully automated methods have been introduced in recent years that show improved precision and accuracy over earlier methods, especially the Friedewald calculation. Each of the atherogenic particles in plasma—very-low, intermediate-, and low-density lipoprotein—as well as lipoprotein (a), contain one molecule of apolipoprotein B (apoB) and thus, plasma total concentration of apoB reflects the number of atherogenic particles. Several studies suggested that the measurement of apoB could improve the prediction of risk of coronary artery disease. Thus, in addition to the newly developed direct assays, alternative calculation procedures have been proposed that also take into consideration total serum apoB concentration for the estimation of LDL-C and the presence of small, dense LDL particles. The new generation of homogenous methods for the measurement of LDL-C and the use of serum apoB concentration for the estimation of LDL-C can contribute to the accurate LDL-C determination.

Roles of Lipoproteins and Apolipoproteins in Particle Formation of Hepatitis C Virus

More than 160 million people worldwide are infected with hepatitis C virus (HCV), and cirrhosis and hepatocellular carcinoma induced by HCV infection are life-threatening diseases. HCV takes advantage of many aspects of lipid metabolism for an efficient propagation in hepatocytes. Due to the morphological and physiological similarities of HCV particles to lipoproteins, lipid-associated HCV particles are named lipoviroparticles. Recent analyses have revealed that exchangeable apolipoproteins directly interact with the viral membrane to generate infectious HCV particles. In this review, we summarize the roles of lipid metabolism in the life cycle of HCV.

Computational studies of plasma lipoprotein lipids

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

Structural analysis of APOB variants, p.(Arg3527Gln), p.(Arg1164Thr) and p.(Gln4494del), causing Familial Hypercholesterolaemia provides novel insights into variant pathogenicity

Familial hypercholesterolaemia (FH) is an inherited autosomal dominant disorder resulting from defects in the low-density lipoprotein receptor (LDLR), in the apolipoprotein B (APOB) or in the proprotein convertase subtilisin/kexin type 9 (PCSK9) genes. In the majority of the cases FH is caused by mutations occurring within LDLR, while only few mutations in APOB and PCSK9 have been proved to cause disease. p.(Arg3527Gln) was the first mutation in APOB being identified and characterized. Recently two novel pathogenic APOB variants have been described: p.(Arg1164Thr) and p.(Gln4494del) showing impaired LDLR binding capacity, and diminished LDL uptake. The objective of this work was to analyse the structure of p.(Arg1164Thr) and p.(Gln4494del) variants to gain insight into their pathogenicity. Secondary structure of the human ApoB100 has been investigated by infrared spectroscopy (IR) and LDL particle size both by dynamic light scattering (DLS) and electron microscopy. The results show differences in secondary structure and/or in particle size of p.(Arg1164Thr) and p.(Gln4494del) variants compared with wild type. We conclude that these changes underlie the defective binding and uptake of p.(Arg1164Thr) and p.(Gln4494del) variants. Our study reveals that structural studies on pathogenic variants of APOB may provide very useful information to understand their role in FH disease.

Lipoprotein assembly and function in an evolutionary perspective

Circulatory fat transport in animals relies on members of the large lipid transfer protein (LLTP) superfamily, including mammalian apolipoprotein B (apoB) and insect apolipophorin II/I (apoLp-II/I). ApoB and apoLp-II/I, constituting the structural (non-exchangeable) basis for the assembly of various lipoproteins, acquire lipids through microsomal triglyceride-transfer protein, another LLTP family member, and bind them by means of amphipathic α-helical and β-sheet structural motifs. Comparative research reveals that LLTPs evolved from the earliest animals and highlights the structural adaptations in these lipid-binding proteins. Thus, in contrast to apoB, apoLp-II/I is cleaved post-translationally by a furin, resulting in the appearance of two non-exchangeable apolipoproteins in the single circulatory lipoprotein in insects, high-density lipophorin (HDLp). The remarkable structural similarities between mammalian and insect lipoproteins notwithstanding important functional differences relate to the mechanism of lipid delivery. Whereas in mammals, partial delipidation of apoB-containing lipoproteins eventually results in endocytic uptake of their remnants, mediated by members of the low-density lipoprotein receptor (LDLR) family, and degradation in lysosomes, insect HDLp functions as a reusable lipid shuttle capable of alternate unloading and reloading of lipid. Also, during muscular efforts (flight activity), an HDLp-based lipoprotein shuttle provides for the transport of lipid for energy generation. Although a lipophorin receptor - a homolog of LDLR - was identified that mediates endocytic uptake of HDLp during specific developmental periods, the endocytosed lipoprotein appears to be recycled in a transferrin-like manner. These data highlight that the functional adaptations in the lipoprotein lipid carriers in mammals and insects also emerge with regard to the functioning of their cognate receptors.

Modified low density lipoprotein and lipoprotein-containing circulating immune complexes as diagnostic and prognostic biomarkers of atherosclerosis and type 1 diabetes macrovascular disease

In atherosclerosis; blood low-density lipoproteins (LDL) are subjected to multiple enzymatic and non-enzymatic modifications that increase their atherogenicity and induce immunogenicity. Modified LDL are capable of inducing vascular inflammation through activation of innate immunity; thus, contributing to the progression of atherogenesis. The immunogenicity of modified LDL results in induction of self-antibodies specific to a certain type of modified LDL. The antibodies react with modified LDL forming circulating immune complexes. Circulating immune complexes exhibit prominent immunomodulatory properties that influence atherosclerotic inflammation. Compared to freely circulating modified LDL; modified LDL associated with the immune complexes have a more robust atherogenic and proinflammatory potential. Various lipid components of the immune complexes may serve not only as diagnostic but also as essential predictive markers of cardiovascular events in atherosclerosis. Accumulating evidence indicates that LDL-containing immune complexes can also serve as biomarker for macrovascular disease in type 1 diabetes.

