Endocytosis of apolipoprotein A-V by members of the low density lipoprotein receptor and the VPS10p domain receptor families

Apolipoprotein A-V (apoA-V) is present in low amounts in plasma and has been found to modulate triacylglycerol levels in humans and in animal models. ApoA-V displays affinity for members of the low density lipoprotein receptor (LDL-R) gene family, known as the classical lipoprotein receptors, including LRP1 and SorLA/LR11. In addition to LDL-A binding repeats, the mosaic receptor SorLA/LR11 also possesses a Vps10p domain. Here we show that apoA-V also binds to sortilin, a receptor from the Vsp10p domain gene family that lacks LDL-A repeats. Binding of apoA-V to sortilin was competed by neurotensin, a ligand that binds specifically to the Vps10p domain. To investigate the biological fate of receptor-bound apoA-V, binding experiments were conducted with cultured human embryonic kidney cells transfected with either SorLA/LR11 or sortilin. Compared with nontransfected cells, apoA-V binding to SorLA/LR11- and sortilin-expressing cells was markedly enhanced. Internalization experiments, live imaging studies, and fluorescence resonance energy transfer analyses demonstrated that labeled apoA-V was rapidly internalized, co-localized with receptors in early endosomes, and followed the receptors through endosomes to the trans-Golgi network. The observed decrease of fluorescence signal intensity as a function of time during live imaging experiments suggested ligand uncoupling in endosomes with subsequent delivery to lysosomes for degradation. This interpretation was supported by experiments with (125)I-labeled apoA-V, demonstrating clear differences in degradation between transfected and nontransfected cells. We conclude that apoA-V binds to receptors possessing LDL-A repeats and Vsp10p domains and that apoA-V is internalized into cells via these receptors. This could be a mechanism by which apoA-V modulates lipoprotein metabolism in vivo.

Intracellular lipid droplet targeting by apolipoprotein A-V requires the carboxyl-terminal segment

The expression of apolipoprotein A-V (apoA-V) in hepatoma cells results in homing of this protein to intracellular lipid droplets. When hepatoma cells transfected with a full-length apoA-V-green fluorescent protein fusion protein were cultured in medium that was not supplemented with oleic acid (OA), intracellular lipid droplet size and number were reduced compared with those of cells supplemented with OA. Confocal microscopy studies revealed that apoA-V associates with lipid droplets under both conditions. To define the structural requirements for apoA-V lipid droplet association, hepatoma cells were transfected with a series of C-terminal truncated apoA-V variants. Confocal microscopy analysis revealed that, in a manner similar to mature full-length apoA-V (343 amino acids), truncation variants apoA-V(1-292), apoA-V(1-237), and apoA-V(1-191) associated with lipid droplets, while apoA-V(1-146) did not. Western blot analysis of the relative abundance of apoA-V in cell lysates versus conditioned medium indicated that apoA-V variants associated with lipid droplets were poorly secreted while apoA-V(1-146) was efficiently secreted. Ultracentrifugation of conditioned medium revealed that, unlike full-length apoA-V, which associates with lipoproteins, apoA-V(1-146) was present solely in the lipoprotein-deficient fraction. Deletion of the N-terminal signal peptide from apoA-V resulted in an inability of the protein to be secreted into the medium, although it associated with lipid droplets. Taken together, these data suggest that the C terminus of apoA-V is essential for lipid droplet association in transfected hepatoma cells and lipoprotein association in conditioned medium while the signal peptide is required for extracellular trafficking of this protein.

