Insight into lipid surface recognition and reversible conformational adaptations of an exchangeable apolipoprotein by multidimensional heteronuclear NMR techniques

A bacterial cytolinker couples positioning of magnetic organelles to cell shape control

Magnetotactic bacteria maneuver within the geomagnetic field by means of intracellular magnetic organelles, magnetosomes, which are aligned into a chain and positioned at midcell by a dedicated magnetosome-specific cytoskeleton, the "magnetoskeleton." However, how magnetosome chain organization and resulting magnetotaxis is linked to cell shape has remained elusive. Here, we describe the cytoskeletal determinant CcfM (curvature-inducing coiled-coil filament interacting with the magnetoskeleton), which links the magnetoskeleton to cell morphology regulation in Magnetospirillum gryphiswaldense Membrane-anchored CcfM localizes in a filamentous pattern along regions of inner positive-cell curvature by its coiled-coil motifs, and independent of the magnetoskeleton. CcfM overexpression causes additional circumferential localization patterns, associated with a dramatic increase in cell curvature, and magnetosome chain mislocalization or complete chain disruption. In contrast, deletion of ccfM results in decreased cell curvature, impaired cell division, and predominant formation of shorter, doubled chains of magnetosomes. Pleiotropic effects of CcfM on magnetosome chain organization and cell morphology are supported by the finding that CcfM interacts with the magnetoskeleton-related MamY and the actin-like MamK via distinct motifs, and with the cell shape-related cytoskeleton via MreB. We further demonstrate that CcfM promotes motility and magnetic alignment in structured environments, and thus likely confers a selective advantage in natural habitats of magnetotactic bacteria, such as aquatic sediments. Overall, we unravel the function of a prokaryotic cytoskeletal constituent that is widespread in magnetic and nonmagnetic spirilla-shaped Alphaproteobacteria.

Analysis of apolipoprotein genes and their involvement in disease response of channel catfish after bacterial infection

Apolipoproteins are protein component of plasma lipoproteins. They exert crucial roles in lipoprotein metabolism and serve as enzyme cofactors, receptor ligands, and lipid transfer carriers in mammals. In teleosts, apolipoproteins are also involved in diverse processes including embryonic and ontogenic development, liver and digestive system organogenesis, and innate immunity. In this study, we identified a set of 19 apolipoprotein genes in channel catfish (Ictalurus punctatus). Phylogenetic analysis and syntenic analysis were conducted to determine their identities and evolutionary relationships. The expression signatures of apolipoproteins in channel catfish were determined in healthy tissues and after infections with two major bacterial pathogens, Edwardsiella ictaluri and Flavobacterium columnare. In healthy channel catfish, most apolipoprotein genes exhibited tissue-specific expression patterns in channel catfish. After ESC and columnaris infections, 5 and 7 apolipoprotein genes were differentially expressed respectively, which presented a pathogen-specific and time-dependent pattern of regulation. After ESC infection, three exchangeable apolipoproteins (apoA-IB, apoC-I, and apoE-B) were suppressed in catfish intestine, while two nonexchangeable apolipoproteins (apoB-A and apoB-B) were slightly up-regulated. After columnaris infection, apoB-B, apoD-B, and apoE-A were significantly down-regulated in catfish gill, while apoF, apoL-IV, apoO-like, and apo-14 kDa showed significantly up-regulation. Taken together, these results suggested that apolipoprotein genes may play significant roles in innate immune responses to bacterial pathogens in channel catfish.

Deletion of the N- or C-Terminal Helix of Apolipophorin III To Create a Four-Helix Bundle Protein

Apolipophorin III (apoLp-III) is an exchangeable apolipoprotein found in insects and plays an important function in lipid transport. The protein has an unusual five-helix bundle architecture, deviating from the common four-helix bundle motif. To understand the role of the additional helix in apoLp-III, the N-terminal or C-terminal helix was deleted to create a putative four-helix bundle protein. While the protein lacking helix-1 could be expressed in bacteria albeit at reduced yields, apoLp-III lacking helix-5 could not be produced. Mutational analysis by truncating helix-5 showed that a minimum segment of approximately one-third of the C-terminal helix is required for protein expression. The variant lacking helix-5 was produced by inserting a methionine residue between helix-4 and -5; subsequent cyanogenbromide cleavage generated the four-helix variant. Both N- and C-terminal helix deletion variants displayed significantly reduced helical content, protein stability, and tertiary structure. Despite the significantly altered structure, the variants were still fully functional. The rate of dimyristoylphosphatidylcholine vesicle solubilization was enhanced 4-5-fold compared to the wild-type protein, and the deletion variants were effective in binding to lipolyzed low density lipoprotein thereby preventing lipoprotein aggregation. These results show that the additional helix of apoLp-III is not essential for lipid binding but is required for proper folding to keep the protein into a stable conformation.

Lipoprotein assembly and function in an evolutionary perspective

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

Possible function of VIPP1 in maintaining chloroplast membranes

A protein designated as VIPP1 is found widely in organisms performing oxygenic photosynthesis, but its precise role in chloroplasts has remained somewhat mysterious. Based on its structural similarity, it presumably has evolved from bacterial Phage shock protein A (PspA) with a C-terminal extension of approximately 40 amino acids. Both VIPP1 and PspA are membrane-associated despite the lack of transmembrane helices. They form an extremely large homo-complex that consists of an oligomeric ring unit. Although PspA is known to respond to membrane stress and although it acts in maintaining proton motive force through membrane repair, the multiple function of VIPP1, such as vesicle budding from inner envelope to deliver lipids to thylakoids, maintenance of photosynthetic complexes in thylakoid membranes, biogenesis of Photosystem I, and protective role of inner envelope against osmotic stress, has been proposed. Whatever its precise function in chloroplasts, it is an important protein because depletion of VIPP1 in mutants severely affects photoautotrophic growth. Recent reports of the relevant literature describe that VIPP1 becomes highly mobile when chloroplasts receive hypotonic stress, and that VIPP1 is tightly bound to lipids, which implies a crucial role of VIPP1 in membrane repair through lipid transfer. This review presents a summary of our current knowledge related to VIPP1, particularly addressing the dynamic behavior of complexes against stress and its property of lipid binding. Those data altogether suggest that VIPP1 acts a priori in chloroplast membrane maintenance through its activity to transfer lipids rather than in thylakoid formation through vesicles. This article is part of a Special Issue titled: Chloroplast Biogenesis.

