Recombinant locust apolipophorin III: characterization and NMR spectroscopy

Foam fractionation of a recombinant biosurfactant apolipoprotein

Locusta migratoria apolipophorin III (apoLp-III) possesses the ability to exist as a water soluble amphipathic α-helix bundle and a lipid surface seeking apolipoprotein. The intrinsic ability of apoLp-III to transform phospholipid vesicles into reconstituted discoidal high-density lipoproteins (rHDL) has led to myriad applications. To improve the yield of recombinant apoLp-III, studies were performed in a bioreactor. Induction of apoLp-III expression generated a protein product that is secreted from E. coli into the culture medium. Interaction of apoLp-III with gas and liquid components in media produced large quantities of thick foam. A continuous foam fractionation process yielded a foamate containing apoLp-III as the sole major protein component. The yield of recombinant apoLp-III was ~0.2 g / liter bacterial culture. Mass spectrometry analysis verified the identity of the target protein and indicated no modifications or changes to apoLp-III occurred as a result of foam fractionation. The functional ability of apoLp-III to induce rHDL formation was evaluated by incubating foam fractionated apoLp-III with phosphatidylcholine vesicles. FPLC size exclusion chromatography revealed a single major population of particles in the size range of rHDL. The results described offer a novel approach to bioreactor-based apoLp-III production that takes advantage of its intrinsic biosurfactant properties.

Calcium-induced release of cytochrome c from cardiolipin nanodisks: Implications for apoptosis

Miniature bilayer membranes comprised of phospholipid and an apolipoprotein scaffold, termed nanodisks (ND), have been used in binding studies. When ND formulated with cardiolipin (CL), but not phosphatidylcholine, were incubated with cytochrome c, FPLC gel filtration chromatography provided evidence of a stable binding interaction. Incubation of CL ND with CaCl₂ resulted in a concentration-dependent increase in sample turbidity caused by ND particle disruption. Prior incubation of CL ND with cytochrome c increased CL ND sensitivity to CaCl₂-induced effects. Centrifugation of CaCl₂-treated CL ND samples yielded pellet and supernatant fractions. Whereas the ND scaffold protein, apolipophorin III, was recovered in the pellet fraction along with CL, the majority of the cytochrome c pool was in the supernatant fraction. Moreover, when cytochrome c CL ND were incubated with CaCl₂ at concentrations below the threshold to induce ND particle disruption, FPLC analysis showed that cytochrome c was released. Pre-incubation of CL ND with CaCl₂ under conditions that do not disrupt ND particle integrity prevented cytochrome c binding to CL ND. Thus, competition between Ca²⁺ and cytochrome c for a common binding site on CL modulates cytochrome c binding and likely plays a role in its dissociation from CL-rich cristae membranes in response to apoptotic stimuli.

Fragments of Locusta migratoria apoLp-III provide insight into lipid binding

Apolipophorin III (apoLp-III) from Locusta migratoria is an exchangeable apolipoprotein with a critical role in lipid transport in insects. The protein is composed of a bundle of five amphipathic α-helices which undergo a large conformational change upon lipid binding. To better understand the apoLp-III lipid binding interaction, the protein was cleaved by cyanogen bromide upon introduction of a S92M mutation, generating an N-terminal fragment corresponding to the first three helices (NT_H1–3) and a C-terminal fragment of the last two helices (CT_H4–5). MALDI-TOF analysis of the HPLC purified fragments provided masses of 9863.8 Da for NT_H1–3 and 7497.0 Da for CT_H4–5 demonstrating that the intended fragments were obtained. Circular dichroism spectra revealed a decrease in helical content from 82% for the intact protein to 57% for NT_H1–3 and 41% for CT_H4–5. The fragments adopted considerably higher α-helical structure in the presence of trifluoroethanol or phospholipids. Equimolar mixing of the two fragments did not result in changes in helical content or tryptophan fluorescence, indicating recombination into the native protein fold did not occur. The rate of protein induced dimyristoylphosphatidylcholine vesicle solubilization increased 15-fold for NT_H1–3 and 100-fold for CT_H4–5 compared to the intact protein. Despite the high activity in phospholipid vesicle interaction, CT_H4–5 did not protect phospholipase-treated low-density lipoprotein from aggregation. In contrast, NT_H1–3 provided protection to lipoprotein aggregation similar to the intact protein, indicating that specific amino acid residues in this part of apoLp-III are essential for lipoprotein binding interaction.