Glycosylation of Asn-76 in mouse GPIHBP1 is critical for its appearance on the cell surface and the binding of chylomicrons and lipoprotein lipase

GPIHBP1 is a glycosylphosphatidylinositol-anchored protein in the lymphocyte antigen 6 (Ly-6) family that recently was identified as a platform for the lipolytic processing of triglyceride-rich lipoproteins. GPIHBP1 binds both LPL and chylomicrons and is expressed on the luminal face of microvascular endothelial cells. Here, we show that mouse GPIHBP1 is N-glycosylated at Asn-76 within the Ly-6 domain. Human GPIHBP1 is also glycosylated. The N-linked glycan could be released from mouse GPIHBP1 with N-glycosidase F, endoglycosidase H, or endoglycosidase F1. The glycan was marginally sensitive to endoglycosidase F2 digestion but resistant to endoglycosidase F3 digestion, suggesting that the glycan on GPIHBP1 is of the oligomannose type. Mutating the N-glycosylation site in mouse GPIHBP1 results in an accumulation of GPIHBP1 in the endoplasmic reticulum and a markedly reduced amount of the protein on the cell surface. Consistent with this finding, cells expressing a nonglycosylated GPIHBP1 lack the ability to bind LPL or chylomicrons. Eliminating the N-glycosylation site in a truncated soluble version of GPIHBP1 causes a modest reduction in the secretion of the protein. These studies demonstrate that N-glycosylation of GPIHBP1 is important for the trafficking of GPIHBP1 to the cell surface.

Normal binding of lipoprotein lipase, chylomicrons, and apo-AV to GPIHBP1 containing a G56R amino acid substitution

GPIHBP1 is an endothelial cell protein that serves as a platform for lipoprotein lipase-mediated processing of triglyceride-rich lipoproteins within the capillaries of heart, adipose tissue, and skeletal muscle. The absence of GPIHBP1 causes severe chylomicronemia. A hallmark of GPIHBP1 is the ability to bind lipoprotein lipase, chylomicrons, and apolipoprotein (apo-) AV. A homozygous G56R mutation in GPIHBP1 was recently identified in two siblings with chylomicronemia, and the authors of that study suggested that the G56R substitution was responsible for the hyperlipidemia. In this study, we created a human GPIHBP1 expression vector, introduced the G56R mutation, and tested the ability of the mutant GPIHBP1 to reach the cell surface and bind lipoprotein lipase, chylomicrons, and apo-AV. Our studies revealed that the G56R substitution did not affect the ability of GPIHBP1 to reach the cell surface, nor did the amino acid substitution have any discernible effect on the binding of lipoprotein lipase, chylomicrons, or apo-AV.

Amphotericin B induces interdigitation of apolipoprotein stabilized nanodisk bilayers

Amphotericin B nanodisks (AMB-ND) are ternary complexes of AMB, phospholipid and apolipoprotein organized as discrete nanometer scale disk-shaped bilayers. In gel filtration chromatography experiments, empty ND lacking AMB elute as a single population of particles with a molecular weight in the range of 200 kDa. AMB-ND formulated at a 4:1 phospholipid:AMB weight ratio separated into two peaks. One peak eluted at the position of control ND lacking AMB while the second peak, containing all of the AMB present in the original sample, eluted in the void volume. When ND prepared with increased AMB were subjected to gel filtration chromatography an increased proportion of phospholipid and apolipoprotein was recovered in the void volume with AMB. Native gradient gel electrophoresis corroborated the gel filtration chromatography data and electron microscopy studies revealed an AMB concentration-dependent heterogeneity in ND particle size. Stability studies revealed that introduction of AMB into ND decreases the ability of apoA-I to resist denaturation. Atomic force microscopy experiments showed that AMB induces compression of ND bilayer thickness while infrared spectroscopy analysis revealed that the presence of AMB does not induce extreme lipid disorder or alter the mean angle of the molecular axis along fatty acyl chains of ND phospholipids. Taken together the results are consistent with AMB-induced bilayer interdigitation, a phenomenon that likely contributes to AMB-dependent pore formation in susceptible membranes.

Apolipoprotein E isoform-specific binding to the low-density lipoprotein receptor

Apolipoprotein E (apoE) is a ligand for members of the low-density lipoprotein receptor (LDLR) family and functions in plasma cholesterol homeostasis. A fluorescence-based assay has been employed in molecular studies of receptor-ligand interactions. Competition experiments revealed isoform-specific differences in binding of lipid-associated apoE N terminal (NT) domain to a recombinant soluble LDLR (sLDLR). In a similar manner, lipid--associated-but not lipid-free--full-length apoE3 showed binding activity to sLDLR. The molecular chaperone, receptor-associated protein, inhibited apoE3-NT-phospholipid complex binding to sLDLR. Kinetic studies of apoE3-NT-phospholipid complex interaction with sLDLR revealed time-dependent effects of apoE-NT isoform binding to sLDLR. The results reveal a discerning method for study of the molecular basis of ligand interactions that likely influence receptor function in maintenance of whole body cholesterol homeostasis.