Lipid transfer particle from the silkworm, Bombyx mori, is a novel member of the apoB/large lipid transfer protein family

Lipid transfer particle (LTP) is a high-molecular-weight, very high-density lipoprotein known to catalyze the transfer of lipids between a variety of lipoproteins, including both insects and vertebrates. Studying the biosynthesis and regulation pathways of LTP in detail has not been possible due to a lack of information regarding the apoproteins. Here, we sequenced the cDNA and deduced amino acid sequences for three apoproteins of LTP from the silkworm (Bombyx mori). The three subunit proteins of the LTP are coded by two genes, apoLTP-II/I and apoLTP-III. ApoLTP-I and apoLTP-II are predicted to be generated by posttranslational cleavage of the precursor protein, apoLTP-II/I. Clusters of amphipathic secondary structure within apoLTP-II/I are similar to Homo sapiens apolipoprotein B (apoB) and insect lipophorins. The apoLTP-II/I gene is a novel member of the apoB/large lipid transfer protein gene family. ApoLTP-III has a putative conserved juvenile hormone-binding protein superfamily domain. Expression of apoLTP-II/I and apoLTP-III genes was synchronized and both genes were primarily expressed in the fat body at the stage corresponding to increased lipid transport needs. We are now in a position to study in detail the physiological role of LTP and its biosynthesis and assembly.

Increased very low density lipoprotein (VLDL) secretion, hepatic steatosis, and insulin resistance

Insulin resistance (IR) affects not only the regulation of carbohydrate metabolism but all aspects of lipid and lipoprotein metabolism. IR is associated with increased secretion of VLDL and increased plasma triglycerides, as well as with hepatic steatosis, despite the increased VLDL secretion. Here we link IR with increased VLDL secretion and hepatic steatosis at both the physiologic and molecular levels. Increased VLDL secretion, together with the downstream effects on high density lipoprotein (HDL) cholesterol and low density lipoprotein (LDL) size, is proatherogenic. Hepatic steatosis is a risk factor for steatohepatitis and cirrhosis. Understanding the complex inter-relationships between IR and these abnormalities of liver lipid homeostasis will provide insights relevant to new therapies for these increasing clinical problems.

Three-dimensional cryoEM reconstruction of native LDL particles to 16Å resolution at physiological body temperature

Background

Low-density lipoprotein (LDL) particles, the major carriers of cholesterol in the human circulation, have a key role in cholesterol physiology and in the development of atherosclerosis. The most prominent structural components in LDL are the core-forming cholesteryl esters (CE) and the particle-encircling single copy of a huge, non-exchangeable protein, the apolipoprotein B-100 (apoB-100). The shape of native LDL particles and the conformation of native apoB-100 on the particles remain incompletely characterized at the physiological human body temperature (37°C).

Methodology/Principal Findings

To study native LDL particles, we applied cryo-electron microscopy to calculate 3D reconstructions of LDL particles in their hydrated state. Images of the particles vitrified at 6°C and 37°C resulted in reconstructions at ∼16 Å resolution at both temperatures. 3D variance map analysis revealed rigid and flexible domains of lipids and apoB-100 at both temperatures. The reconstructions showed less variability at 6°C than at 37°C, which reflected increased order of the core CE molecules, rather than decreased mobility of the apoB-100. Compact molecular packing of the core and order in a lipid-binding domain of apoB-100 were observed at 6°C, but not at 37°C. At 37°C we were able to highlight features in the LDL particles that are not clearly separable in 3D maps at 6°C. Segmentation of apoB-100 density, fitting of lipovitellin X-ray structure, and antibody mapping, jointly revealed the approximate locations of the individual domains of apoB-100 on the surface of native LDL particles.

Conclusions/Significance

Our study provides molecular background for further understanding of the link between structure and function of native LDL particles at physiological body temperature.

A molecular dynamics approach for the association of apolipoproteinB-100 and chondroitin-6-sulfate

A force field has been previously designed for a dodecasaccharide chain of chondroitin-6-sulfate (C6S) and has proved to yield valuable data going from basic conformational properties to a more detailed H-bonding network. This force field is further used here to unravel the interaction of C6S with its pathological counterpart in low density lipoprotein (LDL) particles. In particular, well-selected peptide fragment p2 (residues 3359-3377) also identified as the principal proteoglycan binding site (PPBS) of the major protein in LDL, apolipoproteinB-100 (apoB-100), was chosen. We study here the interaction between C6S and p2. The role of arginine and lysine, positively charged amino acids of p2, in the crucial interaction of C6S with LDL is highlighted. The secondary structure of p2 is shown to affect the efficiency of the interaction, as the α-helical structure of p2 allows optimal interaction with C6S also in dynamic conditions. One point mutation in p2 appeared to affect consequently p2-C6S interaction.