New insights into the determination of HDL structure by apolipoproteins: Thematic review series: high density lipoprotein structure, function, and metabolism

Apolipoprotein (apo)A-I is the principal protein component of HDL, and because of its conformational adaptability, it can stabilize all HDL subclasses. The amphipathic α-helix is the structural motif that enables apoA-I to achieve this functionality. In the lipid-free state, the helical segments unfold and refold in seconds and are located in the N-terminal two thirds of the molecule where they are loosely packed as a dynamic, four-helix bundle. The C-terminal third of the protein forms an intrinsically disordered domain that mediates initial binding to phospholipid surfaces, which occurs with coupled α-helix formation. The lipid affinity of apoA-I confers detergent-like properties; it can solubilize vesicular phospholipids to create discoidal HDL particles with diameters of approximately 10 nm. Such particles contain a segment of phospholipid bilayer and are stabilized by two apoA-I molecules that are arranged in an anti-parallel, double-belt conformation around the edge of the disc, shielding the hydrophobic phospholipid acyl chains from exposure to water. The apoA-I molecules are in a highly dynamic state, and they stabilize discoidal particles of different sizes by certain segments forming loops that detach reversibly from the particle surface. The flexible apoA-I molecule adapts to the surface of spherical HDL particles by bending and forming a stabilizing trefoil scaffold structure. The above characteristics of apoA-I enable it to partner with ABCA1 in mediating efflux of cellular phospholipid and cholesterol and formation of a heterogeneous population of nascent HDL particles. Novel insights into the structure-function relationships of apoA-I should help reveal mechanisms by which HDL subclass distribution can be manipulated.

Pyrene: a probe to study protein conformation and conformational changes

The review focuses on the unique spectral features of pyrene that can be utilized to investigate protein structure and conformation. Pyrene is a fluorescent probe that can be attached covalently to protein side chains, such as sulfhydryl groups. The spectral features of pyrene are exquisitely sensitive to the microenvironment of the probe: it exhibits an ensemble of monomer fluorescence emission peaks that report on the polarity of the probe microenvironment, and an additional band at longer wavelengths, the appearance of which reflects the presence of another pyrene molecule in spatial proximity (~10 Å). Its high extinction coefficient allows us to study labeled proteins in solution at physiologically relevant concentrations. The environmentally- and spatially-sensitive features of pyrene allow monitoring protein conformation, conformational changes, protein folding and unfolding, protein-protein, protein-lipid and protein-membrane interactions.

Apolipophorin III: a lipid-triggered molecular switch

Apolipophorin III (apoLp-III) is a low molecular weight exchangeable apolipoprotein that plays an important role in the enhanced neutral lipid transport during insect flight. The protein exists in lipid-free and lipid-bound states. The lipid-bound state is the active form of the protein and occurs when apoLp-III associates with lipid-enriched lipophorins. ApoLp-III is well characterized in two evolutionally divergent species: Locusta migratoria and Manduca sexta. The two apolipoproteins interact in a similar manner with model phospholipid vesicles, and transform them into discoidal particles. Their low intrinsic stability in the lipid-free state likely facilitates interaction with lipid surfaces. Low solution pH also favors lipid binding interaction through increased exposure of hydrophobic surfaces on apoLp-III. While secondary structure is maintained under acidic conditions, apoLp-III tertiary structure is altered, adopting molten globule-like characteristics. In studies of apoLp-III interaction with natural lipoproteins, we found that apoLp-III is readily displaced from the surface of L. migratoria low-density lipophorin by recombinant apoLp-III proteins from either L. migratoria or M. sexta. Thus, despite important differences between these two apoLp-IIIs (amino acid sequence, presence of carbohydrate), their functional similarity is striking. This similarity is also illustrated by the recently published NMR solution structure of M. sexta apoLp-III wherein its molecular architecture closely parallels that of L. migratoria apoLp-III.

Structure-guided protein engineering modulates helix bundle exchangeable apolipoprotein properties

Apolipoprotein (apo) E plays a major role in lipid metabolism by mediating cellular uptake of lipoprotein particles through interaction with members of the low density lipoprotein (LDL) receptor family. The primary region of apoE responsible for receptor binding has been limited to a cluster of basic amino acids between residues 134 and 150, located in the fourth helix of the N-terminal domain globular helix bundle structure. To investigate structural and functional requirements of this "receptor binding region" we engineered an apolipoprotein chimera wherein residues 131-151 of human apoE were substituted for residues 146-166 (helix 5) of Manduca sexta apolipophorin III (apoLp-III). Recombinant hybrid apolipoprotein was expressed in Escherichia coli, isolated, and characterized. Hybrid apolipoprotein and apoE3-N-terminal, but not apoLp-III, bound to heparin-Sepharose. Far UV circular dichroism spectroscopy revealed the presence of predominantly alpha-helix secondary structure, and stability studies revealed a urea denaturation midpoint of 1.05 m, similar to wild-type apoLp-III. Hybrid apolipoprotein-induced dimyristoylphosphatidylcholine (DMPC) bilayer vesicle solubilization activity was significantly enhanced compared with either parent protein, consistent with detection of solvent-exposed hydrophobic regions on the protein in fluorescent dye binding experiments. Unlike wild-type apoLp-III.DMPC complexes, disc particles bearing the hybrid apolipoprotein competed with 125ILDL for binding to the LDL receptor on cultured human skin fibroblasts. We conclude that a hybrid apolipoprotein containing a key receptor recognition element of apoE preserves the structural integrity of the parent protein while conferring a new biological activity, illustrating the potential of helix swapping to introduce desirable biological properties into unrelated or engineered apolipoproteins.

NMR solution structure and dynamics of an exchangeable apolipoprotein, Locusta migratoria apolipophorin III