Expressed protein ligation using an N-terminal cysteine containing fragment generated in vivo from a pelB fusion protein

Advances in expressed protein ligation (EPL) methods that permit specific introduction of unique modifications into proteins have facilitated protein engineering, structure-function and protein interaction studies. An EPL-generated hybrid exchangeable apolipoprotein has been constructed from recombinant fragments of apolipoprotein E (apoE) and apolipophorin III (apoLp-III). A recombinant fusion protein comprised of human apoE N-terminal residues 1-111, a modified Saccharomyces cerevisiae intein and a chitin binding domain was subjected to 2-mercaptoethanesulfonic acid (MESNA) induced cleavage to generate apoE(1-111)-MESNA. A second fusion protein was comprised of a bacterial pelB leader peptide fused to a variant form of Galleria mellonella apoLp-III residues 1-91. The N-terminal pelB leader sequence directed the newly synthesized fusion protein to the Escherichia coli perisplamic space where endogenous leader peptidase cleavage generated the desired N-terminal cysteine-containing protein fragment. The resulting apoLp-III fragment, which contained no sequence tags or tails, escaped the bacteria and accumulated in the culture medium. When cultured in M9 minimal medium, Asp1Cys apoLp-III(1-91) was produced in high yield and was the sole major protein in the culture supernatant. Ligation reactions with apoE(1-111)-MESNA yielded an engineered hybrid apolipoprotein. The results document the utility of the pelB fusion protein system for generating active N-terminal cysteine containing proteins for EPL applications.

Characterization of apolipoprotein A-V structure and mode of plasma triacylglycerol regulation

PURPOSE OF REVIEW

Cardiovascular disease is the leading cause of death in the USA, and hypertriglyceridemia represents an independent risk factor contributing to its premature onset. Apolipoprotein (apo)A-V has been shown to be a potent regulator of plasma triacylglycerol. We highlight structural aspects of apoA-V and discuss recent findings that provide mechanistic insight into its function as a regulator of plasma triacylglycerol metabolism.

RECENT FINDINGS

Recent findings indicate that apoA-V is comprised of two independently folded domains. Fluorescence spectroscopy and truncation analysis revealed that the carboxyl-terminal region functions in apoA-V lipid binding, consistent with its known association with plasma lipoproteins. An indirect triacylglycerol-modulating effect of apoA-V has been attributed to heparan sulfate proteoglycan binding, as confirmed by structural studies. Furthermore, apoA-V has been shown to interact with cell surface receptors, potentially facilitating lipoprotein particle endocytosis.

SUMMARY

Several features of apoA-V, including extremely low plasma concentration, lack of correlation with plasma cholesterol levels despite its association with HDL, and insolubility at neutral pH in the absence of lipid, are unlike those of other exchangeable apolipoproteins. Current and future studies of apoA-V will help to shed light on the molecular basis whereby this protein functions to modulate plasma lipid homeostasis.

The C Terminus of Apolipoprotein A-V Modulates Lipid-binding Activity

Human apolipoprotein A-V (apoA-V) is a potent modulator of plasma triacylglycerol (TG) levels. To probe different regions of this 343-amino-acid protein, four single Trp apoA-V variants were prepared. The variant with a Trp at position 325, distal to the tetraproline sequence at residues 293-296, displayed an 11-nm blue shift in wavelength of maximum fluorescence emission upon lipid association. To evaluate the structural and functional role of this C-terminal segment, a truncated apoA-V comprising amino acids 1-292 was generated. Far UV circular dichroism spectra of full-length apoA-V and apoA-V-(1-292) were similar, with approximately 50% alpha-helix content. In guanidine HCl denaturation experiments, both full-length and truncated apoA-V yielded biphasic profiles consistent with the presence of two structural domains. The denaturation profile of the lower stability component (but not the higher stability component) was affected by truncation. Truncated apoA-V displayed an attenuated ability to solubilize l-alpha-dimyristoylphosphatidylcholine phospholipid vesicles compared with full-length apoA-V, whereas a peptide corresponding to the deleted C-terminal segment displayed markedly enhanced kinetics. The data support the concept that the C-terminal region is not required for apoA-V to adopt a folded protein structure, yet functions to modulate apoA-V lipid-binding activity; therefore, this concept may be relevant to the mechanism whereby apoA-V influences plasma TG levels.