Structure and lipid interactions of an anti-inflammatory and anti-atherogenic 10-residue class G(*) apolipoprotein J peptide using solution NMR

The surprising observation that a 10-residue class G(⁎) peptide from apolipoprotein J, [113-122]apoJ, possesses anti-inflammatory and anti-atherogenic properties prompted us to delineate its structural characteristics in the presence of normal and oxidized lipid. Towards this, we have determined high-resolution structure of [113-122]apoJ in solution using nuclear magnetic resonance (NMR) spectroscopy and studied its interaction with lipids, including oxidized lipids, using a number of biophysical methods. Circular dichroism and NMR studies established that in the presence of dodecylphosphocholine (DPC) micelle, this peptide adopts amphipathic α-helical structure. The observed Nuclear Overhauser effects indicate that the amphipathic helical structure of the peptide is stabilized by the N-terminal acetyl and C-terminal amide blocking groups. We used isothermal titration calorimetry to measure binding enthalpy of the peptide with DPC micelle, an oxidized lipid, 1-(palmitoyl)-2-(5-keto-6-octene-dioyl) phosphatidylcholine (KOdiA-PC), and the mixture of these two lipids (5mol% KOdiA-PC in DPC micelle). We find that the peptide binding with DPC micelle is associated with an enthalpy change (-16.75±0.16 Kcal/mol) much larger than that resulting from the binding with KodiA-PC (-3.67±0.13 Kcal/mol). Incorporation of a small amount of KOdiA-PC (5mol%) in DPC micelle also results in the lowering of peptide binding enthalpy (-13.43±0.18 Kcal/mol). These results are consistent with overall negative charge and altered conformational properties of oxidized sn-2 chain of KOdiA-PC. Our results have unambiguously established the amphipathic α-helical structure of [113-122]apoJ peptide in the presence of DPC micelle as well as its ability to bind oxidized lipid. These in vitro results help explain the previously observed anti-inflammatory and anti-atherosclerotic properties of this peptide.

Interfacial properties of apolipoprotein B292-593 (B6.4-13) and B611-782 (B13-17). Insights into the structure of the lipovitellin homology region in apolipoprotein B

The N-terminal sequence of apolipoprotein B (apoB) is critical in triacylglycerol-rich lipoprotein assembly. The first 17% of apoB (B17) is thought to consist of three domains: B5.9, a beta-barrel, B6.4-13, a series of 17 alpha-helices, and B13-17, a putative beta-sheet. B5.9 does not bind to lipid, while B6.4-13 and B13-17 contain hydrophobic interfaces that can interact with lipids. To understand how B6.4-13 and B13-17 might interact with triacylglycerol during lipoprotein assembly, the interfacial properties of both peptides were studied at the triolein/water interface. Both B6.4-13 and B13-17 are surface active. Once bound, the peptides can be neither exchanged nor pushed off the interface. Some residues of the peptides can be ejected from the interface upon compression but readsorb on expansion. B13-17 binds to the interface more strongly. The maximum pressure the peptide can withstand without being partially ejected (Pi(max)) is 19.2 mN/m for B13-17 compared to 16.7 mN/m for B6.4-13. B13-17 is purely elastic at the interface, while B6.4-13 forms a viscous-elastic film. When they are spread at an air/water interface, the limiting area and the collapse pressures are 16.6 A(2)/amino acid and 31 mN/m for B6.4-13 and 17.8 A(2)/amino acid and 35 mN/m for B13-17, respectively. The alpha-helical B6.4-13 contains some hydrophobic helices that stay bound and prevent the peptide from leaving the surface. The beta-sheets of B13-17 bind irreversibly to the surface. We suggest that during lipoprotein assembly, the N-terminal apoB starts recruiting lipid as early as B6.4, but additional sequences are essential for formation of a lipid pocket that can stabilize lipoprotein emulsion particles for secretion.

Model of human low-density lipoprotein and bound receptor based on cryoEM

Human plasma low-density lipoproteins (LDL), a risk factor for cardiovascular disease, transfer cholesterol from plasma to liver cells via the LDL receptor (LDLr). Here, we report the structures of LDL and its complex with the LDL receptor extracellular domain (LDL.LDLr) at extracellular pH determined by cryoEM. Difference imaging between LDL.LDLr and LDL localizes the site of LDLr bound to its ligand. The structural features revealed from the cryoEM map lead to a juxtaposed stacking model of cholesteryl esters (CEs). High density in the outer shell identifies protein-rich regions that can be accounted for by a single apolipoprotein (apo B-100, 500 kDa) leading to a model for the distribution of its alpha-helix and beta-sheet rich domains across the surface. The structural relationship between the apo B-100 and CEs appears to dictate the structural stability and function of normal LDL.

Surface study of apoB1694-1880, a sequence that can anchor apoB to lipoproteins and make it nonexchangeable

Apolipoprotein B (apoB) is a nonexchangeable apolipoprotein. During lipoprotein assembly, it recruits phospholipids and triacylglycerols (TAG) into TAG-rich lipoprotein particles. It remains bound to secreted lipoproteins during lipid metabolism in plasma. The beta1 region (residues 827-1880) of apoB has a high amphipathic beta strand (AbetaS) content and is proposed to be one region anchoring apoB to lipoproteins. The AbetaS-rich region between apoB37 and apoB41 (residues 1694-1880) was cloned, expressed, and purified. The interfacial properties were studied at the triolein/water (TO/W) and air/water (A/W) interfaces. ApoB[37-41] is surface-active and adsorbs to the TO/W interface. After adsorption the unbound apoB[37-41] was removed from the aqueous phase. Adsorbed apoB[37-41] did not desorb and could not be forced off by increasing the surface pressure up to 23 mN/m. ApoB[37-41] adsorbed on the TO/W interface was completely elastic when compressed and expanded by +/-13% of its area. On an A/W interface, the apoB[37-41] monolayer became solid when compressed to 4 mN/m pressure indicating extended beta-sheet formation. It could be reversibly compressed and expanded between low pressure and its collapse pressure (35 mN/m). Our studies confirm that the AbetaS structure of apoB[37-41] is a lipid-binding motif that can irreversibly anchor apoB to lipoproteins.

