Apolipoprotein C-I modulates the interaction of apolipoprotein E with beta-migrating very low density lipoproteins (beta-VLDL) and inhibits binding of beta-VLDL to low density lipoprotein receptor-related protein

The binding of native rabbit beta-very low density lipoproteins (beta-VLDL) to the low density lipoprotein receptor-related protein (LRP) requires incubation with exogenous apolipoprotein (apo) E. Inclusion of a mixture of the C apolipoproteins in the incubation inhibits this binding. In the present study, the ability of the individual C apolipoproteins (C-I, C-II, and C-III) to block binding of beta-VLDL to the LRP was examined by measuring cholesteryl ester formation in mutant fibroblasts that lack low density lipoprotein receptors or by measuring binding to the LRP using ligand blotting. In each assay, both apoC-I and apoC-II inhibited binding; apoC-I was the more effective inhibitor. Apolipoprotein C-III had no effect on binding activity, regardless of its sialylation level. Binding of human apoE to rabbit beta-VLDL in the absence or presence of human apoC-I, apoC-II, and monosialo-apoC-III was also determined, by gel filtration and sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The results of these studies are consistent with a mechanism in which exogenous human apoE displaces the endogenous apoE and the beta-VLDL particle becomes enriched with apoE (by 4.2-fold in this study). At this higher apoE content, the beta-VLDL bound to the LRP. Inclusion of apoC-I, apoC-II, or apoC-III in the incubation mixture resulted in a differential displacement of apoE from the beta-VLDL; however, at the concentrations examined, only apoC-I and apoC-II were capable of displacing sufficient apoE to abolish binding to LRP.     

Structure and expression of dog apolipoprotein A-I, E, and C-I mRNAs: implications for the evolution and functional constraints of apolipoprotein structure

Dog apolipoprotein (apo) C-I, A-I, and E cDNA clones were identified in a dog liver cDNA library in lambda gt10 by hybridization to synthetic oligonucleotide probes with the corresponding human DNA sequences. The longest clone for each apolipoprotein was completely sequenced. The apoC-I cDNA sequence predicts a protein of 62 residue mature peptide preceded by a 26 amino acid signal peptide. The apoA-I cDNA sequence predicts a 242 residue mature peptide, a 6 residue pro-segment, and an 18 residue signal peptide. The apoE cDNA, which lacks the signal peptide region, predicts a mature peptide of 291 amino acid residues. Slot blot hybridization of total RNA isolated from various dog tissues to dog apoC-I, A-I, and E cDNA probes indicates that apoC-I mRNA is detectable in liver only, apoA-I mRNA is present in liver and small intestine, though the concentration in the latter tissue is only approximately 15% of that in the liver, and apoE mRNA is present in multiple tissues including liver, jejunum, urinary bladder, ileum, colon, brain, kidney, spleen, pancreas, and testis with relative concentrations (%) of 100, 17.5, 7.5, 6.9, 5.9, 5.5, 5.0, 3.3, 1.0, and 1.0, respectively. These tissue distributions indicate that nascent lipoprotein particles produced in the dog small intestine would contain apoA-I and apoE but not apoC-I. The widespread tissue distribution of apoE mRNA indicates that like other mammals, peripheral synthesis of apoE contributes significantly to the total apoE pool in dog. We next compared the cDNA sequences among different vertebrate species for apoC-I (human and dog), A-I (human, rat, dog, rabbit and chicken), and E (human, rat, dog and rabbit) and calculated the rate of nucleotide substitution for each gene. Our results indicate that apoC-I has evolved rather rapidly and that on the whole, apoA-I is more conservative than apoE, contradictory to an earlier suggestion. ApoA-I is also more conservative than a region (residues 4204-4536) at the carboxyl-terminal portion, but less conservative than a region (residues 595-979) at the amino-terminal portion of apoB-100. Some regions in each of the apolipoproteins studied are better conserved than others and the rate of evolution of individual regions seems to be related to the stringency of functional requirements. Finally, we estimate that the human apoC-I pseudogene arose more than 35 million years ago, becoming nonfunctional soon after its formation.     