Apolipoprotein A-V association with intracellular lipid droplets

Apolipoprotein A-V (apoA-V) plays a key role in the regulation of triglyceride (TG) metabolism. Given the very low concentration of apoA-V in plasma, we hypothesized that apoA-V may influence plasma TG levels by affecting the assembly and/or secretion of apoB-containing lipoproteins. When apoA-V was overexpressed in cultured Hep3B cells, neither the amount of apoB secreted nor the density distribution of apoB-containing lipoproteins was affected. Fluorescence microscopy and cell lysate immunoprecipitation studies revealed that apoA-V is not associated with apoB intracellularly, yet immunoprecipitation of apoA-V from the cell culture medium resulted in coprecipitation of apoB. These data suggest that the apoA-V association with apoB-containing lipoproteins is a postsecretory event. Confocal fluorescence microscopy revealed the presence of apoA-V in distinct cellular structures. Based on Nile Red staining, we identified these structures to be intracellular lipid droplets. These data suggest that apoA-V has a unique association with cellular lipids and, therefore, may be involved in the storage or mobilization of intracellular lipids.

Peptide stabilized amphotericin B nanodisks

Nanometer scale apolipoprotein A-I stabilized phospholipid disk complexes (nanodisks; ND) have been formulated with the polyene antibiotic amphotericin B (AMB). The present studies were designed to evaluate if a peptide can substitute for the function of the apolipoprotein component of ND with respect to particle formation and stability. An 18-residue synthetic amphipathic alpha-helical peptide, termed 4F (Ac-D-W-F-K-A-F-Y-D-K-V-A-E-K-F-K-E-A-F-NH(2)), solubilized vesicles comprised of egg phosphatidylcholine (egg PC), dipentadecanoyl PC or dimyristoylphosphatidylcholine (DMPC) at rates greater than or equal to solubilization rates observed with human apolipoprotein A-I (apoA-I; 243 amino acids). Characterization studies revealed that interaction with DMPC induced a near doubling of 4F tryptophan fluorescence emission quantum yield (excitation 280 nm) and a approximately 7 nm blue shift in emission wavelength maximum. Inclusion of AMB in the vesicle substrate resulted in formation of 4F AMB-ND. Spectra of AMB containing particles revealed the antibiotic is a highly effective quencher of 4F tryptophan fluorescence emission, giving rise to a Ksv=7.7 x 10(4). Negative stain electron microscopy revealed that AMB-ND prepared with 4F possessed a disk shaped morphology similar to ND prepared without AMB or prepared with apoA-I. In yeast and pathogenic fungi growth inhibition assays, 4F AMB-ND was as effective as apoA-I AMB-ND. The data indicate that AMB-ND generated using an amphipathic peptide in lieu of apoA-I form a discrete population of particles that possess potent biological activity. Given their intrinsic versatility, peptides may be preferred for scale up and clinical application of AMB-ND.