Circulatory lipid transport: lipoprotein assembly and function from an evolutionary perspective

Circulatory transport of neutral lipids (fat) in animals relies on members of the large lipid transfer protein (LLTP) superfamily, including mammalian apolipoprotein B (apoB) and insect apolipophorin II/I (apoLp-II/I). Latter proteins, which constitute the structural basis for the assembly of various lipoproteins, acquire lipids through microsomal triglyceride transfer protein (MTP)--another LLTP family member--and bind them by means of amphipathic structures. Comparative research reveals that LLTPs have evolved from the earliest animals and additionally highlights the structural and functional adaptations in these lipid carriers. For instance, in contrast to mammalian apoB, the insect apoB homologue, apoLp-II/I, is post-translationally cleaved by a furin, resulting in their appearance of two non-exchangeable apolipoproteins in the insect low-density lipoprotein (LDL) homologue, high-density lipophorin (HDLp). An important difference between mammalian and insect lipoproteins relates to the mechanism of lipid delivery. Whereas in mammals, endocytic uptake of lipoprotein particles, mediated via members of the LDL receptor (LDLR) family, results in their degradation in lysosomes, the insect HDLp was shown to act as a reusable lipid shuttle which is capable of reloading lipid. Although the recent identification of a lipophorin receptor (LpR), a homologue of LDLR, reveals that endocytic uptake of HDLp may constitute an additional mechanism of lipid delivery, the endocytosed lipoprotein appears to be recycled in a transferrin-like manner. Binding studies indicate that the HDLp-LpR complex, in contrast to the LDL-LDLR complex, is resistant to dissociation at endosomal pH as well as by treatment with EDTA mimicking the drop in Ca(2+) concentration in the endosome. This remarkable stability of the ligand-receptor complex may provide a crucial key to the recycling mechanism. Based on the binding and dissociation capacities of mutant and hybrid receptors, the specific binding interaction of the ligand-binding domain of the receptor with HDLp was characterized. These structural similarities and functional adaptations of the lipid transport systems operative in mammals and insects are discussed from an evolutionary perspective.

Reconstituting initial events during the assembly of apolipoprotein B-containing lipoproteins in a cell-free system

The synthesis of apolipoprotein B (apoB) dictates the formation of chylomicrons and very low-density lipoproteins, two major lipoprotein precursors in the human plasma. Despite its biological significance, the mechanism of the assembly of these apoB-containing lipoproteins remains elusive. An essential obstacle is the lack of systems that allow fine dissection of key components during assembly, including nascent apoB peptide, lipids in defined forms, chaperones, and microsomal triglyceride transfer protein (MTP). In this study, we used a prokaryotic cell-free expression system to reconstitute early events in the assembly of apoB-containing lipoprotein that involve the N-terminal domains of apoB. Our study shows that N-terminal domains larger than 20.5% of apoB (B20.5) have an intrinsic ability to remodel vesicular phospholipid bilayers into discrete protein-lipid complexes. The presence of appropriate lipid substrates during apoB translation plays a pivotal role for successful lipid recruitment, and similar lipid recruitment fails to occur if the lipids are added posttranslationally. Cotranslational presence of MTP can dramatically promote the folding of B6.4-20.5 and B6.4-22. Furthermore, apoB translated in the presence of MTP retains its phospholipid recruitment capability posttranslationally. Our data suggest that during the synthesis of apoB, the N-terminal domain has a short window for intrinsic phospholipid recruitment, the time frame of which is predetermined by the environment where apoB synthesis occurs. The presence of MTP prolongs this window of time by acting as a chaperone. The absence of either proper lipid substrate or MTP may result in the improper folding of apoB and, consequently, its degradation.

Charged amino acid residues 997-1000 of human apolipoprotein B100 are critical for the initiation of lipoprotein assembly and the formation of a stable lipidated primordial particle in McA-RH7777 cells

We previously demonstrated that a portion, or perhaps all, of the residues between 931 and 1000 of apolipoprotein (apo) B100 are required for the initiation of apoB-containing particle assembly. Based on our structural model of the first 1000 residues of apoB (designated as apoB:1000), we hypothesized that this domain folds into a three-sided lipovitellin-like "lipid pocket" via a hairpin-bridge mechanism. We proposed that salt bridges are formed between four tandem charged residues 717-720 in the turn of the hairpin bridge and four tandem complementary residues 997-1000 located at the C-terminal end of the model. To identify the specific motif within residues 931 and 1000 that is critical for apoB particle assembly, apoB:956 and apoB:986 were produced. To test the hairpin-bridge hypothesis, the following mutations were made: 1) residues 997-1000 deletion (apoB:996), 2) residues 717-720 deletion (apoB:1000Delta717-720), and 3) substitution of charged residues 997-1000 with alanines (apoB:996 + 4Ala). Characterization of particles secreted by stable transformants of McA-RH7777 cells demonstrated the following. 1) ApoB:956 did not form stable particles and was secreted as large lipid-rich aggregates. 2) ApoB:986 formed both a lipidated particle that was denser than HDL(3) and large lipid-rich aggregates. 3) Compared with wild-type apoB:1000, apoB:1000Delta717-720 displayed the following: (i) significantly diminished capacity to form intact lipidated particles and (ii) increased propensity to form large lipid-rich aggregates. 4) In striking contrast to wild-type apoB:1000, (i) apoB:996 and apoB:996 + 4Ala were highly susceptible to intracellular degradation, (ii) only a small proportion of the secreted proteins formed stable HDL(3)-like lipoproteins, and (iii) a majority of the secreted proteins formed large lipid-rich aggregates. We conclude that the first 1000 amino acid residues of human apoB100 are required for the initiation of nascent apoB-containing lipoprotein assembly, and residues 717-720 and 997-1000 play key roles in this process, perhaps via a hairpin-bridge mechanism.