Mechanisms of inhibition by apolipoprotein C of apolipoprotein E-dependent cellular metabolism of human triglyceride-rich lipoproteins through the low density lipoprotein receptor pathway

The mechanism of inhibition by apolipoprotein C of the uptake and degradation of triglyceride-rich lipoproteins from human plasma via the low density lipoprotein (LDL) receptor pathway was investigated in cultured human skin fibroblasts. Very low density lipoprotein (VLDL) density subfractions and intermediate density lipoprotein (IDL) with or without added exogenous recombinant apolipoprotein E-3 were used. Total and individual (C-I, C-II, C-III-1, and C-III-2) apoC molecules effectively inhibited apoE-3-mediated cell metabolism of the lipoproteins through the LDL receptor, with apoC-I being most effective. When the incubation was carried out with different amounts of exogenous apoE-3 and exogenous apoC, it was shown that the ratio of apoE-3 to apoC determined the uptake and degradation of VLDL. Excess apoE-3 overcame, at least in part, the inhibition by apoC. ApoC, in contrast, did not affect LDL metabolism. Neither apoA-I nor apoA-II, two apoproteins that do not readily associate with VLDL, had any effect on VLDL cell metabolism. The inhibition of VLDL and IDL metabolism cannot be fully explained by interference of association of exogenous apoE-3 with or displacement of endogenous apoE from the lipoproteins. IDL is a lipoprotein that contains both apoB-100 and apoE. By using monoclonal antibodies 4G3 and 1D7, which specifically block cell interaction by apoB-100 and apoE, respectively, it was possible to assess the effects of apoC on either apoprotein. ApoC dramatically depressed the interaction of IDL with the fibroblast receptor through apoE, but had only a moderate effect on apoB-100. The study thus demonstrates that apoC inhibits predominantly the apoE-3-dependent interaction of triglyceride-rich lipoproteins with the LDL receptor in cultured fibroblasts and that the mechanism of inhibition reflects association of apoC with the lipoproteins and specific concentration-dependent effects on apoE-3 at the lipoprotein surface.     

NMR structure and dynamics of a receptor-active apolipoprotein E peptide

Apolipoprotein E (apoE) is important in lipid metabolism due to its interaction with members of the low density lipoprotein (LDL) receptor family. ApoE is able to interact with the LDL receptor only when it is bound to lipid particles. To address structural aspects of this phenomenon, a receptor-active apoE peptide, encompassing the receptor-binding region of the protein, was studied by NMR in the presence of the lipid-mimicking agent trifluoroethanol. In 50% trifluoroethanol, apoE-(126-183) forms a continuous amphipathic alpha-helix over residues Thr(130)-Glu(179). Detailed NMR relaxation analysis indicates a high degree of plasticity for the residues surrounding 149-159. This intrinsic flexibility imposes a curvature to the peptide that may be important in terms of interaction of apoE with various sized lipid particles and the LDL receptor. Residues 165-179 of apoE may act as a molecular switch whereby these residues are unstructured in the absence of lipids and prevent interaction with the LDL receptor. In the presence of lipids, these residues become helical resulting in a receptor-active conformation of the protein. Furthermore, the electrostatic characteristics and geometric features of apoE-(126-183) suggest that apoE binds to the LDL receptor by interacting with more than one of the receptor ligand-binding repeats.     

Apolipoprotein E Structure and Substrate and Receptor-Binding Activities of Triglyceride-Rich Human Plasma Lipoproteins in Normo- and Hypertriglyceridemia

Cysteine-arginine interchanges along the primary sequence of human plasma apolipoprotein E (apoE) play an important role in determining its biological functions due to a high mutation frequency of cytosine in CGX triplet that codes 33 of 34 apolipoprotein arginine residues. The contribution of apoE secondary structure to apolipoprotein-lipid interaction is described. The significance of apolipoprotein in triglyceride synthesis, lipoprotein lipolysis, and receptor-mediated clearance of lipolytic remnants of triglyceride-rich lipoproteins is discussed as well. The metabolic flow of lipoproteins in normo- and hypertriglyceridemia can be described by separate compartments that contribute to lipoprotein interaction with at least six different receptors: 1) low density lipoprotein (LDL) receptor; 2) LDL receptor-related protein (LRP); 3) apoB(48) macrophage receptor for hypertriglyceridemic very low density lipoproteins (VLDL); 4) scavenger receptors; 5) VLDL receptor; 6) lipolysis-stimulated receptor. The contribution of the exposure of apoE molecules on the surface of triglyceride-rich particles sensitive both to lipolysis and plasma triglyceride content to the interaction with LDL receptor and LRP is emphasized.     

