Cryo-electron microscopy reveals human low density lipoprotein substructure

Radical Oxygen Species, Oxidized Low-Density Lipoproteins, and Lectin-like Oxidized Low-Density Lipoprotein Receptor 1: A Vicious Circle in Atherosclerotic Process

Atherosclerosis is a complex condition that involves the accumulation of lipids and subsequent plaque formation in the arterial intima. There are various stimuli, cellular receptors, and pathways involved in this process, but oxidative modifications of low-density lipoprotein (ox-LDL) are particularly important in the onset and progression of atherosclerosis. Ox-LDLs promote foam-cell formation, activate proinflammatory pathways, and induce smooth-muscle-cell migration, apoptosis, and cell death. One of the major receptors for ox-LDL is LOX-1, which is upregulated in several cardiovascular diseases, including atherosclerosis. LOX-1 activation in endothelial cells promotes endothelial dysfunction and induces pro-atherogenic signaling, leading to plaque formation. The binding of ox-LDLs to LOX-1 increases the generation of reactive oxygen species (ROS), which can induce LOX-1 expression and oxidize LDLs, contributing to ox-LDL generation and further upregulating LOX-1 expression. This creates a vicious circle that is amplified in pathological conditions characterized by high plasma levels of LDLs. Although LOX-1 has harmful effects, the clinical significance of inhibiting this protein remains unclear. Further studies both in vitro and in vivo are needed to determine whether LOX-1 inhibition could be a potential therapeutic target to counteract the atherosclerotic process.

High Hydrostatic Pressure Induces a Lipid Phase Transition and Molecular Rearrangements in Low-Density Lipoprotein Nanoparticles

Low-density lipoproteins (LDL) are natural lipid transporter in human plasma whose chemically modified forms contribute to the progression of atherosclerosis and cardiovascular diseases accounting for a vast majority of deaths in westernized civilizations. For the development of new treatment strategies, it is important to have a detailed picture of LDL nanoparticles on a molecular basis. Through the combination of X-ray and neutron small-angle scattering (SAS) techniques with high hydrostatic pressure (HHP) this study describes structural features of normolipidemic, triglyceride-rich and oxidized forms of LDL. Due to the different scattering contrasts for X-rays and neutrons, information on the effects of HHP on the internal structure determined by lipid rearrangements and changes in particle shape becomes accessible. Independent pressure and temperature variations provoke a phase transition in the lipid core domain. With increasing pressure an inter-related anisotropic deformation and flattening of the particle are induced. All LDL nanoparticles maintain their structural integrity even at 3000 bar and show a reversible response toward pressure variations. The present work depicts the complementarity of pressure and temperature as independent thermodynamic parameters and introduces HHP as a tool to study molecular assembling and interaction processes in distinct lipoprotein particles in a nondestructive manner.

LDL dinitrosyl iron complex: A new transferrin-independent route for iron delivery in hepatocytes

In view of the interrelations between NO, Fe, and LDL in the cardiovascular system it appears interesting to find out, if the lipoprotein particles undergo the process of iron-nitrosylation, commonly observed for other proteins and what is the biological fate of iron-nitrosylated LDL particles. Iron-nitrosylated LDL preparation containing Fe(NO)₂ motif (DNICLDL) was obtained and characterized for the first time. In order to test its interactions with potential target cells, DNICLDL was administered to the hepatoma HepG2 cells. The effects were referred to those induced by native LDL (nLDL) and oxidized LDL (oxLDL) particles. DNICLDL administration considerably increased total iron content in the studied cell line, but did not influence the level of calcein-chelatable ions. DNICLDL was found to be low toxic to cells. The study suggests that DNICLDL might be a potential transducer of iron. © 2017 BioFactors, 44(2):192-201, 2018.

Advantages and Limitations of Current Imaging Techniques for Characterizing Liposome Morphology

There are currently a number of imaging techniques available for evaluating the morphology of liposomes and other nanoparticles, with each having its own advantages and disadvantages that should be considered when interpreting data. Controlling and validating the morphology of nanoparticles is of key importance for the effective clinical translation of liposomal formulations. There are a number of physical characteristics of liposomes that determine their in vivo behavior, including size, surface characteristics, lamellarity, and homogeneity. Despite the great importance of the morphology of nanoparticles, it is generally not well-characterized and is difficult to control. Appropriate imaging techniques provide important details regarding the morphological characteristics of nanoparticles, and should be used in conjunction with other methods to assess physicochemical parameters. In this review, we will discuss the advantages and limitations of available imaging techniques used to evaluate liposomal formulations.