Interaction of a model apolipoprotein, apoLp-III, with an oil-phospholipid interface

Lipid droplets are "small" organelles that play an important role in de novo synthesis of new membrane, and steroid hormones, as well as in energy storage. The way proteins interact specifically with the oil-(phospho-)lipid monolayer interface of lipid droplets is a relatively unexplored but crucial question. Here, we use our home built liquid droplet tensiometer to mimic intracellular lipid droplets and study protein-lipid interactions at this interface. As model neutral lipid binding protein, we use apoLp-III, an amphipathic α-helix bundle protein. This domain is also found in proteins from the perilipin family and in apoE. Protein binding to the monolayer is studied by the decrease in the oil/water surface tension. Previous work used POPC (one of the major lipids found on lipid droplets) to form the phospholipid monolayer on the triolein surface. Here we expand this work by incorporating other lipids with different physico-chemical properties to study the effect of charge and lipid head-group size. This study sheds light on the affinity of this important protein domain to interact with lipids.

Expression of the C-terminal domain of human apolipoprotein A-I using a chimeric apolipoprotein

Human apolipoprotein A-I (apoA-I) is the most abundant protein in high-density lipoprotein, an anti-atherogenic lipid-protein complex responsible for reverse cholesterol transport. The protein is composed of an N-terminal helix bundle domain, and a small C-terminal (CT) domain. To facilitate study of CT-apoA-I, a novel strategy was employed to produce this small domain in a bacterial expression system. A protein construct was designed of insect apolipophorin III (apoLp-III) and residues 179-243 of apoA-I, with a unique methionine residue positioned between the two proteins and an N-terminal His-tag to facilitate purification. The chimera was expressed in E. coli, purified by Ni-affinity chromatography, and cleaved by cyanogen bromide. SDS-PAGE revealed the presence of three proteins with masses of 7 kDa (CT-apoA-I), 18 kDa (apoLp-III), and a minor 26 kDa band of uncleaved chimera. The digest was reloaded on the Ni-affinity column to bind apoLp-III and uncleaved chimera, while CT-apoA-I was washed from the column and collected. Alternatively, CT-apoA-I was isolated from the digest by reversed-phase HPLC. CT-apoA-I was α-helical, highly effective in solubilizing phospholipid vesicles and disaggregating LPS micelles. However, CT-apoA-I was less active compared to full-length apoA-I in protecting lipolyzed low density lipoproteins from aggregating, and disrupting phosphatidylglycerol bilayer vesicles. Thus the novel expression system produced mg quantities of functional CT-apoA-I, facilitating structural and functional studies of this critical domain of apoA-I.

Transfer of C-terminal residues of human apolipoprotein A-I to insect apolipophorin III creates a two-domain chimeric protein with enhanced lipid binding activity

Apolipophorin III (apoLp-III) is an insect apolipoprotein (18kDa) that comprises a single five-helix bundle domain. In contrast, human apolipoprotein A-I (apoA-I) is a 28kDa two-domain protein: an α-helical N-terminal domain (residues 1-189) and a less structured C-terminal domain (residues 190-243). To better understand the apolipoprotein domain organization, a novel chimeric protein was engineered by attaching residues 179 to 243 of apoA-I to the C-terminal end of apoLp-III. The apoLp-III/apoA-I chimera was successfully expressed and purified in E. coli. Western blot analysis and mass spectrometry confirmed the presence of the C-terminal domain of apoA-I within the chimera. While parent apoLp-III did not self-associate, the chimera formed oligomers similar to apoA-I. The chimera displayed a lower α-helical content, but the stability remained similar compared to apoLp-III, consistent with the addition of a less structured domain. The chimera was able to solubilize phospholipid vesicles at a significantly higher rate compared to apoLp-III, approaching that of apoA-I. The chimera was more effective in protecting phospholipase C-treated low density lipoprotein from aggregation compared to apoLp-III. In addition, binding interaction of the chimera with phosphatidylglycerol vesicles and lipopolysaccharides was considerably improved compared to apoLp-III. Thus, addition of the C-terminal domain of apoA-I to apoLp-III created a two-domain protein, with self-association, lipid and lipopolysaccharide binding properties similar to apoA-I. The apoA-I like behavior of the chimera indicate that these properties are independent from residues residing in the N-terminal domain of apoA-I, and that they can be transferred from apoA-I to apoLp-III.