Two copies of the human apolipoprotein C-I gene are linked closely to the apolipoprotein E gene

The gene for human apolipoprotein (apo) C-I was selected from human genomic cosmid and lambda libraries. Restriction endonuclease analysis showed that the gene for apoC-I is located 5.5 kilobases downstream of the gene for apoE. A copy of the apoC-I gene, apoC-I', is located 7.5 kilobases downstream of the apoC-I gene. Both genes contain four exons and three introns; the apoC-I gene is 4653 base pairs long, the apoC-I' gene 4387 base pairs. In each gene, the first intron is located 20 nucleotides upstream from the translation start signal; the second intron, within the codon of Gly-7 of the signal peptide region; and the third intron, within the codon for Arg39 of the mature plasma protein coding region. The upstream apoC-I gene encodes the known apoC-I plasma protein and differs from the downstream apoC-I' gene in about 9% of the exon nucleotide positions. The most important difference between the exons results in a change in the codon for Gln-2 of the signal peptide region, which introduces a translation stop signal in the downstream gene. Major sequence differences are found in the second and third introns of the apoC-I and apoC-I' genes, which contain 9 and 7.5 copies, respectively, of Alu family sequences. The apoC-I gene is expressed primarily in the liver, and it is activated when monocytes differentiate into macrophages. In contrast, no mRNA product of the apoC-I' gene can be detected in any tissue, suggesting that it may be a pseudogene. The similar structures and the proximity of the apoE and apoC-I genes suggest that they are derived from a common ancestor. Furthermore, they may be considered to be constituents of a family of seven apolipoprotein genes (apoE, -C-I, -C-II, -C-III, -A-I, -A-II, and -A-IV) that have a common evolutionary origin.     

Apolipoprotein E gene mapping and expression: localization of the structural gene to human chromosome 19 and expression of ApoE mRNA in lipoprotein- and non-lipoprotein-producing tissues

Apolipoprotein E (apoE) binds to specific cell-surface receptors and appears to be an important determinant in lipoprotein metabolism in man. Cloned human apoE cDNA (pAE155) was used as a probe in chromosome mapping studies to detect the structural gene sequences in human--Chinese hamster cell hybrids. Southern blot analysis of HincII-digested DNAs from 13 hybrids localized the gene to human chromosome 19. This observation indicates that apoE is syntenic to at least two other genes related to lipid metabolism, those for the low-density lipoprotein (LDL) receptor (the LDLR) and apoC-II. The cloned apoE cDNA was further used to detect the presence of apoE mRNA in RNA extracts of various human and baboon tissues. Northern gel analysis using the 32P-labeled pAE155 as a probe demonstrated the presence of hybridizable apoE mRNAs in human liver and in baboon liver, intestine, spleen, kidney, adrenal gland, and brain but not in baboon skeletal muscle. The apoE mRNAs appear to be intact and migrate on an agarose gel under denaturing conditions at approximately 18 S. To assay for the biological activity of the apoE mRNAs in these tissues, they were translated in a reticulocyte lysate system in vitro. Immunoprecipitation with an apoE-specific antiserum followed by sodium dodecyl sulfate gel electrophoresis and fluorography demonstrated that immunoreactive apoE with the expected apparent size was a product of translation of mRNAs from baboon liver, intestine, kidney, spleen, and brain but not that from baboon skeletal muscle.(ABSTRACT TRUNCATED AT 250 WORDS)     

Linkage analysis of the genetic determinants of high density lipoprotein concentrations and composition: evidence for involvement of the apolipoprotein A-II and cholesteryl ester transfer protein loci

We have tested for evidence of linkage between the genetic loci determining concentrations and composition of plasma high density lipoproteins (HDL) with the genes for the major apolipoproteins and enzymes participating in lipoprotein metabolism. These genes include those encoding various apolipoproteins (apo), including apoA-I, apoA-II, apoA-IV, apoB, apoC-I, apoC-II, apoC-III, apoE, and apo(a), cholesteryl ester transfer protein (CETP), HDL-binding protein, lipoprotein lipase, and the low density lipoprotein (LDL) receptor. Polymorphisms of these genes, and nearby highly polymorphic simple sequence repeat markers, were examined by quantitative sib-pair linkage analysis in 30 coronary artery disease families consisting of a total of 366 individuals. Evidence for linkage was observed between a marker locus D16S313 linked to the CETP locus and a locus determining plasma HDL-cholesterol concentration (P = 0.002), and the genetic locus for apoA-II and a locus determining the levels of the major apolipoproteins of HDL, apoA-I and apoA-II (P = 0.009 and 0.02, respectively). HDL level was also influenced by the variation at the apo(a) locus on chromosome 6 (P = 0.02). Thus, these data indicate the simultaneous involvement of at least two different genetic loci in the determination of the levels of HDL and its associated lipoproteins.     

