Role of secondary structure in protein-phospholipid surface interactions: reconstitution and denaturation of apolipoprotein C-I:DMPC complexes

Rapid, simple, and effective strategy to produce monoclonal antibodies targeting protein structures using hybridoma technology

BACKGROUND

Monoclonal antibodies are essential in life science research and developing antibody drugs and test drugs. Various methods have been developed to obtain monoclonal antibodies, among which hybridoma technology continues to be widely used. However, developing a rapid and efficient method for obtaining conformation-specific antibodies using hybridoma technology remains challenging. We previously developed the membrane-type immunoglobulin-directed hybridoma screening (MIHS) method, which is a flow cytometry-based screening technique based on the interaction between the B-cell receptor expressed on the hybridoma cell surface and the antigen protein, to obtain conformation-specific antibodies.

RESULTS

In this study, we proposed a streptavidin-anchored ELISA screening technology (SAST) as a secondary screening method that retains the advantages of the MIHS method. Anti-enhanced green fluorescent protein monoclonal antibodies were generated as a model experiment, and their structural recognition abilities were examined. Examination of the reaction profiles showed that all monoclonal antibodies obtained in this study recognize the conformational epitopes of the protein antigen. Furthermore, these monoclonal antibodies were classified into two groups: those with binding activities against partially denatured proteins and those with complete loss of binding activities. Next, when screening monoclonal antibodies by the MIHS method as the first screening, we found that monoclonal antibodies with stronger binding constants may be selected by double-staining for hybridomas with fluorescently labeled target antigens and fluorescently labeled B cell receptor antibodies.

CONCLUSIONS

The proposed two-step screening method, which incorporates MIHS and SAST, constitutes a rapid, simple, and effective strategy to obtain conformation-specific monoclonal antibodies generated through hybridoma technology. The novel monoclonal antibody screening strategy reported herein could accelerate the development of antibody drugs and antibody tests.

Proteogenomic Review of the Changes in Primate apoC-I during Evolution

Apolipoprotein C-I has evolved more rapidly than any of the other soluble apolipoproteins. During the course of primate evolution, the gene for this apolipoprotein was duplicated. Prompted by our observation that the two resulting genes encode two distinct forms of apoC-I in great apes, we have reviewed both the genomic and proteomic data to examine what changes have occurred during the course of primate evolution. We have found data showing that one of the duplicated genes, known to be a pseudogene in humans, was also a pseudogene in Denisovans and Neandertals. Using genomic and proteomic data for primates, we will provide in this review evidence that the duplication took place after the divergence of New World monkeys from the human lineage and that the formation of the pseudogene took place after the divergence of the bonobos and chimpanzees from the human lineage.

Changes in helical content or net charge of apolipoprotein C-I alter its affinity for lipid/water interfaces

Amphipathic α-helices mediate binding of exchangeable apolipoproteins to lipoproteins. To probe the role of α-helical structure in protein-lipid interactions, we used oil-drop tensiometry to characterize the interfacial behavior of apolipoprotein C-I (apoC-I) variants at triolein/water (TO/W) and 1-palmitoyl-2-oleoylphosphatidylcholine/triolein/water (POPC/TO/W) interfaces. ApoC-I, the smallest apolipoprotein, has two amphipathic α-helices. Mutants had single Pro or Ala substitutions that resulted in large differences in helical content in solution and on phospholipids. The ability of apoC-I to bind TO/W and POPC/TO/W interfaces correlated strongly with α-helical propensity. On binding these interfaces, peptides with higher helical propensity increased surface pressure to a greater extent. Likewise, peptide exclusion pressure at POPC/TO/W interfaces increased with greater helical propensity. ApoC-I retention on TO/W and POPC/TO/W interfaces correlated strongly with phospholipid-bound helical content. On compression of these interfaces, peptides with higher helical content were ejected at higher pressures. Substitution of Arg for Pro in the N-terminal α-helix altered net charge and reduced apoC-I affinity for POPC/TO/W interfaces. Our results suggest that peptide-lipid interactions drive α-helix binding to and retention on lipoproteins. Point mutations in small apolipoproteins could significantly change α-helical propensity or charge, thereby disrupting protein-lipid interactions and preventing the proteins from regulating lipoprotein catabolism at high surface pressures.