Circulating Lipoproteins: A Trojan Horse Guiding Squalenoylated Drugs to LDL-Accumulating Cancer Cells

Selective delivery of anticancer drugs to rapidly growing cancer cells can be achieved by taking advantage of their high receptor-mediated uptake of low-density lipoproteins (LDLs). Indeed, we have recently discovered that nanoparticles made of the squalene derivative of the anticancer agent gemcitabine (SQGem) strongly interacted with the LDLs in the human blood. In the present study, we showed both in vitro and in vivo that such interaction led to the preferential accumulation of SQGem in cancer cells (MDA-MB-231) with high LDL receptor expression. As a result, an improved pharmacological activity has been observed in MDA-MB-231 tumor-bearing mice, an experimental model with a low sensitivity to gemcitabine. Accordingly, we proved that the use of squalene moieties not only induced the gemcitabine insertion into lipoproteins, but that it could also be exploited to indirectly target cancer cells in vivo.

Conjugation of squalene to gemcitabine as unique approach exploiting endogenous lipoproteins for drug delivery

Once introduced in the organism, the interaction of nanoparticles with various biomolecules strongly impacts their fate. Here we show that nanoparticles made of the squalene derivative of gemcitabine (SQGem) interact with lipoproteins (LPs), indirectly enabling the targeting of cancer cells with high LP receptors expression. In vitro and in vivo experiments reveal preeminent affinity of the squalene-gemcitabine bioconjugates towards LP particles with the highest cholesterol content and in silico simulations further display their incorporation into the hydrophobic core of LPs. To the best of our knowledge, the use of squalene to induce drug insertion into LPs for indirect cancer cell targeting is a novel concept in drug delivery. Interestingly, not only SQGem but also other squalene derivatives interact similarly with lipoproteins while such interaction is not observed with liposomes. The conjugation to squalene represents a versatile platform that would enable efficient drug delivery by simply exploiting endogenous lipoproteins.

The interaction of nanoparticles with a range of biomolecules once they have been injected within the body can affect their performance. Here, the authors demonstrate that squalene nanomaterials conjugated with anticancer drugs can interact with lipoproteins and can be used to target cancer cells.

Effect of Trimethylamine N-Oxide on Interfacial Electrostatics at Phospholipid Monolayer-Water Interfaces and Its Relevance to Cardiovascular Disease

Trimethylamine N-oxide (TMAO), a metabolite of choline containing dietary nutrients which are abundant in red meat, egg, and other animal foods, increases the risk of cardiovascular disease (e.g., atherosclerosis) by boosted accumulation of fatty deposits on artery wall. Hence, for the molecular level elucidation of the pathogenesis of atherosclerosis, it is important to understand the effect of TMAO at the endothelial cell membrane-blood interface (artery wall). Heterodyne-detected vibrational sum frequency generation (HD-VSFG) study of a zwitterionic phosphatidylcholine (PC) lipid monolayer-water interface (mimic of endothelial membrane-blood interface) shows that the interfacial water becomes increasingly H-up oriented in the presence of TMAO in the aqueous phase, revealing a dramatic change in the interfacial electrostatics. Examinations of charged lipid interfaces show that TMAO screens anionic phosphate less effectively than cationic choline, which confirms that TMAO increases the relative influence of the anionic phosphate by preferential screening of the cationic choline at the zwitterionic PC lipid interface where the phosphate and choline groups are simultaneously present. Together, it is conceivable that at an elevated TMAO level in serum would modify the electrostatics at the endothelial cell membrane-blood interface (artery wall), which may affect the influx/efflux of fatty deposits on artery wall, setting the stage for atherosclerosis.

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.

Engineering nanomaterials to address cell-mediated inflammation in atherosclerosis

Atherosclerosis is an inflammatory disorder with a pathophysiology driven by both innate and adaptive immunity and a primary cause of cardiovascular disease (CVD) worldwide. Vascular inflammation and accumulation of foam cells and their products induce maturation of atheromas, or plaques, which can rupture by metalloprotease action, leading to ischemic stroke or myocardial infarction. Diverse immune cell populations participate in all stages of plaque maturation, many of which directly influence plaque stability and rupture via inflammatory mechanisms. Current clinical treatments for atherosclerosis focus on lowering serum levels of low-density lipoprotein (LDL) using therapeutics such as statins, administration of antithrombotic drugs, and surgical intervention. Strategies that address cell-mediated inflammation are lacking, and consequently have recently become an area of considerable research focus. Nanomaterials have emerged as highly advantageous tools for these studies, as they can be engineered to target specific inflammatory cell populations, deliver therapeutics of wide-ranging solubilities and enhance analytical methods that include imaging and proteomics. Furthermore, the highly phagocytic nature of antigen presenting cells (APCs), a diverse cell population central to the initiation of immune responses and inflammation, make them particularly amenable to targeting and modulation by nanoscale particulates. Nanomaterials have therefore become essential components of vaccine formulations and treatments for inflammation-driven pathologies like autoimmunity, and present novel opportunities for immunotherapeutic treatments of CVD. Here, we review recent progress in the design and use of nanomaterials for therapeutic assessment and treatment of atherosclerosis. We will focus on promising new approaches that utilize nanomaterials for cell-specific imaging, gene therapy and immunomodulation.