Plasma lipoproteins: apolipoprotein structure and function

Plasma lipoprotein metabolism is regulated and controlled by the specific apolipoprotein (apo-) constituents of the various lipoprotein classes. The major apolipoproteins include apoE, apoB, apoA-I, apoA-II, apoA-IV, apoC-I, apoC-II, and apoC-III. Specific apolipoproteins function in the regulation of lipoprotein metabolism through their involvement in the transport and redistribution of lipids among various cells and tissues, through their role as cofactors for enzymes of lipid metabolism, or through their maintenance of the structure of the lipoprotein particles. The primary structures of most of the apolipoproteins are now known, and various functional domains of these proteins are being mapped using selective chemical modification, synthetic peptides, and monoclonal antibodies. Furthermore, the establishment of structure-function relationships has been greatly advanced by the identification of genetically determined variants of specific apolipoproteins that are associated with a disorder of lipoprotein metabolism. Future studies will rely heavily on the use of recombinant DNA technology and site-specific mutagenesis to elucidate further the correlations between structure and function and the role of specific apolipoproteins in lipoprotein metabolism.     

Structure and evolution of the apolipoprotein multigene family

We present the complementary DNA and deduced amino acid sequence of rat apolipoprotein A-II (apoA-II), and the results of a detailed statistical analysis of the nucleotide and amino acid sequences of all the apolipoprotein gene sequences published to date: namely, those of human and rat apoA-I, apoA-II and apoE, rat apoA-IV, and human apoC-I, C-II and C-III. Our results indicate that the apolipoprotein genes have very similar genomic structures, each having a total of three introns at the same locations. Using the exon/intron junctions as reference points, we have obtained an alignment of the coding regions of all the genes studied. It appears that the mature peptide regions of these genes are almost completely made up of tandem repeats of 11 codons. The part of mature peptide region encoded by exon 3 contains a common block of 33 codons, whereas the part encoded by exon 4 contains a much more variable number of internal repeats of 11 codons. These genes have apparently evolved from a primordial gene through multiple partial (internal) and complete gene duplications. On the basis of the degree of homology of the various sequences, and the pattern of the internal repeats in these genes, we propose an evolutionary tree for the apolipoprotein genes and give rough estimates of the divergence times between these genes. Our results show that apoA-II has evolved extremely rapidly and that apoA-I and apoE also have evolved at high rates but some regions are better conserved than the others. The rate of evolution of individual regions seems to be related to the stringency of their functional requirements.     

Apolipoprotein E activates Akt pathway in neuro-2a in an isoform-specific manner

Apolipoprotein E (apoE) is a ligand for members of the low density lipoprotein (LDL) receptor family, receptors highly expressed in neurons. A study of one of the mechanisms by which apoE might affect neuronal cell metabolism is reported herein. ApoE can induce Akt/protein kinase B phosphorylation in Neuro-2a via two different pathways. Both pathways are mediated by phosphatidylinositol 3-kinase and cAMP-dependent protein kinase. The first pathway is stimulated by apoE3 and E4, but not by E2, after a 1-h incubation. The process requires the binding of apoE to the heparan sulfate proteoglycan/LDL receptor-related protein complex. The second pathway is activated after a 2-h incubation of the cells, in another isoform-dependent manner (E2 = E3 dbl greater-than sign E4) and is mediated by calcium. Our results suggest that apoE might affect cell metabolism and survival in neurons in an isoform-specific manner by inducing novel signaling pathways.     

Apolipoproteins C-I, C-II, and C-III: kinetics of association with model membranes and intermembrane transfer