We report here the NMR structure and backbone dynamics of an exchangeable apolipoprotein, apoLp-III, from the insect Locusta migratoria. The NMR structure adopts an up-and-down elongated five-helix bundle, which is similar to the x-ray crystal structure of this protein. A short helix, helix 4', is observed that is perpendicular to the bundle and fully solvent-exposed. NMR experimental parameters confirm the existence of this short helix, which is proposed to serve as a recognition helix for apoLp-III binding to lipoprotein surfaces. The L. migratoria apoLp-III helix bundle displays several characteristic structural features that regulate the reversible lipoprotein binding activity of apoLp-III. The buried hydrophilic residues and exposed hydrophobic residues readily adjust the marginal stability of apoLp-III, facilitating the helix bundle opening. Specifically, upon lipoprotein binding the locations and orientations of the buried hydrophilic residues modulate the apoLp-III helix bundle to adopt a possible opening at the hinge that is opposite the recognition short helix, helix 4'. The backbone dynamics provide additional support to the recognition role of helix 4' and this preferred conformational adaptation of apoLp-III upon lipid binding. In this case, the lipid-bound open conformation contains two lobes linked by hinge loops. One lobe contains helices 2 and 3, and the other lobe contains helices 1, 4, and 5. This preferred bundle opening is different from the original proposal on the basis of the x-ray crystal structure of this protein (Breiter, D. R., Kanost, M. R., Benning, M. M., Wesenberg, G., Law, J. H., Wells, M. A., Rayment, I., and Holden, H. M. (1991) Biochemistry 30, 603-608), but it efficiently uses helix 4' as the recognition short helix. The buried interhelical H-bonds are found to be mainly located between the two lobes, potentially providing a specific driving force for the helix bundle recovery of apoLp-III from the lipid-bound open conformation. Finally, we compare the NMR structures of Manduca sexta apoLp-III and L. migratoria apoLp-III and present a united scheme for the structural basis of the reversible lipoprotein binding activity of apoLp-III.

Lipid-triggered conformational switch of apolipophorin III helix bundle to an extended helix organization

Apolipophorin III (ApoLp-III) from the Sphinx moth, Manduca sexta, is an 18kDa protein that binds reversibly to hydrophobic surfaces generated on metabolizing lipoprotein particles. It is comprised of amphipathic alpha-helices (H1-H5) organized in an up-and-down topology forming a helix bundle in the lipid-free state. Upon interaction with lipids, apoLp-III has been proposed to undergo a dramatic conformational change, involving helix bundle opening about putative hinge loops such that H1, H2 and H5 move away from H3 and H4. In the present study, we examine the relative spatial disposition of H1 and H5 on discoidal phospholipid complexes and spherical lipoproteins. Cysteine residues were engineered at position 8 in H1 and/or at position 138 in H5 in apoLp-III (which otherwise lacks Cys) yielding A8C-, A138C- and A8C/A138C-apoLp-III. Tethering of H1 and H5 by a disulfide bond between A8C and A138C abolished the ability of apoLp-III to transform phospholipid vesicles to discoidal particles, or to interact with lipoproteins, demonstrating that these helices are required to reposition during lipid interaction. Site-specific labeling of A8C/A138C-apoLp-III with N-(1-pyrene)maleimide in the lipid-free state resulted in intramolecular pyrene "excimer" fluorescence emission indicative of spatial proximity between these sites. Upon association with dimyristoylphosphatidylcholine (DMPC) discoidal complexes, the intramolecular excimer was replaced by intermolecular excimer fluorescence due to proximity between pyrene moieties on A8C and A138C in neighboring apoLp-III molecules on the discoidal particle. No excimer emission was observed in the case of pyrene-A8C-apoLp-III/DMPC or pyrene-A138C-apoLp-III/DMPC complexes. However, equimolar mixing of the two labeled single-cysteine mutants prior to disc formation resulted in excimer emission. In addition, intramolecular pyrene excimer formation was diminished upon binding of pyrene-A8C/A138C-apoLp-III to spherical lipoproteins. The data are consistent with repositioning of H1 away from H5 upon encountering a lipid surface, resulting in an extended conformation of apoLp-III that circumscribes the discoidal bilayer particle.

A major IgE epitope-containing grass pollen allergen domain from Phl p 5 folds as a four-helix bundle

Phl p 5, a 29 kDa major allergen from timothy grass pollen, is one of the most reactive members of group 5 allergens. Its sequence comprises two repeats of a novel alanine-rich motif (AR) whose structure and allergenic response are still mostly unknown. We report here a structural characterization of an immunodominant fragment of Phl p 5, Phl p 5(56-165) which comprises the first AR repeat. Recombinant (r)Phl p 5(56-165) was expressed in Escherichia coli, purified to homogeneity and shown to be sufficient to react with serum IgE from 90% of grass pollen allergic patients. Using NMR spectroscopy, we show conclusively that the fragment forms a compact globular domain which is, however, prone to degradation with time. The rPhl p 5(56-165) fold consists of a four-helix bundle held together by hydrophobic interactions between the aromatic rings and aliphatic side chains. This evidence gives clear indications about the structure of the full-length Phl p 5 and provides a rational basis for finding ways to stabilize the fold and designing therapeutic vaccines against grass pollen allergy.

Purification and characterization of apolipophorin III from immune hemolymph of Heliothis virescens pupae

Apolipophorin III (ApoLp-III) from Heliothis virescens pupae was purified by heat-treatment followed by Sephadex G-50 filtration and reverse phase-HPLC. The molecular mass of the purified ApoLp-III was determined as 17965.9+/-5 Da by mass spectrometry. The N-terminal sequence confirmed the protein as ApoLp-III with homology of 56-83% to other insect ApoLp-III molecules. The amino acid spatial arrangement of the predicted alpha-helix 1 of Heliothis ApoLp-III was nearly identical to that of the amphipatic alpha-helix 1 of Manduca sexta ApoLp-III. The absorption spectrum from 240-340 nm of the Heliothis ApoLp-III was the same as the UV spectra of ApoLp-III from Manduca sexta and Galleria mellonella, showing absorption maxima at 280, 268, 264 and 259 nm. These results indicated that the primary structure of ApoLp-III is conserved in lepidopterans. The Heliothis ApoLp-III was not a glycoprotein and showed hemagglutination activity against rabbit red blood cells. This hemagglutination activity was abolished by Tween 80, but not by six different carbohydrates. Hydrophobic interaction of ApoLp-III with red blood cells agreed with structural studies since ApoLp-III binds lipid through hydrophobic interaction after conformational change. Bacterial injection apparently increased the amount of ApoLp-III in immune hemolymph when compared with normal hemolymph, and may indicate that ApoLp-III plays a role in insect immunity.

Adipokinetic hormones of insect: release, signal transduction, and responses

Flight activity of insects provides an attractive yet relatively simple model system for regulation of processes involved in energy metabolism. This is particularly highlighted during long-distance flight, for which the locust constitutes a well-accepted model insect. Peptide adipokinetic hormones (AKHs) are synthesized and stored by neurosecretory cells of the corpus cardiacum, a neuroendocrine gland connected with the insect brain. The actions of these hormones on their fat body target cells trigger a number of coordinated signal transduction processes which culminate in the mobilization of both carbohydrate (trehalose) and lipid (diacylglycerol). These substrates fulfill differential roles in energy metabolism of the contracting flight muscles. The molecular mechanism of diacylglycerol transport in insect blood involving a reversible conversion of lipoproteins (lipophorins) has revealed a novel concept for lipid transport in the circulatory system. In an integrative approach, recent advances are reviewed on the consecutive topics of biosynthesis, storage, and release of insect AKHs, AKH signal transduction mechanisms and metabolic responses in fat body cells, and the dynamics of reversible lipophorin conversions in the insect blood.