New insights into the structure of apolipoprotein B from low-density lipoproteins and identification of a novel YGP-like protein in hen egg yolk

Apoproteins of low-density lipoproteins (LDL) and soluble proteins (livetins) contained in hen egg yolk plasma have been demonstrated as being essential to the interfacial and emulsifying properties of yolk. The knowledge of their structure is necessary to better understand these properties. Purified protein fractions were separated by SDS-PAGE or 2D-PAGE and identified through the LC-MS/MS of their trypsin peptides. Hen blood apolipoprotein B gives rise to nine different apoproteins in LDL after maturation and proteolysis. Among these apoproteins, two protein fragments appeared to be less accessible to proteases and could be enriched in beta-sheets and firmly associated with lipids. Plasma soluble proteins were constituted by approximately 45% of yolk immunoglobulins with a high heterogeneity of the variable regions of both heavy and light chains, 41% of glycoproteins constituted by YGP42 and YGP40, 14% of albumins, and one new minor protein we called YGP30, showing 75% similarity to YGP40.

Systematic comparison of two novel, thiol-reactive prosthetic groups for 18F labeling of peptides and proteins with the acylation agent succinimidyl-4-[18F]fluorobenzoate ([18F]SFB)

A systematic comparison of 4-[18F]fluorobenzaldehyde-O-(2-{2-[2-(pyrrol-2,5-dione-1-yl)ethoxy]-ethoxy}-ethyl)oxime ([18F]FBOM) and 4-[18F]fluorobenzaldehyde-O-[6-(2,5-dioxo-2,5-dihydro-pyrrol-1-yl)-hexyl]oxime ([18F]FBAM) as prosthetic groups for the mild and efficient 18F labeling of cysteine-containing peptides and proteins with the amine-group reactive acylation agent, succinimidyl-4-[18F]fluorobenzoate ([18F]SFB), is described. All three prosthetic groups were prepared in a remotely controlled synthesis module. Synthesis of [18F]FBOM and [18F]FBAM was accomplished via oxime formation through reaction of appropriate aminooxy-functionalized labeling precursors with 4-[18F]fluorobenzaldehyde. The obtained radiochemical yields were 19% ([18F]FBOM) and 29% ([18F]FBAM), respectively. Radiolabeling involving [18F]FBAM and [18F]FBOM was exemplified by the reaction with cysteine-containing tripeptide glutathione (GSH), a cysteine-containing dimeric neurotensin derivative, and human native low-density lipoprotein (nLDL) as model compounds. Radiolabeling with the acylation agent [18F]SFB was carried out using a dimeric neurotensin derivative and nLDL. Both thiol-group reactive prosthetic groups show significantly better labeling efficiencies for the peptides in comparison with the acylation agent [18F]SFB. The obtained results demonstrate that [18F]FBOM is especially suited for the labeling of hydrophilic cysteine-containing peptides, whereas [18F]FBAM shows superior labeling performance for higher molecular weight compounds as exemplified for nLDL apolipoprotein constituents. However, the acylation agent [18F]SFB is the preferred prosthetic group for labeling nLDL under physiological conditions.

Insect lipoprotein biogenesis depends on an amphipathic β cluster in apolipophorin II/I and is stimulated by microsomal triglyceride transfer protein

Lipoproteins transport lipids in the circulation of an evolutionally wide diversity of animals. The pathway for lipoprotein biogenesis has been revealed to a large extent in mammals only, in which apolipoprotein B (apoB) acquires lipids via the assistance of microsomal triglyceride transfer protein (MTP) and binds them by means of amphipathic protein structures. To investigate whether this is a common mechanism for lipoprotein biogenesis in animals, we studied the structural elements involved in the assembly of the insect lipoprotein, lipophorin. LOCATE sequence analysis predicted that the insect lipoprotein precursor, apolipophorin II/I (apoLp-II/I), contains clusters of amphipathic alpha-helices and beta-strands, organized along the protein as N-alpha(1)-beta-alpha(2)-C, reminiscent of a truncated form of apoB. Recombinant expression of a series of C-terminal truncation variants of Locusta migratoria apoLp-II/I in an insect cell (Sf9) expression system revealed that the formation of a buoyant high density lipoprotein requires the amphipathic beta cluster. Coexpression of apoLp-II/I with the MTP homolog of Drosophila melanogaster affected insect lipoprotein biogenesis quantitatively as well as qualitatively, as the secretion of apoLp-II/I proteins was increased several-fold and the buoyant density of the secreted lipoprotein decreased concomitantly, indicative of augmented lipidation. Based on these findings, we propose that, despite specific modifications, the assembly of lipoproteins involves MTP as well as amphipathic structures in the apolipoprotein carrier, both in mammals and insects. Thus, lipoprotein biogenesis in animals appears to rely on structural elements that are of early metazoan origin.