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.

Insertion of apoLp-III into a lipid monolayer is more favorable for saturated, more ordered, acyl-chains

Neutral lipid transport in mammals is complicated involving many types of apolipoprotein. The exchangeable apolipoproteins mediate the transfer of hydrophobic lipids between tissues and particles, and bind to cell surface receptors. Amphipathic α-helices form a common structural motif that facilitates their lipid binding and exchangeability. ApoLp-III, the only exchangeable apolipoprotein found in insects, is a model amphipathic α-helix bundle protein and its three dimensional structure and function mimics that of the mammalian proteins apoE and apoAI. Even the intracellular exchangeable lipid droplet protein TIP47/perilipin 3 contains an α-helix bundle domain with high structural similarity to that of apoE and apoLp-III. Here, we investigated the interaction of apoLp-III from Locusta migratoria with lipid monolayers. Consistent with earlier work we find that insertion of apoLp-III into fluid lipid monolayers is highest for diacylglycerol. We observe a preference for saturated and more highly ordered lipids, suggesting a new mode of interaction for amphipathic α-helix bundles. X-ray reflectivity shows that apoLp-III unfolds at a hydrophobic interface and flexible loops connecting the amphipathic α-helices stay in solution. X-ray diffraction indicates that apoLp-III insertion into diacylglycerol monolayers induces additional ordering of saturated acyl-chains. These results thus shed important new insight into the protein-lipid interactions of a model exchangeable apolipoprotein with significant implications for its mammalian counterparts.

SERINE proteinase like activity in apolipophorin III from the hemolymph of desert locust, Schistocerca gregaria

Apolipophorin III (apoLp-III) has been known as a lipid transport protein of insects. Recent studies indicated the involvement of apoLp-III in immune reactions and in the control of cell destruction, but no enzymatic activity has so far been detected. In the present study, a protease from the hemolymph of Schistocerca gregaria was purified to homogeneity and its enzymatic activity was examined. Identity as chymotrypsin-like proteinase was established by its high affinity toward bulky aromatic substrates and its catalytic specificity for amide or ester bonds on the synthetic substrates, Suc-Ala-Ala-Pro-Xaa-AMC (where Xaa was Phe, Tyr, Trp, and Lys, and AMC is 7-amino-4-methyl-coumarin) and thiolbenzyl ester substrate Suc-Ala-Ala-Pro-Phe-SBzl. The sensitivity for serine protease and chymotrypsin-specific covalent inhibitors, PMSF, TPCK, and noncovalent inhibitors SGCI, showed that it is a chymotrypsin-like proteinase. It showed its maximum activity at pH 8.0 and 55°C for the hydrolysis of Suc-Ala-Ala-Pro-Tyr-AMC. According to similarities in the amino terminal sequence, molar mass (19 kDa) and retention on reversed-phase analytical high-performance liquid chromatography (HPLC) column, this protein is S. gregaria homologue of Locusta migratoria apoLp-III. Our data suggest that apoLp-III also has an inherent proteolytic activity. Results indicated that S. gregaria apoLp-III is a good catalyst and could be used as a biotechnological tool in food processing and in agricultural biotechnology.