Glycosylphosphatidylinositol-anchored high-density lipoprotein-binding protein 1 plays a critical role in the lipolytic processing of chylomicrons

The triglycerides in chylomicrons are hydrolyzed by lipoprotein lipase (LpL) along the luminal surface of the capillaries. However, the endothelial cell molecule that facilitates chylomicron processing by LpL has not yet been defined. Here, we show that glycosylphosphatidylinositol-anchored high-density lipoprotein-binding protein 1 (GPIHBP1) plays a critical role in the lipolytic processing of chylomicrons. Gpihbp1-deficient mice exhibit a striking accumulation of chylomicrons in the plasma, even on a low-fat diet, resulting in milky plasma and plasma triglyceride levels as high as 5000 mg/dl. Normally, Gpihbp1 is expressed highly in heart and adipose tissue, the same tissues that express high levels of LpL. In these tissues, GPIHBP1 is located on the luminal face of the capillary endothelium. Expression of GPIHBP1 in cultured cells confers the ability to bind both LpL and chylomicrons. These studies strongly suggest that GPIHBP1 is an important platform for the LpL-mediated processing of chylomicrons in capillaries.

All-trans-retinoic acid nanodisks

Nanodisks are nanoscale, disk-shaped phospholipid bilayers whose edge is stabilized by association of apolipoprotein molecules. Self-assembled ND particles enriched with all-trans-retinoic acid (ATRA) (phospholipid:ATRA molar ratio = 5.5:1) were generated wherein all reaction components were solubilized. ATRA-ND migrated as a single band (Stokes' diameter approximately 20 nm) on native gradient polyacrylamide gel electrophoresis. ATRA, phospholipid and apolipoprotein co-eluted from a Sepharose 6B gel filtration column, consistent with stable integration of ATRA into the ND particle milieu. Spectroscopic analysis of ATRA-ND in buffer yielded an absorbance spectrum characteristic of ATRA. ATRA-ND mediated time-dependent inhibition of cultured HepG2 cell growth more effectively than free ATRA. The nanoscale size of the formulation particles and the stable integration of biologically active ATRA suggest ND represent a potentially useful vehicle for solubilization and in vivo delivery of ATRA.

Apolipoprotein A-V Interaction with Members of the Low Density Lipoprotein Receptor Gene Family†

Apolipoprotein A-V is a potent modulator of plasma triacylglycerol levels. To investigate the molecular basis for this phenomenon we explored the ability of apolipoprotein A-V, in most experiments complexed to disks of dimyristoylphosphatidylcholine, to interact with two members of the low density lipoprotein receptor family, the low density lipoprotein receptor-related protein and the mosaic type-1 receptor, SorLA. Experiments using surface plasmon resonance showed specific binding of both free and lipid-bound apolipoprotein A-V to both receptors. The binding was calcium dependent and was inhibited by the receptor associated protein, a known ligand for members of the low density lipoprotein receptor family. Preincubation with heparin decreased the receptor binding of apolipoprotein A-V, indicating that overlap exists between the recognition sites for these receptors and for heparin. A double mutant, apolipoprotein A-V (Arg210Glu/Lys211Gln), showed decreased binding to heparin and decreased ability to bind the low density lipoprotein receptor-related protein. Association of apolipoprotein A-V with the low density lipoprotein receptor-related protein or SorLA resulted in enhanced binding of human chylomicrons to receptor-covered sensor chips. Our results indicate that apolipoprotein A-V may influence plasma lipid homeostasis by enhancing receptor-mediated endocytosis of triacylglycerol-rich lipoproteins.

Anionic phospholipids inhibit apolipoprotein E--low-density lipoprotein receptor interactions

Apolipoprotein E (apoE) is a ligand for members of the low-density lipoprotein receptor (LDLR) family. Lipid-free apoE is not recognized by LDLR, yet interaction with lipid confers receptor recognition properties. Although lipid interaction is known to induce a conformational change in apoE, it is not known if the lipid composition of the resulting complex influences binding. Using reconstituted lipoprotein particles of apoE3 N-terminal (NT) domain and dimyristoylphosphatidylcholine (DMPC), maximal LDLR binding was observed at DMPC:apoE3-NT ratios >2.5:1 (w/w). ApoE3-NT lipid particles prepared with egg sphingomyelin were functional as LDLR ligands while complexes formed with the anionic phospholipids dimyristoylphosphatidylglycerol or dimyristoylphosphatidylserine (DMPS) were not. In the case of apoE3-NT, lipid particles comprised of a mixture of DMPC and DMPS, a DMPS concentration dependent inhibition of LDLR binding activity was observed. Thus, in addition to affecting apoE conformational status, the lipid composition of ligand particles can modulate LDLR binding activity.