Structural basis for the conformational adaptability of apolipophorin III, a helix-bundle exchangeable apolipoprotein

The high-resolution NMR structure of apolipophorin III from the sphinx moth, Manduca sexta, has been determined in the lipid-free state. We show that lipid-free apolipophorin III adopts a unique helix-bundle topology that has several characteristic structural features. These include a marginally stable, up-and-down helix bundle that allows for concerted opening of the bundle about "hinged" loops upon lipid interaction and buried polar/ionizable residues and buried interhelical H-bonds located in the otherwise hydrophobic interior of the bundle that adjust protein stability and facilitate lipid-induced conformational opening. We suggest that these structural features modulate the conformational adaptability of the lipid-free helix bundle upon lipid binding and control return of the open conformation to the original lipid-free helix-bundle state. Taken together, these data provide a structural rationale for the ability of exchangeable apolipoproteins to reversibly interact with circulating lipoprotein particles.

Essential role of the conformational flexibility of helices 1 and 5 on the lipid binding activity of apolipophorin-III

It has been recently postulated that the conformational flexibility of helices 1 and 5 of Locusta migratoria apoLp-III could play an important role in early steps of binding of this apolipoprotein to a lipid surface (Soulages, J. L., and Arrese, E. L. (2000) J. Biol. Chem. 275, 17501-17509). To test this model, we have designed a double Cys mutant in which a disulfide bond linking helices 1 and 5 could be formed, resulting in an apolipoprotein with reduced conformational flexibility of its N- and C-terminal helices. Substitution of Thr(18) and Ala(147) by Cys residues provided a protein that under nonreducing conditions was fully oxidized. The far-UV CD spectra of this mutant in the reduced and oxidized states indicated that their secondary structures were identical to the structure of the wild type recombinant apoLp-III, which contains no Cys residues. Near-UV CD studies confirmed the formation of a disulfide bond and the absence of structural perturbations. The lipid binding activity of the reduced mutant, as determined by its ability to form discoidal lipoproteins, was nearly identical to that of the wild type protein. Contrarily, the disulfide form of the mutant was not able to form discoidal lipoproteins with liposomes of either dimirystoylphosphatidylcholine or dimyristoylphosphatidylglycerol. It is concluded that the separation of the helices 1 and 5 constitutes one of the key steps along the complex pathway for the formation of the final apolipoprotein lipid-bound state. It is inferred that the conformational flexibility of helices 1 and 5 is a key property of apoLp-III, allowing the exposure of hydrophobic protein regions and the interaction of the hydrophobic faces of the amphipathic alpha-helices with the lipoprotein lipid surface.

Modulation of the lipid binding properties of the N-terminal domain of human apolipoprotein E3

Apolipoprotein E (apoE) plays a critical role in plasma lipid homeostasis through its function as a ligand for the low-density lipoprotein (LDL) receptor family. Receptor recognition is mediated by residues 130-150 in the independently folded, 22-kDa N-terminal (NT) domain. This elongated globular four-helix bundle undergoes a conformational change upon interaction with an appropriate lipid surface. Unlike other apolipoproteins, apoE3 NT failed to fully protect human LDL from aggregation induced by treatment with phospholipase C. Likewise, in dimyristoylglycerophosphocholine (Myr2Gro-PCho) vesicle transformation assays, 100 microg apoE3 NT induced only 15% reduction in vesicle (250 microg) light scattering intensity after 30 min. ApoE3 NT interaction with modified lipoprotein particles or Myr2Gro-PCho vesicles was concentration-dependent whereas the vesicle transformation reaction was unaffected by buffer ionic strength. In studies with the anionic phospholipid dimyristoylglycerophosphoglycerol, apoE3 NT-mediated vesicle transformation rates were enhanced > 10-fold compared with Myr2Gro-PCho and activity decreased with increasing buffer ionic strength. Solution pH had a dramatic effect on the kinetics of apoE3 NT-mediated Myr2Gro-PCho vesicle transformation with increased rates observed as a function of decreasing pH. Fluorescence studies with a single tryptophan containing apoE3 NT mutant (L155W) revealed increased solvent exposure of the protein interior at pH values below 4.0. Similarly, fluorescent dye binding experiments with 8-anilino-1-naphthalene sulfonate revealed increased exposure of apoE3 NT hydrophobic interior as a function of decreasing pH. These studies indicate that apoE3 NT lipid binding activity is modulated by lipid surface properties and protein tertiary structure.

Structural models of human apolipoprotein A-I: a critical analysis and review

Human apolipoprotein (apo) A-I has been the subject of intense investigation because of its well-documented anti-atherogenic properties. About 70% of the protein found in high density lipoprotein complexes is apo A-I, a molecule that contains a series of highly homologous amphipathic alpha-helices. A number of significant experimental observations have allowed increasing sophisticated structural models for both the lipid-bound and the lipid-free forms of the apo A-I molecule to be tested critically. It seems clear, for example, that interactions between amphipathic domains in apo A-I may be crucial to understanding the dynamic nature of the molecule and the pathways by which the lipid-free molecule binds to lipid, both in a discoidal and a spherical particle. The state of the art of these structural studies is discussed and placed in context with current models and concepts of the physiological role of apo A-I and high-density lipoprotein in atherosclerosis and lipid metabolism.

Spectroscopic evidence for amyloid-like interfacial self-assembly of hydrophobin Sc3

Amphipathic fungal proteins called hydrophobins are able to self-assemble into insoluble supramolecular structures at hydrophobic/hydrophilic interfaces, but the molecular mechanism and underlying protein conformation changes are not known. Secondary-structure prediction indicated that hydrophobin Sc3 is an all-beta protein. Many amyloidogenic proteins self-assemble into insoluble amyloid fibrils while undergoing a change to an all-beta conformation. In this study we show that two dyes, thioflavin T, and Congo red, which are widely used for specific detection of stacked beta sheets, interact with Sc3 assemblies in the same way as with the amyloid beta-sheet fibrils. We conclude that Sc3, and probably other hydrophobins too, self-assemble at interfaces in the same manner as amyloidogenic proteins, i.e., through beta-sheet stacking.