The physiological and molecular regulation of lipoprotein assembly and secretion

Triglycerides are insoluble in water and yet are transported at milligram per millilitre concentrations in the bloodstream. This is made possible by the ability of the liver and intestine to assemble lipid-protein emulsions (i.e. lipoproteins), which transport hydrophobic molecules. The assembly of triglyceride-rich lipoproteins requires the coordination of protein and lipid synthesis, which occurs on the cytoplasmic surface of the endoplasmic reticulum (ER), and their concerted assembly and translocation into the luminal ER secretory pathway as nascent lipoprotein particles. The availability of lipid substrate for triglyceride production and the machinery for lipoprotein assembly are highly sensitive to nutritional, hormonal, and genetic modulation. Disorders in lipid metabolism or an imbalance between lipogenesis and lipoprotein assembly can lead to hyperlipidemia and/or hepatic steatosis. We selectively review recently-identified machinery, such as transcription factors and nuclear hormone receptors, which provide new clues to the regulation of lipoprotein secretion.

Structural analysis of reconstituted lipoproteins containing the N-terminal domain of apolipoprotein B

Apolipoproteins play a central role in lipoprotein metabolism, and are directly implicated in cardiovascular diseases, but their structural characterization has been complicated by their structural flexibility and heterogeneity. Here we describe the structural characterization of the N-terminal region of apolipoprotein B (apoB), the major protein component of very low-density lipoprotein and low-density lipoprotein, in the presence of phospholipids. Specifically, we focus on the N-terminal 6.4-17% of apoB (B6.4-17) complexed with the phospholipid dimyristoylphosphatidylcholine in vitro. In addition to circular dichroism spectroscopy and limited proteolysis, our strategy incorporates nanogold-labeling of the protein in the reconstituted lipoprotein complex followed by visualization and molecular weight determination with scanning transmission electron microscopy imaging. Based on the scanning transmission electron microscopy imaging analysis of approximately 1300 individual particles where the B6.4-17 is labeled with nanogold through a six-His tag, most complexes contain either two or three B6.4-17 molecules. Circular dichroism spectroscopy and limited proteolysis of these reconstituted particles indicate that there are no large conformational changes in B6.4-17 upon lipoprotein complex formation. This is in contrast to the large structural changes that occur during apolipoprotein A-I-lipid interactions. The method described here allows a direct measurement of the stoichiometry and molecular weight of individual particles, rather than the average of the entire sample. Thus, it represents a useful strategy to characterize the structure of lipoproteins, which are not structurally uniform, but can still be defined by an ensemble of related patterns.

Theoretical model of human apolipoprotein B100 tertiary structure

Low density lipoprotein (LDL) particles are the main cholesterol carriers in human plasma. The organization of the particle, composed of apolar lipids and phospholipid monolayer stabilized by apolipoprotein B100 (apoB), is highly complex and still unknown. ApoB is an extremely large protein (4563 amino acids) and very little is known about its structure. A 3D model of the N-terminal region has been recently proposed and has provided interesting insights about the physico-chemical properties of the protein and putative interaction zones with lipids. In the present article, we propose the first tentative 3D modelling for most remaining residues. All predicted features emerging from the models are confronted with agreement to experimental data available. Using different up-to-date prediction methods, we decomposed the protein into eight domains and predicted 3D structure for each of them. The analysis of hydrophobic patches, polar regions, coupled with functional predictions based on the 3D models, gives new clues to understanding of the functional role of apoB. We suggest precise regions putatively involved in the lipid interactions, and discuss the position of apoB on the LDL particle. Finally, we propose relative organization of the domains, providing a shape quite compatible with the low resolution electron microscopy map.

Defining lipid-interacting domains in the N-terminal region of apolipoprotein B

Apolipoprotein B (apoB) is a nonexchangeable apolipoprotein that dictates the synthesis of chylomicrons and very low density lipoproteins. ApoB is the major protein in low density lipoprotein, also known as the "bad cholesterol" that is directly implicated in atherosclerosis. It has been suggested that the N-terminal domain of apoB plays a critical role in the formation of apoB-containing lipoproteins through the initial recruitment of phospholipids in the endoplasmic reticulum. However, very little is known about the mechanism of lipoprotein nucleation by apoB. Here we demonstrate that a strong phospholipid remodeling function is associated with the predicted alpha-helical and C-sheet domains in the N-terminal 17% of apoB (B17). Using dimyristoylphosphatidylcholine (DMPC) as a model lipid, these domains can convert multilamellar DMPC vesicles into discoidal-shaped particles. The nascent particles reconstituted from different apoB domains are distinctive and compositionally homogeneous. This phospholipid remodeling activity is also observed with egg phosphatidylcholine (egg PC) and is therefore not DMPC-dependent. Using kinetic analysis of the DMPC clearance assay, we show that the identified phospholipid binding sequences all map to the surface of the lipid binding pocket in the B17 model based on the homologous protein, lipovitellin. Since both B17 and microsomal triglyceride transfer protein (MTP), a critical chaperone during lipoprotein assembly, are homologous with lipovitellin, the identification of these phospholipid remodeling sequences in B17 provides important insights into the potential mechanism that initiates the assembly of apoB-containing lipoproteins.