Lipid-induced extension of apolipoprotein E helix 4 correlates with low density lipoprotein receptor binding ability

Apolipoprotein E (apoE) serves as a ligand for the low density lipoprotein receptor (LDLR) only when bound to lipid. The N-terminal domain of lipid-free apoE exists as globular 4-helix bundle that is conferred with LDLR recognition ability after undergoing a lipid binding-induced conformational change. To investigate the structural basis for this phenomenon, site-directed spin label electron paramagnetic resonance spectroscopy experiments were conducted, focusing on the region near the C-terminal end of helix 4 (Ala-164). Using C112S apoE-N-terminal as template, a series of single cysteine substitution variants (at sequence positions 161, 165, 169, 173, 176, and 181) were produced, isolated, and labeled with the nitroxide probe, methane thiosulfonate. Electron paramagnetic resonance analysis revealed that lipid association induced fixed secondary structure in a region of the molecule known to exist as random coil in the lipid-free state. In a complementary approach, site-directed fluorescence analysis using an environmentally sensitive probe indicated that the lipid-induced transition of this region of the protein to alpha helix was accompanied by relocation to a more hydrophobic environment. In studies with full-length apoE single Cys variants, a similar random coil to stable backbone transition was observed, consistent with the concept that lipid interaction induced an extension of helix 4 beyond the boundary defining its lipid-free conformation. This structural transition likely represents a key conformational change necessary for manifestation of the LDLR recognition properties of apoE.

Role of leucine zipper motif in apoE3 N-terminal domain lipid binding activity

The N terminal domain of human apolipoprotein E3 (apoE3-NT) functions as a ligand for members of the low-density lipoprotein receptor (LDLR) family. Whereas lipid-free apoE3-NT adopts a stable four-helix bundle conformation, a lipid binding induced conformational change is required for LDLR recognition. To investigate the role of a leucine zipper motif identified in the helix bundle on lipid binding activity, three leucine residues in helix 2 (Leu63, Leu71 and Leu78) were replaced by alanine. Recombinant "leucine to alanine" (LA) apoE3-NT was produced in E. coli, isolated and characterized. Stability studies revealed a transition midpoint of guanidine hydrochloride induced denaturation of 2.7 M and 2.1 M for wild type (WT) and LA apoE3-NT, respectively. Results from fluorescent dye binding assays revealed that, compared to WT apoE3-NT, LA apoE3-NT has an increased content of solvent exposed hydrophobic surfaces. In phospholipid vesicle solubilization assays, LA apoE3-NT was more effective than WT apoE3-NT at inducing a time-dependent decrease in dimyristoylphosphatidylglycerol vesicle light scattering intensity. Likewise, in lipoprotein binding assays, LA apoE3-NT protected human low-density lipoprotein from phospholipase C induced aggregation to a greater extent than WT apoE3-NT. On the other hand, LA apoE3-NT and WT apoE3-NT were equivalent in terms of their ability to bind a soluble LDLR fragment. The results suggest that the leucine zipper motif confers stability to the apoE3-NT helix bundle state and may serve to modulate lipid binding activity of this domain and, thereby, influence the conformational transition associated with manifestation of LDLR binding activity.

Nanodisk-associated amphotericin B clears Leishmania major cutaneous infection in susceptible BALB/c mice