Conformational reorganization of the four-helix bundle of human apolipoprotein E in binding to phospholipid

Conformational reorganization of the amino-terminal four-helix bundle (22-kDa fragment) of apolipoprotein E (apoE) in binding to the phospholipid dimyristoylphosphatidylcholine (DMPC) to form discoidal particles was investigated by introducing single, double, and triple interhelical disulfide bonds to restrict the opening of the bundle. Interaction of apoE with DMPC was assessed by vesicle disruption, turbidimetric clearing, and gel filtration assays. The results indicate that the formation of apoE.DMPC discoidal particles occurs in a series of steps. A triple disulfide mutant, in which all four helices were tethered, did not form complexes but could release encapsulated 5-(6)-carboxylfluorescein from DMPC vesicles, indicating that the initial interaction does not involve major reorganization of the helical bundle. Initial interaction is followed by the opening of the four-helix bundle to expose the hydrophobic faces of the amphipathic helices. In this step, helices 1 and 2 and helices 3 and 4 preferentially remain paired, since these disulfide-linked mutants bound to DMPC in a manner similar to that of the 22-kDa fragment of apoE4. In contrast, mutants in which helices 2 and 3 and/or helices 1 and 4 paired bound poorly to DMPC. However, all single and double helical pairings resulted in the formation of larger discs than were formed by the 22-kDa fragment, indicating that further reorganization of the helices occurs following the initial opening of the four-helix bundle in which the protein assumes its final lipid-bound conformation. In support of this rearrangement, reducing the disulfide bonds converted the large disulfide mutant discs to normal size.

Dynamics and hydration of the alpha-helices of apolipophorin III

Apolipophorin III (apoLp-III) is an exchangeable apolipoprotein whose structure is represented as a bundle of five amphipathic alpha-helices. In order to study the properties of the helical domains of apolipophorin III, we designed and obtained five single-tryptophan mutants of Locusta migratoria apoLp-III. The proteins were studied by UV absorption spectroscopy, time-resolved and steady-state fluorescence spectroscopy, and circular dichroism. Fluorescence anisotropy, near-UV CD and solute fluorescence quenching studies indicate that the Trp residues in helices 1 (N-terminal) and 5 (C-terminal) have the highest conformational flexibility. These two residues also showed the highest degree of hydration. Trp residues in helices 3 and 4 display the lowest mobility, as assessed by fluorescence anisotropy and near UV CD. The Trp residue in helix 2 is protected from the solvent but shows high mobility. As inferred from the properties of the Trp residues, helices 1 and 5 appear to have the highest conformational flexibility. Helix 2 has an intermediate mobility, whereas helices 3 and 4 appear to constitute a highly ordered domain. From the configuration of the helices in the tertiary structure of the protein, we estimated the relative strength of the five interhelical interactions of apoLp-III. These interactions can be ordered according to their apparent stabilizing strengths as: helix 3-helix 4 > helix 2-helix 3 > helix 4-helix 1 approximately helix 2-helix 5 > helix 1-helix 5. A new model for the conformational change that is expected to occur upon binding of the apolipoprotein to lipid is proposed. This model is significantly different from the currently accepted model (Breiter, D. R., Kanost, M. R., Benning, M. M., Wesemberg, G., Law, J. H., Wells, M. A., Rayment, I., and Holden, M. (1991) Biochemistry 30, 603-608). The model presented here predicts that the relaxation of the tertiary structure and the concomitant exposure of the hydrophobic core take place through the disruption of the weak interhelical contacts between helices 1 and 5. To some extent, the weakness of the helix 1-helix 5 interaction would be due to the parallel arrangement of these helices.

Conformational flexibility in the apolipoprotein E amino-terminal domain structure determined from three new crystal forms: implications for lipid binding

An amino-terminal fragment of human apolipoprotein E3 (residues 1-165) has been expressed and crystallized in three different crystal forms under similar crystallization conditions. One crystal form has nearly identical cell dimensions to the previously reported orthorhombic (P2(1)2(1)2(1)) crystal form of the amino-terminal 22 kDa fragment of apolipoprotein E (residues 1-191). A second orthorhombic crystal form (P2(1)2(1)2(1) with cell dimensions differing from the first form) and a trigonal (P3(1)21) crystal form were also characterized. The structures of the first orthorhombic and the trigonal form were determined by seleno-methionine multiwavelength anomalous dispersion, and the structure of the second orthorhombic form was determined by molecular replacement using the structure from the trigonal form as a search model. A combination of modern experimental and computational techniques provided high-quality electron-density maps, which revealed new features of the apolipoprotein E structure, including an unambiguously traced loop connecting helices 2 and 3 in the four-helix bundle and a number of multiconformation side chains. The three crystal forms contain a common intermolecular, antiparallel packing arrangement. The electrostatic complimentarity observed in this antiparallel packing resembles the interaction of apolipoprotein E with the monoclonal antibody 2E8 and the low density lipoprotein receptor. Superposition of the model structures from all three crystal forms reveals flexibility and pronounced kinks in helices near one end of the four-helix bundle. This mobility at one end of the molecule provides new insights into the structural changes in apolipoprotein E that occur with lipid association.

Spectroscopic characterization of the conformational adaptability of Bombyx mori apolipophorin III

Apolipophorin III (apoLp-III) from the silkmoth, Bombyx mori, has been over-expressed in Escherichia coli, purified and characterized. Far-UV CD spectroscopic analysis revealed 65% alpha-helix secondary structure. Near-UV CD spectra obtained in buffer or complexed with dimyristoylglycerophosphocholine (DMPC), provided evidence that apoLp-III alpha-helices reorient upon interaction with lipid, indicative of a protein conformational change. In guanidine hydrochloride (GdnHCl) denaturation studies, a transition midpoint of 0.33 M was observed, corresponding to a DeltaGDH2O = 2.46 kcal. mol-1. Fluorescence studies of the sole tryptophan residue (Trp40) in apoLp-III revealed an emission lambdamax = 327 nm. Compared to free tryptophan, Stern-Volmer constants (KSV) for acrylamide and KI quenching of Trp40 fluorescence were decreased by 20-fold and sevenfold, respectively. In studies of apoLp-III-DMPC disc complexes, far-UV CD spectroscopy revealed an increase in alpha-helix content to approximately 85% and a ninefold increase in the GdnHCl-induced denaturation transition midpoint to 3 M. In studies of lipid interaction, apoLp-III was shown to disrupt both negatively charged and zwitterionic phospholipid bilayer vesicles, transforming them into discoidal complexes. Characterization of apoLp-III-DMPC discs, using 5-doxyl or 12-doxyl stearic acid as lipid-based quenching agents, revealed that Trp40 localizes near the phospholipid polar head groups. KSV values for acrylamide and KI quenching of intrinsic fluorescence of apoLp-III-DMPC discs indicate that Trp40 is embedded in the lipid milieu, with little or no accessibility to the aqueous quenchers. Given the large amount of alpha-helix in apoLp-III, the data presented support a model in which amphipathic alpha-helical segments are stabilized by helix-helix interactions and lipid association induces a protein conformational change which results in substitution of helix-helix interactions for helix-lipid contacts.