The apoproteins (apo) C-I, C-II, and C-III are low molecular weight amphiphilic proteins that are associated with the lipid surface of the plasma chylomicron, very low density lipoprotein (VLDL), and high-density lipoprotein (HDL) subfractions. Purified apoC-I spontaneously reassociates with VLDL, HDL, and single-bilayer vesicles (SBV) of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine. ApoC-I also transfers reversibly from VLDL to HDL and from VLDL and HDL to SBV. The kinetics of association of the individual apoC proteins with SBV are second order overall and first order with respect to lipid and protein concentrations. At 37 degrees C, the rates of association were 2.5 x 10(10), 4.0 x 10(10) and 3.8 x 10(10) M-1 s-1 for apoC-I, apoC-II, and apoC-III, respectively. Arrhenius plots of association rate vs temperature were linear and yielded activation energies of 11.0 (apoC-I), 9.0 (apoC-II), and 10.6 kcal/mol (apoC-III). The kinetics of vesicle to vesicle apoprotein transfer are biexponential for intermembrane transfer, indicating two concurrent transfer processes. Rate constants at 37 degrees C for the fast component of dissociation were 11.7, 9.5, and 9.9 s-1, while rate constants for the slow component were 1.3, 0.6, and 0.9 s-1 for apoC-I, apoC-II, and apoC-III, respectively. The dissociation constants, Kd, of apoC-I, apoC-II, and apoC-III bound to the surface monolayer of phospholipid-coated latex beads were 0.5, 1.4, and 0.5 microM, respectively. These studies show that the apoC proteins are in dynamic equilibrium among phospholipid surfaces on a time scale that is rapid compared to lipolysis, lipid transfer, and lipoprotein turnover.     

The recycling of apolipoprotein E and its amino-terminal 22 kDa fragment: evidence for multiple redundant pathways

A portion of apolipoprotein E (apoE) internalized by hepatocytes is spared degradation and is recycled. To investigate the intracellular routing of recycling apoE, primary hepatocyte cultures from LDL receptor-deficient mice and mice deficient in receptor-associated protein [a model of depressed expression of LDL receptor-related protein (LRP)] were incubated with human VLDL containing 125I-labeled human recombinant apoE3. Approximately 30% of the internalized intact apoE was recycled after 4 h. The N-terminal 22 kDa fragment of apoE was also resecreted, demonstrating that this apoE domain contains sufficient sequence to recycle. The 22 kDa fragment has reduced affinity for lipoproteins, suggesting that apoE recycling is linked to the ability of apoE to bind directly to a recycling receptor. Finally, apoE was found to recycle equally well in the presence of brefeldin A, a drug that blocks transport from the endoplasmic reticulum and leads to collapse of the Golgi stacks. Our studies demonstrate that apoE recycling occurs 1) in the absence of the LDL receptor or under conditions of markedly reduced LRP expression; 2) when apoE lacks the carboxyl-terminal domain, which allows binding to the lipoprotein; and 3) in the absence of an intact Golgi apparatus. We conclude that apoE recycling occurs through multiple redundant pathways.     

Recycling of apolipoprotein E in mouse liver

Following the internalization of low density lipoprotein (LDL) by the LDL receptor within cells, both the lipid and the protein components of LDL are completely degraded within the lysosomes. Remnant lipoproteins are also internalized by cells via the LDL receptor as well as other receptors, but the events following the internalization of these complexes, which use apolipoprotein E (apoE) as their ligand for receptor capture, have not been defined. There is evidence that apoE-containing beta-very low density lipoproteins follow differential intracellular routing depending on their size and apoE content and that apoE internalized with lipoproteins can be resecreted by cultured hepatocytes and fibroblasts. In the present studies, we addressed the question of apoE sparing or recycling as a physiologic phenomenon. Remnant lipoproteins (d < 1.019 g/ml) from normal mouse plasma were iodinated and injected into normal C57BL/6 mice. Livers were collected at 10, 30, 60, and 120 min after injection, and hepatic Golgi fractions were prepared for gel electrophoresis analysis. Golgi preparations were analyzed for galactosyltransferase enrichment (>40-fold above cell homogenate) and by appearance of the Golgi stacks and vesicles on electron microscopy. Iodinated apoE was consistently found in the Golgi fractions peaking at 10 min and disappearing by 2 h after injection. Although traces of apoB48 were present in the Golgi fractions, the apoE/apoB ratio in the Golgi was 50-fold higher compared with serum. Quantitatively similar results were obtained when the very low density lipoprotein remnants were injected into mice deficient in either apoE or the LDL receptor, indicating that the phenomenon of apoE recycling is not influenced by the production of endogenous apoE and is not dependent on the presence of LDL receptors. In addition, radioactive apoE in the Golgi fractions was part of d = 1.019-1.21 g/ml complexes, indicating an association of recycled apoE with either newly formed lipoproteins or the internalized complexes. These studies show that apoE recycling is a physiologic phenomenon in vivo and establish the presence of a unique pathway of intracellular processing of apoE-containing remnant lipoproteins.     