Nanometer-scale, apolipoprotein-stabilized phospholipid bilayer disk complexes (nanodisks [ND]) harboring the toxic and poorly soluble antileishmanial agent amphotericin B (AMB) were examined for efficacy in treatment of Leishmania major-infected BALB/c mice (Mus musculus). L. major-infected mice were intraperitoneally (i.p.) treated with AMB-ND in 0-, 1-, and 5-mg/kg doses at 24 h, 48 h, and 4, 7, 14, and 21 days postinfection in two experiments. L. major-infected mice were i.p. treated with phosphate-buffered saline, 5 mg/kg AMB-ND, or 5 mg/kg lipid-associated amphotericin B (liposomal amphotericin B, AmBisome) at 24 h, 48 h, and 10, 20, 30, and 40 days postinfection in one experiment. Parasite numbers, footpad lesion size progression, and development of cytokine responses were assayed at days 7, 15, 30, 50, 140, and 250 or at days 14, 30, 50, 95, and 140 postinfection. Mice administered AMB-ND in 1- or 5-mg/kg doses were significantly protected from L. major, displaying decreases in lesion size and parasite burden, particularly at the 5-mg/kg dosage level. In contrast to the i.p. treated AmBisome group, BALB/c mice treated with i.p. AMB-ND completely cleared an L. major infection by 140 to 250 days postinfection, with no lesions remaining and no parasites isolated from infected animals. Restimulated mixed lymphocyte culture cytokine responses (interleukin-4 [IL-4], IL-12, IL-10, NO, and gamma interferon) were unchanged by AMB-ND administration compared to controls. The marked clearance of Leishmania parasites from a susceptible strain of mice without an appreciable change in the cytokine response suggests that AMB-ND represent a potentially useful formulation for treatment of intrahistiocytic organisms.

Apolipoprotein A-I assumes a "looped belt" conformation on reconstituted high density lipoprotein

Apolipoprotein A-I (apoA-I) plays a central role in the reverse cholesterol transport pathway; however, the structural basis for its antiatherogenic effects remains poorly understood. Here we employ EPR spectroscopy and fluorescence resonance energy transfer to elucidate the conformation and relative alignment of apoA-I monomers on discoidal (9.4 nm) reconstituted high density lipoprotein (rHDL). EPR spectroscopy provided evidence for an extended helical secondary structure. Position 139 since it was the only residue examined to display a dynamic motional character consistent with a flexible loop structure. The EPR spectra of nitroxide probes at positions 133 and 146 exhibit spin coupling, indicating that these positions are proximal to an apoA-I paired counterpart on the perimeter of rHDL. fluorescence resonance energy transfer studies employing engineered apoA-I variants possessing a single tryptophan (energy donor) and/or a single cysteine (whose thiol moiety was covalently labeled with an extrinsic energy acceptor) provided evidence that paired apoA-I molecules around the perimeter of rHDL align in an extended antiparallel conformation. Taken together with the observation that the EPR spectra of nitroxide probes positioned at intervening sequence positions (134-145) do not exhibit spin coupling, this has led us to propose a "looped belt" model, wherein residues 133-146 comprise a flexible loop segment that confers to apoA-I an intrinsic ability to adapt its structure to accommodate changing particle lipid content. Specifically, in the looped belt model, with the exception of amino acids 134-145, apoA-I aligns with its counterpart in a helix 5-helix 5 registry, centered at position 139.

Apolipophorin III: role model apolipoprotein

It has been one-quarter century since the identification of apolipophorin III (apoLp-III) as an important component of insect hemolymph lipid transport processes. Original studies of flight-related lipid transport that led to the discovery of apoLp-III have been followed by detailed studies of its structure and function relations, species distribution as well as its physiological roles beyond lipid transport. The non-exchangeable apoLp-I and -II, which are derived from a common precursor, are structural protein components of the multifunctional lipophorin particle. ApoLp-I/II have been identified as members of a broad lipid-binding protein family based on sequence similarities with their vertebrate counterparts. By contrast, apoLp-III can be found as a lipid-free hemolymph protein that associates with lipophorin during hormone-induced lipid mobilization. Based on structural characterization, apoLp-III belongs to a large family of exchangeable apolipoproteins characterized by segments of amphipathic alpha-helix. The remarkable structural adaptability of apoLp-III can be ascribed to its globular amphipathic alpha-helix bundle conformation wherein hydrophobic lipid-binding regions are stabilized in the absence of lipid by helix-helix interactions. Upon exposure to potential lipid surface-binding sites, the globular helix bundle opens to expose its hydrophobic interior permitting substitution of helix-helix contact in the bundle for helix-lipid interactions. Novel functions of apoLp-III beyond lipid transport have been identified recently. The expanding role of apoLp-III in innate immunity promises to offer exciting research opportunities in the future.