Translocation Efficiency of Apolipoprotein B Is Determined by the Presence of β-Sheet Domains, Not Pause Transfer Sequences

Cotranslational translocation of apoB100 across the endoplasmic reticulum (ER) membrane is inefficient, resulting in exposure of nascent apoB on the cytosolic surface of the ER. This predisposes apoB100 to ubiquitinylation and targeting for proteasomal degradation. It has been suggested that pause transfer sequences (PTS) present throughout apoB cause inefficient translocation. On the other hand, our previous study demonstrated that the translocation efficiency of apoB100 is dependent on the presence of a beta-sheet domain between 29 and 34% of full-length apoB100 (Liang, J.-S., Wu, X., Jiang, H., Zhou, M., Yang, H., Angkeow, P., Huang, L.-S., Sturley, S. L., and Ginsberg, H. N. (1998) J. Biol. Chem. 273, 35216-35221); this region of apoB has no PTS. However, the effects of the beta-sheet domain may require the presence of PTS elsewhere in the N-terminal region of apoB100. To further investigate the roles of PTS and beta-sheet domains in the translocation of apoB100 across the ER, we transfected McArdle RH7777, HepG2, or Chinese hamster ovary cells with human albumin (ALB)/human apoB chimeric cDNA constructs: ALB/B12-17 (two PTS but no beta-sheet), ALB/B29-34 (beta-sheet but no PTS), ALB/B36-41 (two PTS and a beta-sheet), and ALB/B49-54 (neither PTS nor a beta-sheet). ALB/ALB1-40 served as a control. Compared with ALB/ALB1-40, secretion rates of ALB/B12-17, ALB/B29-34, and ALB/B36-41 were reduced. Secretion of ALB/B49-54 was similar to that of ALB/ALB1-40. However, only ALB/B29-34 and ALB/B36-41 had increased proteinase K sensitivity, ubiquitinylation, and increased physical interaction with Sec61alpha. These results indicate that the translocation efficiency of apoB100 is determined mainly by the presence of beta-sheet domains. PTS do not appear to affect translocation, but may affect secretion by other mechanisms.

Modular Structure of Solubilized Human Apolipoprotein B-100

Being intimately involved in cholesterol transport and lipid metabolism human low density lipoprotein (LDL) plays a prominent role in atherogenesis and cardiovascular diseases. The receptor-mediated cellular uptake of LDL is triggered by apolipoprotein B-100 (apoB-100), which represents the single protein moiety of LDL. Due to the size and hydrophobic nature of apoB-100, its structure is not well characterized. Here we present a low resolution structure of solubilized apoB-100. We have used small angle neutron scattering in combination with advanced shape reconstruction algorithms to generate a three-dimensional model of lipid-free apoB-100. Our model clearly reveals that apoB-100 is composed of distinct domains connected by flexible regions. The apoB-100 molecule adopts a curved shape with a central cavity. In comparison to LDL-associated apoB-100, the lipid-free protein is expanded, whereas according to spectroscopic data the secondary structure is widely preserved. Finally, the low resolution model was used as a template to reconstruct a hypothetical domain organization of apoB-100 on LDL, including information derived from a secondary structure prediction.

Protein components of low-density lipoproteins purified from hen egg yolk

To identify apoproteins present in purified low-density lipoproteins from hen egg yolk in relation with their emulsifying properties, they have been separated by SDS-PAGE. We identified two different proteins by liquid chromatography-tandem mass spectrometry analysis of the peptides obtained by the trypsin digestion of protein gel bands. Apovitellenin I was identified as a monomer and a dimer. Its amino acid sequence was totally confirmed, and molecular mass determination by liquid chromatography-mass spectrometry showed that it did not present post-translational modifications but only a slight heterogeneity by the loss of one or two amino acids at the C-terminal part of the protein. Apolipoprotein B was identified into seven bands corresponding to fragments resulting of a processing of the hen blood apo-B protein. The identity of the fragments was determined by the observation of the sequence coverage by trypsin peptides and the sequence alignment with homologous human blood apolipoprotein B-100.

Apolipoprotein B is conformationally flexible but anchored at a triolein/water interface: a possible model for lipoprotein surfaces

Apolipoprotein B (apoB) is one of a unique group of proteins that form and bind to fat droplets, stabilize the emulsified fat, and direct their metabolism. ApoB, secreted on lipoproteins (emulsions), remains bound during lipid metabolism yet exhibits conformational flexibility. It has amphipathic beta-strand (AbetaS)-rich domains and amphipathic alpha-helix (AalphaH)-rich domains. We showed that two consensus AbetaS peptides of apoB bound strongly to hydrophobic interfaces [triolein/water (TO/W) and dodecane/water], were elastic, and were not pushed off the interface when the surface was compressed. In contrast, an AalphaH peptide modeling helical parts of apoB was forced off the TO/W interface by compression and readsorbed when the interface was expanded. In this report, the surface behavior of apoB-100 was studied at the TO/W interface. Solubilized apoB lowered the interfacial tension of TO/W in a concentration-dependent fashion. At equilibrium tension, if the surface was compressed, part of apoB was pushed off but quickly readsorbed when the surface was expanded. Even when the surface area was compressed by approximately 55%, part of the apoB molecule remained bound. The maximum surface pressure that apoB could withstand without being partially ejected was 13 mN/m. ApoB showed high elasticity at the TO/W interface. Based on studies of the consensus AbetaS and AalphaH peptides, we suggest that AbetaSs anchor apoB and are its nonexchangeable motif, whereas its conformational flexibility arises from both the elastic nature of the AbetaS and the ability of AalphaH domains of the molecule to desorb and readsorb rapidly in response to surface pressure changes.