LDL and HDL transfer rates across peripheral microvascular endothelium agree with those predicted for passive ultrafiltration in humans

The mechanisms by which LDLs and HDLs cross the vascular endothelium from plasma into interstitial fluid are not understood, and have never been studied in humans in vivo. We determined whether the plasma-to-lymph clearance rates of LDL and HDL conform with those predicted by passive ultrafiltration through intercellular pores, or if it is necessary to invoke an active process such as receptor-mediated transcytosis. Plasma and afferent peripheral lymph were collected under steady-state conditions from 30 healthy men, and assayed for seven globular proteins of molecular radii 2.89-8.95 nm, complement C3, and apo AI, apo AII, and apo B. Plasma-to-lymph clearance rates of the seven proteins fitted the relation expected for molecules of their size when transported through two populations of pores of radius 4.95 and 20.1 nm. The same model parameters were then found to accurately predict the clearance rates of both HDL and LDL. The apparent clearance of complement C3, previously shown to be secreted by cultured endothelium, exceeded that predicted by the model. We conclude that the transport of HDL and LDL from plasma into interstitial fluid across the peripheral vascular endothelium in healthy humans can be explained by ultrafiltration without invoking an additional active process such as transcytosis.

Ordering and stability in lipid droplets with applications to low-density lipoproteins

In this article, we present a framework for investigating the order-disorder transition in lipid droplets using the standard Ising model. While a single lipid droplet is itself a complex system whose constituent cholesteryl esters each possesses many degrees of freedom, we present justification for using this effective approach to isolate the underlying physics. It is argued that the behavior of the esters confined within lipid droplets is significantly different from that of a bulk system of similar esters, which is adequately described by continuum mean-field theory in the thermodynamic limit. When the droplet's shell is modeled as an elastic membrane, a simple picture emerges for a transition between two ordered phases within the core which is tuned by the strength of interactions between the esters. Triglyceride concentration is proposed as a variable which strongly influences the strength of interactions between cholesteryl esters within droplets. The possible relevance of this mechanism to the well known atherogenic nature of small low-density lipoprotein particles is discussed in detail.

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.

Morphological characterization and fusion properties of triglyceride-rich lipoproteins obtained from cells transduced with hepatitis C virus glycoproteins

The density of hepatitis C virus (HCV) particles circulating in the blood of chronically infected patients and of cell-culture produced HCV is heterogeneous. Specific infectivity and fusion of low density particles are higher than those of high density particles. We recently characterized hybrid particles produced by Caco-2 colon or Huh-7.5 liver cells transduced with HCV E1 and E2 envelope glycoproteins. Caco-2-derived particles, called empty lipo-viral particles (eLVP), are composed of triglyceride-rich lipoproteins positive for apolipoproteins B (i.e. apoB100 and apoB48) and contain HCV E1 and E2. Here we aimed at characterizing the morphology and in vitro fusion properties of eLVP using electron microscopy and fluorescence spectroscopy. They displayed the aspect of beta-lipoproteins, and immunogold labeling confirmed the presence of apoB and HCV E1 and E2 at their surface. These particles are able to fuse with lipid bilayers (liposomes) in a fusion process leading to the coalescence of internal contents of triglyceride-rich lipoproteins particles and liposomes. Fusion was pH-dependent and could be inhibited by either Z-fFG, a peptide known to inhibit viral fusion, or by monoclonal antibodies directed against HCV E2 or the apolipoprotein moiety of the hybrid particle. Interestingly, particles derived from Huh-7.5 cells failed to display equivalent efficient fusion. Optimal fusion activity is, thus, observed when HCV envelope proteins are associated to apoB-positive hybrid particles. Our results, therefore, point to a crucial role of the E1 and E2 proteins in HCV fusion with a subtle interplay with the apolipoprotein part of eLVP.

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.