Lipid transport biochemistry and its role in energy production

Recent advances on the biochemistry of flight-related lipid mobilization, transport, and metabolism are reviewed. The synthesis and release of adipokinetic hormones and their function in activation of fat body triacylglycerol lipase to produce diacylglycerol is discussed. The dynamics of reversible lipoprotein conversions and the structural properties and role of the exchangeable apolipoprotein, apolipophorin III, in this process is presented. The nature and structure of hemolymph lipid transfer particle and the potential role of a recently discovered lipoprotein receptor of the low-density lipoprotein receptor family, in lipophorin metabolism and lipid transport is reviewed.

Examination of the structure/function relationship in the exchangeable apolipoprotein, apolipophorin-III

Exchangeable apolipoproteins are proteins that reversibly bind lipoprotein particles to facilitate their transport in vivo. The structure/function relationship of apolipophorin-III (apo-III), the only insect exchangeable apolipoprotein, has been investigated by examining the association of this protein with lipid vesicles. The importance of a conserved leucine residue, reported to be essential for apo-III binding to lipids, has been evaluated through site-directed mutagenesis. A unique cysteine replaces the conserved leucine at position 30 in recombinant apo-III (L30C protein). This substitution results in the covalent dimerization of the apo-III mutant via a disulfide bond. The cysteine mutation causes no difference in surface hydrophobicity of the L30C proteins when compared to the wild type apo-III. Wild type apo-III, L30C monomer, and L30C dimer associate with dimyristoylphosphatidylcholine (DMAC) vesicles in a similar manner, resulting in a reduction of turbidity of a phospholipid vesicle suspension. Analysis with transmission electron microscopy (TEM) reveals disk-like complexes identical to those previously reported with the wild type apo-III. Because the mutation of the conserved leucine seems to affect the solution behavior and surface hydrophobicity of apo-III, this residue is likely to be exposed to the aqueous environment. However, the similar behaviors of the wild type protein, the L30C monomer, and L30C dimer with respect to the binding of phospholipid vesicles suggest that this residue is not absolutely required for the protein binding to hydrophobic or amphiphilic interfaces.

Insect immune activation by recombinant Galleria mellonella apolipophorin III(1)

Apolipophorin III (apoLp-III) is an exchangeable insect apolipoprotein. Its function, as currently understood, lies in the stabilization of low-density lipophorin particles (LDLp) crossing the hemocoel in phases of high energy consumption to deliver lipids from the fat body to the flight muscle cells. Recent studies with native Galleria mellonella-apoLp-III gave first indications of an unexpected role of that protein in insect immune activation. Here we report the immune activation by the recombinant protein, documenting a newly discovered correlation between lipid physiology and immune defense in insects. The complete cDNA sequence of G. mellonella-apoLp-III was identified by mixed oligonucleotide-primed amplification of cDNA (MOPAC), 3'-RACE-PCR, and cRACE-PCR. The sequence coding for the native protein was ligated into a pET-vector; this construct was transfected into Escherichia coli and overexpressed in the bacteria. Photometric turbidity assays with human low density lipoprotein (LDL) and transmission electron microscopy studies on apoLp-III-stabilized lipid discs revealed the full functionality of the isolated recombinant apoLp-III with regard to its lipid-association ability. For proving its immune-stimulating capacity, apoLp-III was injected into the hemocoel of last instar G. mellonella larvae and the antibacterial activity in cell-free hemolymph was determined 24 h later. As a result, the hemolymph samples of injected insects contained strongly increased antibacterial activities against E. coli as well as clearly enhanced lysozyme-like activities. From Northern blot analysis of total RNA from insects injected with apoLp-III or the bacterial immune provocator lipopolysaccharide, it could be concluded that the transcription rate of apoLp-III mRNA does not vary in comparison to untreated last instar larvae.

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

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

Structural characterization by computer experiments of the lipid-free LDL-receptor-binding domain of apolipoprotein E

The structure and dynamics of the lipid-free LDL-receptor-binding domain of apolipoprotein E (apoE-RBD) has been investigated by Molecular Dynamics Simulations. ApoE-RBD in its monomeric lipid-free form is a singular four-helix bundle made up of four elongated amphipathic helices. Analysis of one 1.5 ns molecular dynamics trajectory of apoE-RBD performed in water indicates that the lipid-free domain adopts a structure that exhibits characteristics found in native proteins: it has very stable helices and presents a compact structure. Yet its interior exhibits a larger number of transient atomic-size cavities relative to that found in other proteins of similar size and its apolar side chains are more mobile. The latter features distinguish the elongated four-helix bundle as a slightly disordered structure, which shows a structural likeness with some de novo designed four-helix bundle proteins and shares with the latter a leucine-rich residue composition. We anticipate that these unique properties compared with other native helix bundles may be related to the postulated ability of apoE-RBD to undergo an opening of its bundle upon interaction with phospholipids. The distribution of empty cavities computed along the trajectory in the interface regions between the different pairs of helices reveals that the tertiary contacts in one of the interfaces are weaker suggesting that this particular interface could be more easily ruptured upon lipid association.

A molecular trigger of lipid binding-induced opening of a helix bundle exchangeable apolipoprotein

Apolipophorin III (apoLp-III) from the sphinx moth, Manduca sexta, is a helix bundle protein that interacts reversibly with lipoproteins. Its five elongated amphipathic alpha-helices are organized in an antiparallel fashion, with helices 3 and 4 connected by a short 6-residue (PDVEKE) linker helix, termed helix 3'. Upon interaction with lipoproteins, apoLp-III opens to expose a continuous hydrophobic interior. It was postulated that helix bundle opening is preceded by an initiation step wherein helix 3' serves to recognize available lipoprotein surface binding sites. To test this hypothesis, helix 3' was replaced by residues that have a propensity to form a type I beta-turn, NPNG. This mutant apoLp-III was defective in lipoprotein binding assays. To define a more precise mode of interaction, the relevance of the presence of the hydrophobic Val-97 flanked by Asp-96 and Glu-98 was investigated by site-directed mutagenesis. V97N and D96N/V97N/E98Q apoLp-III were unable to compete with wild-type apoLp-III to initiate an interaction with lipoproteins, whereas D96N/E98Q apoLp-III was as competent as wild-type apoLp-III. The results suggest that Val-97 is critical, whereas Asp-96 and Glu-98 are irrelevant for initiating binding to lipoproteins. A model of binding is presented wherein apoLp-III is oriented with the helix 3' end of the molecule juxtaposed to the lipoprotein surface. Recognition of lipoprotein surface hydrophobic defects by Val-97 triggers opening of the helix bundle and facilitates formation of a stable binding interaction.