Abnormal low density lipoprotein metabolism in apolipoprotein E deficiency

Apolipoprotein(apo) E deficiency is an inherited disease characterized by type III hyperlipoproteinemia and less than 1% normal plasma apoE concentration. The role of apoE in LDL metabolism was investigated by quantitating the metabolism of radiolabeled normal and apoE-deficient LDL in both normal and apoE-deficient subjects. ApoE deficiency resulted in an accumulation of plasma IDL, and a decreased synthesis of LDL consistent with a block in the conversion of IDL to LDL. The LDL isolated from the apoE-deficient patient was similar to normal LDL in hydrated density, size, and composition. However, the apoE-deficient LDL was kinetically abnormal with delayed catabolism in both normal subjects and the apoE-deficient patient. In addition, the catabolism of normal LDL in the apoE-deficient subject was increased. These results were interpreted as indicating that apoE is necessary for the conversion of IDL to LDL and the formation of kinetically normal LDL. The rapid catabolism of normal LDL in the apoE-deficient patient suggests an up-regulation of the hepatic LDL receptor pathway. Based on these results, apoE is proposed to play an important role in the conversion of IDL to LDL, the formation of kinetically normal LDL, and the regulation of LDL receptor function.     

Structural basis for the recognition and cross-linking of amyloid fibrils by human apolipoprotein E

Apolipoprotein (apo) E is a well characterized lipid-binding protein in plasma that also exists as a common nonfibrillar component of both cerebral and systemic amyloid deposits. A genetic link between a common isoform of apoE, apoE4, and the incidence of late onset Alzheimer disease has drawn considerable attention to the potential roles of apoE in amyloid-related disease. We examined the interactions of apoE with amyloid fibrils composed of apoC-II and the amyloid-beta (Abeta) peptide. Aggregates of apoE with Abeta and apoC-II are found in Alzheimer and atherosclerotic plaques, respectively. Sedimentation velocity and fibril size distribution analysis showed that apoE3 and E4 isoforms bind and noncovalently cross-link apoC-II fibrils in a similar manner. This ability to cross-link apoC-II fibrils was abolished by the dissociation of the apoE tetramer to monomers or by thrombin cleavage to yield separate N- and C-terminal domains. Preparative ultracentrifuge binding studies indicated that apoE and the isolated N- and C-terminal domains of apoE bind with submicromolar affinities to both apoC-II and Abeta fibrils. Fluorescence quenching and resonance energy transfer experiments confirmed that both domains of apoE interact with apoC-II fibrils and demonstrated that the binding of the isolated N-terminal domain of apoE to apoC-II or Abeta fibrils is accompanied by a significant conformational change with helix three of the domain moving relative to helix one. We propose a model involving the interaction of apoE with patterns of aligned residues that could explain the general ability of apoE to bind to a diverse range of amyloid fibrils.     

Structural characterization of a low density lipoprotein receptor-active apolipoprotein E peptide, ApoE3-(126-183)

Apolipoprotein E (apoE) plays a critical role in lipoprotein particle clearance from blood plasma through its interaction with the low density lipoprotein (LDL) receptor and other related receptors. Here, we studied a 58-residue peptide encompassing the receptor binding region of apoE. ApoE3-(126-183) was generated by cyanogen bromide cleavage of recombinant apoE3-(1-183), purified by reversed-phase high pressure liquid chromatography, and characterized by mass spectrometry. Far UV CD spectroscopy of the peptide showed that it is unstructured in aqueous solution. The addition of trifluoroethanol or dodecylphosphocholine induces the peptide to adopt an alpha-helical conformation. ApoE3-(126-183) efficiently transforms dimyristoylphosphatidylglycerol (DMPG) vesicles into peptide-lipid complexes. Analysis of apoE3-(126-183). DMPG complexes by electron microscopy revealed disc-shaped particles with an average diameter of 13 +/- 3 nm. Flotation equilibrium analysis yielded a particle molecular mass of 252 kDa. Far UV CD analysis of apoE3-(126-183).DMPG discs provided evidence that the peptide adopts a helical conformation. Competition binding experiments with (125)I-labeled low density lipoprotein (LDL) were conducted to assess the ability of apoE3-(126-183).DMPG complexes to bind to the LDL receptor. Both N-terminal apoE and the peptide, when complexed with DMPG, competed with (125)I-LDL for binding sites on the surface of cultured human skin fibroblasts. Under the conditions employed, apoE3-(126-183).DMPG complexes were similar to apoE3-(1-183).DMPG discs in their ability to bind to the receptor, demonstrating that the peptide represents a good model to study the interaction between apoE and the LDL receptor. Preliminary NMR results indicated that a high resolution structure of the apoE3-(126-183) peptide is obtainable.