The assembly of apoB-containing lipoproteins: a structural biology point of view

Atherosclerosis is a widespread disease caused by the deposition of lipids on arterial walls. Such lipid plaques in coronary arteries can be fatal. Although many factors related to diet, life-style, etc. contribute to the worsening of the ailment, the primary cause, the lipids in the circulatory system, come from a series of low-density lipoproteins. These lipoproteins are necessary for the transport of lipids to and from different organs. It would be valuable to medicine and the field of drug design if a more detailed understanding of the organization of lipid and protein in these molecules were available. Unfortunately because of heterogeneity in their size and lipid composition, all classes of the low-density serum lipoproteins appear to be not amenable to the most widely used method for obtaining detailed atomic data - X-ray crystallography. However there appears to be a recently identified homolog that is relatively homogeneous, and crystal structures have been obtained. Used as a molecular model, the homolog serves as a source of conformational information that might help to unravel the processes involved in the lipid loading of the low-density lipoproteins. The review attempts to give a brief summary of the structural biology of the serum low-density lipoproteins relative to the molecular model of lipovitellin.

Method of imaging low density lipoproteins by atomic force microscopy

This short paper reports a simple method to image low density lipoproteins (LDL) using atomic force microscopy (AFM). This instrument allows imaging of biological samples in liquid and presents the advantage of needing no sample preparation such as staining or fixation that may affect their general structure. Dimensions (diameter and height) of individual LDL particles were successfully measured. AFM imaging revealed that LDL have a quasi-spherical structure on the x and y axis with an oblate spheroid structure in the z axis (i.e., height). LDLs were found to have an average diameter of 23 +/- 3 nm. The obtained mean height was 10 +/- 2 nm.

Miniaturization of asymmetrical flow field-flow fractionation and application to studies on lipoprotein aggregation and fusion

Asymmetrical flow field-flow fractionation (AsFlFFF), a technique that provides direct measurement of particle size and diffusion coefficient, is converted into miniaturized scale. In comparison with conventional AsFlFFF, the separation of proteins in miniaturized AsFlFFF is achieved within shorter time periods, with smaller sample amounts, and with lower mobile phase consumption. Minimization of the overloading and optimization of the separation efficiency are prerequisites to good results. Miniaturized AsFlFFF is applied to the measurement of particle sizes of high-density lipoprotein (HDL), low-density lipoprotein (LDL), and very low-density lipoprotein (VLDL). The average hydrodynamic diameters at pH 7.4 in 8.5mM phosphate buffer containing 1mM EDTA and 150 mM NaCl are 8.6+/-0.5, 11.2+/-0.2, 22.1+/-0.7, and 48.9+/-7.5 nm for subgroups HDL3, HDL2, LDL, and VLDL, respectively. In addition, the effect of different factors on the aggregation and fusion of LDL particles is studied. LDL particle sizes are unaffected by the addition of up to 300 mM NaCl and by an increase of the carrier solution pH from 3.2 to 7.4, but treatment of LDL with alpha-chymotrypsin, sphingomyelinase, or copper sulfate leads to the formation of aggregated and fused LDL particles.

Lipid composition influences the shape of human low density lipoprotein in vitreous ice

Earlier cryo-electron microscopic studies have indicated that the normal low density lipoprotein (N-LDL) has a discoid shape when its core is in the liquid-crystalline state. In the present study, we investigated whether the shape of LDL depends on the physical state and/or the lipid composition of the lipoprotein core. Using a custom-built freezing device, we vitrified NLDL samples from either above or below the phase-transition temperature of the core (42 and 24 degrees C, respectively). Cryo-electron microscopy revealed no differences between these samples and indicated a discoid shape of the N-LDL particle. In contrast, TG-enriched LDL (T-LDL) did not have discoid features and appeared to be quasi-spherical in preparations that were vitrified from either 42 or 24 degrees C. These results suggest that the shape of NLDL is discoid, regardless of the physical state of its core, whereas T-LDL is more spherical. Aspects that may influence the shape of LDL are discussed.

Preferred orientations of LDL in vitreous ice indicate a discoid shape of the lipoprotein particle

The structure of the human low-density lipoprotein (LDL) was analyzed in vitreous ice using cryo-electron microscopy (cryo-EM). In relatively thick cryo-EM preparations, random orientation of LDL particles produced various types of projections on the microscope screen, including circular projections with a high-density ring and rectangular projections with two high-density bands. However, in especially thin preparations, preferred, non-random orientations of the LDL particle produced only circular projections of the lipoprotein structure. In preparations with high LDL concentrations, ordered two-dimensional arrays, including hexagonal arrangements of circular projections and short stacks of rectangular projections, were observed. These observations are consistent with a discoid shape of the LDL particle, and suggest that surface tension forces may influence orientation of the LDL disc in thin aqueous films. Face-on orientation of LDL in especially thin cryo-EM preparations may explain earlier difficulties in identifying discoid features of the lipoprotein particle, and illustrates that some caution is warranted when attempts are made to reconstruct the three-dimensional structure of LDL from cryo-electron micrographs.