Purification and properties of a small lipid-binding protein from the hemolymph of Triatoma infestans

A small lipid-binding protein (sLBP) was purified from the hemolymph of the blood-sucking bug Triatoma infestans. Its isolation involved size exclusion-high performance liquid chromatography (HPLC) followed by anion exchange chromatography-HPLC. The molecular weight of the protein, as determined by gel permeation chromatography, was 20 kDa. Sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) resolved the protein into a single polypeptide with M(r) approximately equal to 16 kDa. The sLBP contains 6% lipids. Diacylglycerols represent the major lipid class, whereas phosphatidyl-choline, phosphatidyl-ethanolamine, free fatty acids and hydrocarbons were found in minor amounts. The amino acid composition indicated a high content of aspartic and glutamic acids and non-polar aliphatic amino acids. The N-terminal sequence did not resemble the sequence of any other previously reported insect hemolymph protein. Far-UV circular dichroism suggested that sLBP adopts a conformation rich in beta-sheet structure. The presence of this protein in hemolymph, fat body and unfertilized eggs was explored throughout the last nymphal and adult stages of the insect by Western blot assays. These assays indicated that sLBP is particularly abundant in hemolymph. A high concentration of sLBP was also detected in the fat body of the nymphs.

Isolation and characterization of an antifreeze protein from the longhorn sculpin, Myoxocephalus octodecimspinosis

A new type of antifreeze protein was isolated from the serum of the longhorn sculpin, Myoxocephalus octodecimspinosis, by gel filtration and high-performance liquid chromatography. This protein (LS-12) exhibits freezing point depression activity (thermal hysteresis) and ice crystal modification properties similar to those seen for other types of fish antifreeze polypeptide, except that ice crystals grow as hexagonal trapezohedra in the presence of LS-12, rather than hexagonal bipyramids usually seen. Ice crystal etching studies demonstrate that LS-12 does not bind to the hexagonal bipyramidal or secondary prism surfaces reported for the antifreeze polypeptides from winter flounder and shorthorn sculpin, respectively. Circular dichroism studies indicate that LS-12 has an alpha-helix content of about 60% at 1 degreesC, which is in good agreement with a value of about 70% predicted from the amino acid sequence. Limited proteolysis studies and further analysis of the amino acid sequence suggest that LS-12 consists of four amphipathic alpha-helices of similar length which are folded into a four-helix bundle. Based on its size (Mr=12299) and predicted tertiary structure, LS-12 can be regarded as the first example of a new class (type IV) of fish antifreeze protein.

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

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

Primary structure of apolipophorin-III from the greater wax moth, Galleria mellonella

The complete amino acid sequence of apolipophorin-III (apoLp-III), a lipid-binding hemolymph protein from the greater wax moth, Galleria mellonella, was determined by protein sequencing. The mature protein consists of 163 amino acid residues forming a protein of 18,075.5 Da. Its sequence is similar to apoLp-III from other Lepidopteran species, but remarkably different from the apoLp-IIIs of insects from other orders. As shown by mass spectrometric analysis, the protein carries no modifications. Thus, all of its known physiological functions, including its recently discovered immune response-stimulating activity, must reside in the protein itself.

Recombinant locust apolipophorin III: characterization and NMR spectroscopy

Apolipophorin III (apoLp-III) from the locust Locusta migratoria is an exchangeable apolipoprotein that reversibly binds to lipoproteins. During lipid binding the protein has been proposed to undergo a major conformational change. To study the mechanism of lipid binding we have cloned and expressed recombinant protein in bacteria, permitting stable isotope enrichment for heteronuclear NMR spectroscopy and site-directed mutagenesis. The cDNA coding for apoLp-III was subcloned into the pET expression vector and transformed into Escherichia coli cells. Induction of expression resulted in the specific appearance of apoLp-III in the cell culture medium, indicating it escaped the bacteria without lysis. The protein was purified from the cell-free supernatant by reversed-phase HPLC, characterized and compared to the natural protein isolated from locust hemolymph. SDS-PAGE revealed the recombinant protein has a molecular mass of approximately 17 kDa, similar to that of deglycosylated natural apoLp-III. Monoclonal antibodies were used to detect recombinant apoLp-III in the cells as well as in cell-free medium of induced bacterial cultures. Amino acid sequencing and analysis confirmed the identity of the recombinant protein as L. migratoria apoLp-III. Circular dichroism spectroscopy of recombinant and natural apoLp-III showed similar spectra, both displaying high contents of alpha-helical secondary structure. Denaturation studies of lipid-free apoLp-III with guanidine hydrochloride showed that both proteins have similar denaturation midpoints and DeltaG values indicating similar protein stability. The natural and recombinant protein were functional in lipoprotein binding assays. Using recombinant protein, uniformly and specifically labeled with 15N-amino acids, two dimensional 1H-15N heteronuclear single quantum correlation spectra were obtained. The spectra revealed excellent chemical shift dispersion in both the 1H and 15N dimensions with a well defined resonance pattern. Studies with 15N-leucine specifically labeled apoLp-III in the presence and absence of the micelle forming lipid, dodecylphosphocholine, provided evidence for a significant conformational change upon lipid association.