Apolipoprotein-induced conversion of phosphatidylcholine bilayer vesicles into nanodisks

Apolipoprotein mediated formation of nanodisks was studied in detail using apolipophorin III (apoLp-III), thereby providing insight in apolipoprotein-lipid binding interactions. The spontaneous solubilization of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) vesicles occured only in a very narrow temperature range at the gel-liquid-crystalline phase transition temperature, exhibiting a net exothermic interaction based on isothermal titration calorimetry analysis. The resulting nanodisks were protected from proteolysis by trypsin, endoproteinase Glu-C, chymotrypsin and elastase. DMPC solubilization and the simultaneous formation of nanodisks were promoted by increasing the vesicle diameter, protein to lipid ratio and concentration. Inclusion of cholesterol in DMPC dramatically enhanced the rate of nanodisk formation, presumably by stabilization of lattice defects which form the main insertion sites for apolipoprotein α-helices. The presence of fully saturated acyl chains with a length of 13 or 14 carbons in phosphatidylcholine allowed the spontaneous vesicle solubilization upon apolipoprotein addition. Nanodisks with C13:0-phosphatidylcholine were significantly smaller with a diameter of 11.7 ± 3.1nm compared to 18.5 ± 5.6 nm for DMPC nanodisks determined by transmission electron microscopy. Nanodisk formation was not observed when the phosphatidylcholine vesicles contained acyl chains of 15 or 16 carbons. However, using very high concentrations of lipid and protein (>10mg/ml), 1,2,-dipalmitoyl-sn-glycero-3-phosphocholine nanodisks could be produced spontaneously although the efficiency remained low.

Apolipophorin III interaction with model membranes composed of phosphatidylcholine and sphingomyelin using differential scanning calorimetry

Apolipophorin III (apoLp-III) from Locusta migratoria was employed as a model apolipoprotein to gain insight into binding interactions with lipid vesicles. Differential scanning calorimetry (DSC) was used to measure the binding interaction of apoLp-III with liposomes composed of mixtures of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and sphingomyelin (SM). Association of apoLp-III with multilamellar liposomes occurred over a temperature range around the liquid crystalline phase transition (L(alpha)). Qualitative and quantitative data were obtained from changes in the lipid phase transition upon addition of apoLp-III. Eleven ratios of DMPC and SM were tested from pure DMPC to pure SM. Broadness of the phase transition (T(1/2)), melting temperature of the phase transition (T(m)) and enthalpy were used to determine the relative binding affinity to the liposomes. Multilamellar vesicles composed of 40% DMPC and 60% SM showed the greatest interaction with apoLp-III, indicated by large T(1/2) values. Pure DMPC showed the weakest interaction and liposomes with lower percentage of DMPC retained domains of pure DMPC, even upon apoLp-III binding indicating demixing of liposome lipids. Addition of apoLp-III to rehydrated liposomes was compared to codissolved trials, in which lipids were rehydrated in the presence of protein, forcing the protein to interact with the lipid system. Similar trends between the codissolved and non-codissolved trials were observed, indicating a similar binding affinity except for pure DMPC. These results suggested that surface defects due to non-ideal packing that occur at the phase transition temperature of the lipid mixtures are responsible for apolipoprotein-lipid interaction in DMPC/SM liposomes.

Apolipophorin III lysine modification: Effect on structure and lipid binding

Apolipophorin III (apoLp-III) from Locusta migratoria was used as a model to investigate apolipoprotein lipid binding interactions. ApoLp-III contains eight lysine residues, of which seven are located on one side of the protein. To investigate the role of positive charges on lipid binding, lysine residues were acetylated by acetic anhydride. The degree of acetylation was analyzed by SDS-PAGE and MALDI-TOF, indicating a maximum of eight acetyl additions. Modified apoLp-III remained alpha-helical, but displayed a decreased alpha-helical content (from 78 to 54%). Acetylation resulted in a slight increase in protein stability, as indicated by a change in the midpoint of guanidine-HCl induced denaturation from 0.55 (unmodified) to 0.65 M (acetylated apoLp-III). Lipid bound apoLp-III, either acetylated or unmodified, displayed similar increases in helical content and midpoint of guanidine-HCl-induced denaturation of approximately 4 M. The ability to solubilize vesicles of dimyristoylphosphatidylcholine remained unchanged. However, the rate to solubilize dimyristoylphosphatidylglycerol vesicles was reduced two-fold. In addition, a decreased ability to stabilize diacylglycerol-enriched low density lipoproteins was observed. This indicated that lysine residues are not critical for the protein's ability to bind to zwitterionic phospholipids. Since binding interactions with ionic phospholipids and lipoproteins were affected by acetylation, lysine side-chains may play a modulating role in the interaction with more complex lipid surfaces encountered in vivo.