Combined data from LDL composition and size measurement are compatible with a discoid particle shape

The size of LDL is usually reported as particle diameter, with the implicit assumption that it is a spherical particle. On the other hand, data obtained by cryoelectron microscopy and crystallographic analysis suggest that LDL shape may be discoid. We have investigated LDL particle geometry by combining data on LDL lipid composition with size measurement. The mean LDL diameter of 160 samples was measured by high-performance gel-filtration chromatography (HPGC), and particle volume was calculated from its lipid composition. Assuming a spherical shape, diameters calculated from volume correlated poorly with values obtained by HPGC (R(2) = 0.36). Assuming a discoid shape, particle height was calculated from volume and HPGC diameter. Diameter (20.9 +/- 0.5 nm) and height (12.1 +/- 0.8 nm) were not significantly related to each other (r = 0.14, P = 0.09) and accounted for 23% and 77%, respectively, of the variation in particle volume. In multivariate regression models, LDL core lipids were the main determinants of height (R(2) = 0.83), whereas free cholesterol in the shell, which contributes only 5-9% to LDL mass, was the main determinant of diameter (R(2) = 0.54). We conclude that combined data from composition and size measurements are compatible with a discoid particle shape and propose a structural model for LDL in which free cholesterol plays a major role in determining particle shape and diameter.

The physical state of the LDL core influences the conformation of apolipoprotein B-100 on the lipoprotein surface

We assessed the influence of temperature on the secondary structure of apolipoprotein B-100 (apoB) in normal low-density lipoprotein (N-LDL) and triglyceride-rich LDL (T-LDL). Gradual heating from 7 degrees C to the phase-transition temperature of the lipoprotein core ( approximately 28 degrees C and approximately 15 degrees C for N-LDL and T-LDL, respectively) gradually altered the secondary structure of apoB, while further heating from the phase-transition temperature to 45 degrees C had no additional effect. Above the phase-transition temperature of the core, the apoBs of N-LDL and T-LDL had a similar secondary structure. These results indicate that the conformation of apoB on the LDL surface depends strongly on the physical state of the lipoprotein core, and less on the lipid composition of the core per se.

Three-dimensional structure of low density lipoproteins by electron cryomicroscopy

Human low density lipoproteins (LDL) are the major cholesterol carriers in the blood. Elevated concentration of LDL is a major risk factor for atherosclerotic disease. Purified LDL particles appear heterogeneous in images obtained with a 400-kV electron cryomicroscope. Using multivariate statistical and cluster analyses, an ensemble of randomly oriented particle images has been subdivided into homogeneous subpopulations, and the largest subset was used for three-dimensional reconstruction. In contrast to the general belief that below the lipid phase-transition temperature (30 degrees C) LDL are quasi-spherical microemulsion particles with a radially layered core-shell organization, our three-dimensional map shows that LDL have a well-defined and stable organization. Particles consist of a higher-density outer shell and lower-density inner lamellae-like layers that divide the core into compartments. The outer shell consists of apolipoprotein B-100, phospholipids, and some free cholesterol.

A new type of highly polymerized yolk protein from the cochineal insect Dactylopius confusus

A female specific protein was isolated from eggs and female hemolymph of cochineal insects, using density gradient ultracentrifugation, ammonium sulfate precipitation, and size exclusion column chromatography. The protein was found to consist of four different subunits with apparent molecular weights (Mr) 45,000, 49,000, 53,000, and 56,000, respectively. All four subunits were found to be glycosylated; no association of lipids was detected. Size exclusion column chromatography and non-denaturing polyacrylamide gel electrophoresis demonstrated that the native yolk protein exists as large polymers. Electron microscopy showed that these molecules are long, helical ribbons of variable size which are found in both hemolymph and eggs. Using cryo-electron microscopy, it was shown that the ribbons were 14.6 +/- 1.5 nm wide; the helix they form has a repeat distance of 104.9 +/- 11.3 nm and a diameter of 42.1 +/- 5 nm. A clear substructure of the ribbons was recognized. The newly identified protein is the major yolk protein of Dactylopius confusus and no other proteins resembling the more familiar vitellins of other insect species were detected. Moreover, the D. confusus yolk protein appears to be unique both in its subunit structure and in its polymerizing qualities. Thus, the cochineal yolk protein (CYP) is suggested to represent a new type of insect yolk protein.