Phospholipid Transfer Activity of Microsomal Triacylglycerol Transfer Protein Is Sufficient for the Assembly and Secretion of Apolipoprotein B Lipoproteins

Human microsomal triacylglycerol transfer protein (hMTP) is essential for apolipoprotein B (apoB)-lipoprotein assembly and secretion and is known to transfer triacylglycerols, cholesterol esters, and phospholipids. To understand the relative importance of each lipid transfer activity, we compared the ability of hMTP and its Drosophila ortholog (dMTP) to assemble apoB lipoproteins and to transfer various lipids. apoB48 secretion was induced when co-expressed with either hMTP or dMTP in COS cells, and oleic acid supplementation further augmented secretion without altering particle density. C-terminal epitope-tagged dMTP (dMTP-FLAG) facilitated the secretion of apoB polypeptides in the range of apoB48 to apoB72 but was approximately 50% as efficient as hMTP-FLAG. Comparison of lipid transfer activities revealed that although phospholipid transfer was similar in both orthologs, dMTP was unable to transfer neutral lipids. We conclude that the phospholipid transfer activity of MTP is sufficient for the assembly and secretion of primordial apoB lipoproteins and may represent its earliest function evolved for the mobilization of lipid in invertebrates. Identification of MTP inhibitors, which selectively affect transfer of a specific lipid class, may have therapeutic potential.

Integral apolipoproteins increase surface-located triacylglycerol in intact native apoB-100-containing lipoproteins

High-resolution NMR was used to measure the presence and quantity of triacylglycerol (TAG) in the surface of intact native apolipoprotein B-100-containing lipoprotein particles that are made by chickens in response to estrogen treatment and that in hens are deposited in yolk follicles (VLDLy). Integration of 13C NMR resonances shows that intact VLDLy particles contain more surface TAG (5.1 +/- 0.6 mol%, 6.7 +/- 0.8 weight %) than predicted by apolipoprotein-free models using similarly acyl-heterogenous TAG. Change in downfield chemical shift values of surface to core TAG in VLDLy was 0.8 ppm compared with 1.3 ppm in vesicles prepared with purified egg phosphatidylcholine and TAG isolated from the VLDLy, indicating that reduced surface TAG hydration may contribute to the resistance to lipase hydrolysis characteristic of this lipoprotein species. Apolipoprotein-mediated changes in surface lipid composition and lipid hydration provide possible general mechanisms for selectivity in lipoprotein substrate characteristics.

Assembly of lipoprotein particles containing apolipoprotein-B: structural model for the nascent lipoprotein particle

Apolipoprotein B (apoB) is the major protein component of large lipoprotein particles that transport lipids and cholesterol. We have developed a detailed model of the first 1000 residues of apoB using standard sequence alignment programs (ClustalW and MACAW) and the MODELLER6 package for three-dimensional homology modeling. The validity of the apoB model was supported by conservation of disulfide bonds, location of all proline residues in turns and loops, and conservation of the hydrophobic faces of the two C-terminal amphipathic beta-sheets, betaA (residues 600-763) and betaB (residues 780-1000). This model suggests a lipid-pocket mechanism for initiation of lipoprotein particle assembly. In a previous model we suggested that microsomal triglyceride transfer protein might play a structural role in completion of the lipid pocket. We no longer think this likely, but instead propose a hairpin-bridge mechanism for lipid pocket completion. Salt-bridges between four tandem charged residues (717-720) in the turn of the hairpin-bridge and four tandem complementary residues (997-1000) at the C-terminus of the model lock the bridge in the closed position, enabling the deposition of an asymmetric bilayer within the lipid pocket.

Site-specific glycosylation analysis of human apolipoprotein B100 using LC/ESI MS/MS

Human apolipoprotein B100 (apoB100) has 19 potential N-glycosylation sites, and 16 asparagine residues were reported to be occupied by high-mannose type, hybrid type, and monoantennary and biantennary complex type oligosaccharides. In the present study, a site-specific glycosylation analysis of apoB100 was carried out using reversed-phase high-performance liquid chromatography coupled with electrospray ionization tandem mass spectrometry (LC/ESI MS/MS). ApoB100 was reduced, carboxymethylated, and then digested by trypsin or chymotrypsin. The complex mixture of peptides and glycopeptides was subjected to LC/ESI MS/MS, where product ion spectra of the molecular ions were acquired data-dependently. The glycopeptide ions were extracted and confirmed by the presence of carbohydrate-specific fragment ions, such as m/z 204 (HexNAc) and 366 (HexHexNAc), in the product ion spectra. The peptide moiety of glycopeptide was determined by the presence of the b- and y-series ions derived from its amino acid sequence in the product ion spectrum, and the oligosaccharide moiety was deduced from the calculated molecular mass of the oligosaccharide. The heterogeneity of carbohydrate structures at 17 glycosylation sites was determined using this methodology. Our data showed that Asn2212, not previously identified as a site of glycosylation, could be glycosylated. It was also revealed that Asn158, 1341, 1350, 3309, and 3331 were occupied by high-mannose type oligosaccharides, and Asn 956, 1496, 2212, 2752, 2955, 3074, 3197, 3438, 3868, 4210, and 4404 were predominantly occupied by mono- or disialylated oligosaccharides. Asn3384, the nearest N-glycosylation site to the LDL-receptor binding site (amino acids 3359-3369), was occupied by a variety of oligosaccharides, including high-mannose, hybrid, and complex types. These results are useful for understanding the structure of LDL particles and oligosaccharide function in LDL-receptor ligand binding.

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.