Solution NMR and CD spectroscopy of an intrinsically disordered, peripheral membrane protein: evaluation of aqueous and membrane-mimetic solvent conditions for studying the conformational adaptability of the 18.5 kDa isoform of myelin basic protein (MBP)

The stability and secondary structure propensity of recombinant murine 18.5 kDa myelin basic protein (rmMBP, 176 residues) was assessed using circular dichroic and nuclear magnetic resonance spectroscopy (1H-15N HSQC experiments) to determine the optimal sample conditions for further NMR studies (i.e., resonance assignments and protein-protein interactions). Six solvent conditions were selected based on their ability to stabilise the protein, and their tractability to currently standard solution NMR methodology. Selected solvent conditions were further characterised as functions of concentration, temperature, and pH. The results of these trials indicated that 30% TFE-d2 in H2O (v/v), pH 6.5 at 300 K, and 100 mM KCl, pH 6.5 at 277 K were the best conditions to use for future solution NMR studies of MBP. Micelles of DPC were found to be inappropriate for backbone resonance assignments of rmMBP in this instance.

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.

Apolipophorin-III-like protein expressed in the antenna of the red imported fire ant, Solenopsis invicta Buren (Hymenoptera: Formicidae)

Antennal proteins of the male fire ant (Solenopsis invicta) were analyzed by two-dimensional gel electrophoresis, with the objective of identifying pheromone-binding proteins, which have not previously been found in ant antennae. The major low-molecular weight protein found in the male fire ant antenna was subjected to Edman degradation to determine the N-terminal amino acid sequence. Degenerate PCR primers based on this sequence were used to obtain a cDNA sequence corresponding to the full-length protein sequence. In-gel trypsin digestion followed by MALDI-TOF mass spectrometry and HPLC-ESI/MS/MS demonstrated that the protein gel spot contained only the protein corresponding to the cDNA sequence obtained by PCR. The sequence is similar to apolipophorin-III, an exchangeable lipid-binding protein. Fire ant apolipophorin-III is expressed in the antenna as well as the head, thorax and abdomen.

Lipopolysaccharide binding of an exchangeable apolipoprotein, apolipophorin III, from Galleria mellonella

A new role of apolipophorin III (apoLp-III) as an immune activator has emerged recently. To gain insight into this novel function, the interaction of apoLp-III with lipopoly-saccharide (LPS) was investigated. ApoLp-III fromGalleria mellonellawas incubated with LPS fromEscherichia coliO55:B5, and analyzed by non-denaturing polyacrylamide gel electrophoresis (PAGE). Protein staining showed that apoLp-III mobility was significantly reduced. In addition, silver and LPS fluorescent staining demonstrated that LPS mobility was increased upon incubation with apoLp-III. This result suggests association of apoLp-III with LPS. Sodium dodecyl sulfate (SDS) PAGE analysis showed decreased apoLp-III mobility upon LPS addition, indicative of LPS apoLp-III interaction in the presence of SDS. The unique tyrosine residue that resides in apoLp-III was used to provide additional evidence for LPS binding interaction. In the absence of LPS, apoLp-III tyrosine fluorescence was relatively low. However, LPS addition resulted in a progressive increase in the fluorescence intensity, indicating tertiary rearrangement in the environment of tyrosine 142 upon LPS interaction. Other well-characterized apoLp-IIIs were also examined for LPS binding.Manduca sexta,Bombyx moriandLocusta migratoriaapoLp-III were all able to interact with LPS. The ability of apoLp-III to form complexes with LPS supports the proposed role of apoLp-III in innate immunity.

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.

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.

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.

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.

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.

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.