Differential lipid binding of truncation mutants of Galleria mellonella apolipophorin III

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.

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.

Mechanism of Lipid Binding of Human Apolipoprotein E3 by Hydrogen/Deuterium Exchange/Mass Spectrometry and Fluorescence Polarization

BACKGROUND

Human apolipoprotein E3 (apoE3) is an exchangeable apolipoprotein that plays a critical role in maintaining plasma cholesterol/triglyceride homeostasis. The C-terminal (CT) domain of apoE3 (residues 201-299) is composed of amphipathic α-helices C1: W210-S223, C2: V236-E266, and C3: D271-W276, which play a dominant role in mediating high-affinity lipid binding.

OBJECTIVE

The objective is to understand the accessibility of the CT domain at the sub-domain level and the mechanistic details regarding lipid-binding interaction.

METHODS

Hydrogen-deuterium exchange coupled to mass spectrometry (HDX/MS) of recombinant wild type (WT) apoE(201-299), chemical-induced unfolding monitored as changes in fluorescence polarization (FP) of labeled apoE(201-299) bearing a probe at specified sites, and lipid binding studies were carried out.

RESULTS

HDX/MS revealed that residues towards the C-terminal end of the domain display significantly lower %D uptake compared to those towards the center, suggesting extensive protein-protein interaction in this segment. Functional assays showed that locking apoE(201-299) in an inter-molecular disulfide-bonded state at position 209, 223, 255, or 277 significantly decreases its ability to interact with lipids, especially when tethered towards the ends; this could be restored by reduction. Unfolding studies indicate that the C-terminal end offers less resistance to unfolding compared to the central portion of the domain.

CONCLUSION

Taken together, our data suggest that two dimers of CT domain are juxtaposed around helix C3 leading to apoE3 tetramerization, and that dissociation to monomeric units is a required step in lipid binding, with helix C3 likely seeking stability via lipid interaction prior to helices C1 or C2.

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.

The lipid composition of Legionella dumoffii membrane modulates the interaction with Galleria mellonella apolipophorin III

Apolipophorin III (apoLp-III), an insect homologue of human apolipoprotein E (apoE), is a widely used model protein in studies on protein-lipid interactions, and anti-Legionella activity of Galleria mellonella apoLp-III has been documented. Interestingly, exogenous choline-cultured Legionella dumoffii cells are considerably more susceptible to apoLp-III than non-supplemented bacteria. In order to explain these differences, we performed, for the first time, a detailed analysis of L. dumoffii lipids and a comparative lipidomic analysis of membranes of bacteria grown without and in the presence of exogenous choline. (31)P NMR analysis of L. dumoffii phospholipids (PLs) revealed a considerable increase in the phosphatidylcholine (PC) content in bacteria cultured on choline medium and a decrease in the phosphatidylethanolamine (PE) content in approximately the same range. The interactions of G. mellonella apoLp-III with lipid bilayer membranes prepared from PLs extracted from non- and choline-supplemented L. dumoffii cells were examined in detail by means of attenuated total reflection- and linear dichroism-Fourier transform infrared spectroscopy. Furthermore, the kinetics of apoLp-III binding to liposomes formed from L. dumoffii PLs was analysed by fluorescence correlation spectroscopy and fluorescence lifetime imaging microscopy using fluorescently labelled G. mellonella apoLp-III. Our results indicated enhanced binding of apoLp-III to and deeper penetration into lipid membranes formed from PLs extracted from the choline-supplemented bacteria, i.e. characterized by an increased PC/PE ratio. This could explain, at least in part, the higher susceptibility of choline-cultured L. dumoffii to G. mellonella apoLp-III.

Helix 1 tryptophan variants in Galleria mellonella apolipophorin III

Apolipophorin III (apoLp-III) from Galleria mellonella is a critical apolipoprotein aiding in lipid transport and has gained considerable interest for a role in innate immunity. Both functions are likely related and form the rationale to gain a more detailed understanding of the lipid binding properties of this insect apolipoprotein. Tryptophan residues were introduced at positions 16, 20 or 24, all in helix 1 as it may play a critical role in the initial steps of lipid binding. Steady-state fluorescence analysis showed that each tryptophan displayed unique properties, indicating different environments both in lipid-free as in lipid-bound states, and demonstrating potential for use in lipid binding analysis. While α-helical contents of wild-type and the tryptophan variant proteins were similar, W20- and W24-apoLp-III displayed increased protein stability. These variants were significantly slower in their ability to convert phosphatidylcholine vesicles into discoidal lipoproteins, which was employed as a measure for lipid binding. In contrast, W16-apoLp-III displayed decreased protein stability but an order of magnitude higher rate of discoidal lipoprotein formation. This demonstrates an inverse correlation between protein stability and the ability to convert vesicles in discoidal lipoproteins. The most stable W20-apoLp-III variant displayed comprised LDL binding capabilities, indicating a partial loss of function. Thus, there is a delicate balance between helix bundle stability and the ability to bind lipids, and helix 1 may play a critical role in this process.