Bacterial expression and characterization of chicken apolipoprotein A-I

Apolipoprotein (apo) A-I is a 28-kDa exchangeable apolipoprotein that plays a key role in lipoprotein metabolism. It is widely distributed among animal species and is rich in alpha-helical secondary structure. Unlike human apoA-I, which aggregates in the absence of lipid, chicken apoA-I is monomeric in the lipid-free state. To take advantage of this physical characteristic, a bacterial expression system for production of recombinant chicken apoA-I has been developed. The cDNA-encoding chicken apoA-I was cloned into the pET expression vector under the regulation of the lac operon and transformed into Escherichia coli. Recombinant apoA-I protein recovered from the soluble fraction of the bacterial cell pellet was purified to greater than 95% homogeneity by reversed-phase high-performance liquid chromatography. Although immunoblot analysis confirmed the identity of the overexpressed protein, its migration on denaturing polyacrylamide gel electrophoresis was slower than its natural counterpart. To determine if the vector-encoded 18 residue pelB N-terminal leader sequence was not cleaved by the bacterial leader peptidase, isolated recombinant chicken apoA-I was incubated with exogenous leader peptidase. This treatment resulted in an increased electrophoretic mobility, with migration to a position corresponding to plasma-derived chicken apoA-I. Electrospray mass spectrometry indicated a mass of 27,961 +/- 4 Da, in agreement with that predicted for natural chicken apoA-I. Far-UV circular dichroism spectroscopy indicated an alpha-helical content similar to apoA-I isolated from chicken plasma, suggesting that the protein is folded in solution. Fluorescence studies showed that the wavelength of maximum fluorescence emission of the two tryptophan residues in the protein was 331 nm, with no shift occurring following complexation with lipid. Recombinant apoA-I was shown to be functional in lipoprotein binding as well as to possess an ability to transform bilayer vesicles of dimyristoylphosphatidylcholine into discoidal complexes. This is the first report of bacterial expression of an avian apoA-I. Increased availability and the potential for site-directed mutagenesis of this protein will aid in further characterization of apoA-I and the mechanism whereby it functions in cholesterol transport.

Interhelical contacts are required for the helix bundle fold of apolipophorin III and its ability to interact with lipoproteins

Apolipophorin-III (apoLp-III) from the insect, Manduca sexta, is a 166-residue exchangeable apolipoprotein that plays a critical role in the dynamics of plasma lipoprotein interconversions. Our previous work indicated that a 36-residue C-terminal peptide fragment, generated by cyanogen bromide digestion of apoLp-III, was unable to bind to lipid surfaces (Narayanaswami V, Kay CM, Oikawa K, Ryan RO, 1994, Biochemistry 33:13312-13320), and showed no secondary structure in aqueous solution. In this paper, we have performed structural studies of this peptide (E131-Q166) complexed with SDS detergent micelles, or in the presence of the helix-inducing solvent trifluoroethanol (TFE), by two-dimensional 1H NMR spectroscopy. The peptide adopts an alpha-helical structure in the presence of both SDS and 50% TFE. The lipid-bound structure of the peptide, generated from the NMR NOE data, showed an elongated, slightly curved alpha-helix. Despite its high alpha-helix forming propensity, the peptide requires alpha helix-promoting environment to adopt an alpha-helical structure. This indicates the importance of the surrounding chemical environment and implies that, in the absence of lipid, tertiary contacts in the folded protein play a role in maintaining its structural integrity. Furthermore, the data suggest that the amphipathic helix bundle organization serves as a prerequisite structural motif for the reversible lipoprotein-binding activity of M. sexta apoLp-III.

Fluorescence studies of exchangeable apolipoprotein-lipid interactions. Superficial association of apolipophorin III with lipoprotein surfaces

Apolipophorin III (apoLp-III) from the Sphinx moth, Manduca sexta, is an 18-kDa exchangeable apolipoprotein that reversibly associates with lipoprotein particles. In the absence of lipid, apoLp-III exists as an elongated bundle of five amphipathic alpha-helices. Upon lipid association, the protein is postulated to undergo a major conformational change, wherein the bundle opens around hinge loop regions, resulting in exposure of its hydrophobic interior. Fluorescence quenching techniques have been employed to study apoLp-III helix topography and spatial arrangement in phospholipid disc complexes and intact lipoprotein particles. Intrinsic fluorescence of the single tyrosine in apoLp-III was exploited to monitor the location of helix 5 in model disc complexes. To investigate other regions of the protein, site-directed mutagenesis was performed to introduce cysteine residues, replacing Asn-40 (helix 2, N40C) or Leu-90 (helix 3, L90C), thereby providing two mutant apoLp-IIIs, each with a single site for covalent attachment of the extrinsic fluorescent probe, N-(1-pyrene) maleimide. In the lipid-free state, pyrene-N40C- and pyrene-L90C-apoLp-III were highly accessible to the negatively charged aqueous quencher KI, yielding Ksv values of 27.1 and 19.8 M-1, respectively. Upon binding to the surface of a spherical lipoprotein particle, Ksv values for KI decreased by about 90% for both pyrene-labeled apoLp-IIIs, indicating a significant change in the local microenvironment of the fluorophores. A lesser decrease in Ksv was observed when the pyrene-labeled apoLp-IIIs were bound to phospholipid disc complexes. When spin-labeled fatty acids 5-doxylstearic acid and 12-doxylstearic acid were used as lipophilic quenchers, tyrosine and pyrene fluorescence were more effectively quenched by 5-doxylstearic acid in both phospholipid bilayer disc complexes and spherical lipoprotein particles. These data provide insight into the spatial topography of apoLp-III alpha-helices in phospholipid disc complexes and support the concept that interaction with spherical lipoprotein particles results in superficial contact of apoL-III helical segments with the monolayer surface, providing a basis for its reversible binding ability.

Cloning and expression of apolipophorin-III from the common cutworm, Spodoptera litura

We have cloned apolipophorin-III (apoLp-III) cDNA from adult fat body of Spodoptera litura. The sequence encodes a 188 amino acid polypeptide including a 22 amino acid leader peptide. The circular dichroism spectrum from the purified apoLp-III indicated a considerable content of alpha-helix. Sequence alignment showed that S. litura apoLp-III has a relatively high degree of sequence identity with the apoLps-III of lepidopteran, Manduca sexta (72%), Galleria mellonella (67%), Bombyx mori (60%). These alignments with four lepidopteran apoLps-III showed highly identical residues and conservative replacements at a degree of 86%. Levels of mRNA from last instar larval fat body and adult fat body were compared through Northern blot analysis using 32-P-labeled 704 bp apoLp-III cDNA probe. A 850 bp mRNA was detected in both stages and mRNA level of day 1 adult fat body was much higher than that of last instar larval fat body. The tissue-distribution of apoLp-III mRNA in adult ovary and testis was also examined and we confirmed the presence of apoLp-III mRNA in ovary and testis although apoLp-III was expressed in these tissues at very low levels compared with the adult fat body.

Antifreeze proteins

Antifreeze proteins comprise a structurally diverse class of proteins that inhibit the growth of ice. Recently, new AFP types have been discovered; more active AFPs have been isolated; antecedents have been recognized supporting the notion of recent, multiple origins; and detailed structures have emerged leading to models for their adsorption to ice.