Genomic organization, sequence characterization and expression analysis of Tenebrio molitor apolipophorin-III in response to an intracellular pathogen, Listeria monocytogenes

Apolipophorin III (apoLp-III) is a well-known hemolymph protein having a functional role in lipid transport and immune response of insects. We cloned full-length cDNA encoding putative apoLp-III from larvae of the coleopteran beetle, Tenebrio molitor (TmapoLp-III), by identification of clones corresponding to the partial sequence of TmapoLp-III, subsequently followed with full length sequencing by a clone-by-clone primer walking method. The complete cDNA consists of 890 nucleotides, including an ORF encoding 196 amino acid residues. Excluding a putative signal peptide of the first 20 amino acid residues, the 176-residue mature apoLp-III has a calculated molecular mass of 19,146Da. Genomic sequence analysis with respect to its cDNA showed that TmapoLp-III was organized into four exons interrupted by three introns. Several immune-related transcription factor binding sites were discovered in the putative 5'-flanking region. BLAST and phylogenetic analyses reveal that TmapoLp-III has high sequence identity (88%) with Tribolium castaneum apoLp-III but shares little sequence homologies (<26%) with other apoLp-IIIs. Homology modeling of Tm apoLp-III shows a bundle of five amphipathic alpha helices, including a short helix 3'. The 'helix-short helix-helix' motif was predicted to be implicated in lipid binding interactions, through reversible conformational changes and accommodating the hydrophobic residues to the exterior for stability. Highest level of TmapoLp-III mRNA was detected at late pupal stages, albeit it is expressed in the larval and adult stages at lower levels. The tissue specific expression of the transcripts showed significantly higher numbers in larval fat body and adult integument. In addition, TmapoLp-III mRNA was found to be highly upregulated in late stages of L. monocytogenes or E. coli challenge. These results indicate that TmapoLp-III may play an important role in innate immune responses against bacterial pathogens in T. molitor.

Biochemical and biophysical characterization of recombinant rat apolipoprotein E: similarities to human apolipoprotein E3

Apolipoprotein E (apoE) is an anti-atherogenic protein that plays a critical role in maintaining plasma cholesterol and triglyceride homeostasis by virtue of its ability to act as a ligand for the low-density lipoprotein receptor (LDLr) family of proteins. In this study, we characterized the biochemical and biophysical features of recombinant rat apoE, in comparison with those of human apoE3. Rat apoE was overexpressed in Escherichia coli using a codon optimized system and purified by affinity chromatography. SDS-PAGE and RP-HPLC of rat apoE confirmed the purity, while immunoblot verified the identity and cross-reactivity with the LDLr-binding region of apoE3. The α-helical content was calculated to be ~45% by circular dichroism spectroscopy. The protein exists in a predominantly tetrameric form in lipid-free state. Chemical denaturation studies reveal that the unfolding pattern is biphasic with mid points of denaturation corresponding to 0.8 and 2.2 M guanidine hydrochloride, suggesting the presence of two domains. Rat apoE converts DMPC vesicles to smaller DMPC/apoE complexes with a first order rate constant of 0.12 min(-1). It has the ability to bind the LDLr and to heparin. Our studies indicate that although its sequence resembles apoE4, an isoform of apoE3, rat apoE displays the biophysical behavior of apoE3.

Characterization of the apoLp-III/LPS complex: insight into the mode of binding interaction

Apolipoproteins are able to associate with lipopolysaccharides (LPS), potentially providing protection against septic shock. To gain insight into the molecular details of this binding interaction, apolipophorin III (apoLp-III) from Galleria mellonella was used as a model. The binding of apoLp-III to LPS was optimal around 37-40 °C, close to the LPS phase transition temperature. ApoLp-III formed complexes with LPS from E. coli (serotype O55:B5) with a diameter of ~20 nm and a molecular weight of ~390 kDa, containing four molecules of apoLp-III and 24 molecules of LPS. The LPS-bound form of the protein was substantially more resistant to guanidine-induced denaturation compared to unbound protein. The denaturation profile displayed a multiphase character with a steep drop in secondary structure between 0 and 1 M guanidine-HCl and a slower decrease above 1 M guanidine-HCl. In contrast, apoLp-III bound to detoxified LPS was only slightly more resistant to guanidine-HCl induced denaturation compared to unbound protein. Analysis of size-exclusion FPLC elution profiles of mixtures of apoLp-III with LPS or detoxified LPS indicated a much weaker binding interaction with detoxified LPS compared to intact LPS. These results indicate that apoLp-III initially interacts with exposed carbohydrate regions, but that the lipid A region is required for a more stable LPS binding interaction.

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.

Modification by acrolein, a component of tobacco smoke and age-related oxidative stress, mediates functional impairment of human apolipoprotein E

Oxidative damage to proteins such as apolipoprotein B-100 increases the atherogenicity of low-density lipoproteins (LDL). However, little is known about the potential oxidative damage to apolipoprotein E (apoE), an exchangeable antiatherogenic apolipoprotein. ApoE plays an integral role in lipoprotein metabolism by regulating the plasma cholesterol and triglyceride levels. Hepatic uptake of lipoproteins is facilitated by apoE's ability to bind with cell surface heparan sulfate proteoglycans and to lipoprotein receptors via basic residues in its 22 kDa N-terminal domain (NT). We investigated the effect of acrolein, an aldehydic product of endogenous lipid peroxidation and a tobacco smoke component, on the conformation and function of recombinant human apoE3-NT. Acrolein caused oxidative modification of apoE3-NT as detected by Western blot with acrolein-lysine-specific antibodies, and tertiary conformational alterations. Acrolein modification impairs the ability of apoE3-NT to interact with heparin and the LDL receptor. Furthermore, acrolein-modified apoE3-NT displayed a 5-fold decrease in its ability to interact with lipid surfaces. Our data indicate that acrolein disrupts the functional integrity of apoE3, which likely interferes with its role in regulating plasma cholesterol homeostasis. These observations have implications regarding the role of apoE in the pathogenesis of smoking- and oxidative stress-mediated cardiovascular and cerebrovascular diseases.

Apolipophorin III: lipopolysaccharide binding requires helix bundle opening

Apolipophorin III (apoLp-III) is a prototypical apolipoprotein used for structure-function studies. Besides its crucial role in lipid transport, apoLp-III is able to associate with fungal and bacterial membranes and stimulate cellular immune responses. We recently demonstrated binding interaction of apoLp-III of the greater wax moth, Galleria mellonella, with lipopolysaccharides (LPS). In the present study, the requirement of helix bundle opening for LPS binding interaction was investigated. Using site-directed mutagenesis, two cysteine residues were introduced in close spatial proximity (P5C/A135C). When the helix bundle was locked by disulfide bond formation, the tethered helix bundle failed to associate with LPS. In contrast, the mutant protein regained its ability to bind upon reduction with dithiothreitol. Thus, helix bundle opening is a critical event in apoLp-III binding interaction with LPS. This mechanism implies that the hydrophobic interior of the protein interacts directly with LPS, analogous to that observed for lipid interaction.

Tyrosine fluorescence analysis of apolipophorin III–lipopolysaccharide interaction

Apolipophorin III (apoLp-III) is an exchangeable apolipoprotein that binds to lipopolysaccharides (LPS). Polyacrylamide gel electrophoresis analysis demonstrated that apoLp-III from Galleria mellonella associated with various truncated LPS variants, including lipid A. Subsequent binding studies were performed employing the intrinsic tyrosine fluorescence properties of apoLp-III, which is highly quenched in the unbound state. A marked increase in tyrosine fluorescence intensity was observed upon binding to LPS or detoxified LPS, indicating a new microenvironment for Tyr-142. This also implies that the LPS carbohydrate region is involved in LPS binding. Dissociation constants (Kd) measured by apoLp-III titration were estimated at approximately 1 microM. Increasing the ionic strength did not decrease the Kd, neither did LPS phosphate removal. In addition, truncation apoLp-III mutants, lacking two complete helices, were still able to associate with LPS. This indicates that the association of apoLp-III with LPS may not be governed by charge but by hydrophobic interactions.

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.