Folded functional lipid-poor apolipoprotein A-I obtained by heating of high-density lipoproteins: relevance to high-density lipoprotein biogenesis

HDL (high-density lipoproteins) remove cell cholesterol and protect from atherosclerosis. The major HDL protein is apoA-I (apolipoprotein A-I). Most plasma apoA-I circulates in lipoproteins, yet ~5% forms monomeric lipid-poor/free species. This metabolically active species is a primary cholesterol acceptor and is central to HDL biogenesis. Structural properties of lipid-poor apoA-I are unclear due to difficulties in isolating this transient species. We used thermal denaturation of human HDL to produce lipid-poor apoA-I. Analysis of the isolated lipid-poor fraction showed a protein/lipid weight ratio of 3:1, with apoA-I, PC (phosphatidylcholine) and CE (cholesterol ester) at approximate molar ratios of 1:8:1. Compared with lipid-free apoA-I, lipid-poor apoA-I showed slightly altered secondary structure and aromatic packing, reduced thermodynamic stability, lower self-associating propensity, increased adsorption to phospholipid surface and comparable ability to remodel phospholipids and form reconstituted HDL. Lipid-poor apoA-I can be formed by heating of either plasma or reconstituted HDL. We propose the first structural model of lipid-poor apoA-I which corroborates its distinct biophysical properties and postulates the lipid-induced ordering of the labile C-terminal region. In summary, HDL heating produces folded functional monomolecular lipid-poor apoA-I that is distinct from lipid-free apoA-I. Increased adsorption to phospholipid surface and reduced C-terminal disorder may help direct lipid-poor apoA-I towards HDL biogenesis.

Apolipoprotein C-I binds more strongly to phospholipid/triolein/water than triolein/water interfaces: a possible model for inhibiting cholesterol ester transfer protein activity and triacylglycerol-rich lipoprotein uptake

Apolipoprotein C-I (apoC-I) is an important constituent of high-density lipoprotein (HDL) and is involved in the accumulation of cholesterol ester in nascent HDL via inhibition of cholesterol ester transfer protein and potential activation of lecithin:cholesterol acyltransferase (LCAT). As the smallest exchangeable apolipoprotein (57 residues), apoC-I transfers between lipoproteins via a lipid-binding motif of two amphipathic α-helices (AαHs), spanning residues 7-29 and 38-52. To understand apoC-I's behavior at hydrophobic lipoprotein surfaces, oil drop tensiometry was used to compare the binding to triolein/water (TO/W) and palmitoyloleoylphosphatidylcholine/triolein/water (POPC/TO/W) interfaces. When apoC-I binds to either interface, the surface tension (γ) decreases by ~16-18 mN/m. ApoC-I can be exchanged at both interfaces, desorbing upon compression and readsorbing on expansion. The maximal surface pressures at which apoC-I begins to desorb (Π(max)) were 16.8 and 20.7 mN/m at TO/W and POPC/TO/W interfaces, respectively. This suggests that apoC-I interacts with POPC to increase its affinity for the interface. ApoC-I is more elastic on POPC/TO/W than TO/W interfaces, marked by higher values of the elasticity modulus (ε) on oscillations. At POPC/TO/W interfaces containing an increasing POPC:TO ratio, the pressure at which apoC-I begins to be ejected increases as the phospholipid surface concentration increases. The observed increase in apoC-I interface affinity due to higher degrees of apoC-I-POPC interactions may explain how apoC-I can displace larger apolipoproteins, such as apoE, from lipoproteins. These interactions allow apoC-I to remain bound to the interface at higher Π values, offering insight into apoC-I's rearrangement on triacylglycerol-rich lipoproteins as they undergo Π changes during lipoprotein maturation by plasma factors such as lipoprotein lipase.

Disruption of the C-terminal helix by single amino acid deletion is directly responsible for impaired cholesterol efflux ability of apolipoprotein A-I Nichinan

Apolipoprotein A-I (apoA-I) Nichinan, a naturally occurring variant with DeltaE235 in the C terminus, is associated with low plasma HDL levels. Here, we investigated the tertiary structure, lipid-binding properties, and ability to induce cellular cholesterol efflux of apoA-I Nichinan and its C-terminal peptide. Thermal and chemical denaturation experiments demonstrated that the DeltaE235 mutation decreased the protein stability compared with wild type (WT). ApoA-I Nichinan exhibited capabilities to bind to or solubilize lipid vesicles that are intermediate to that of WT and a L230P/L233P/Y236P variant in which the C-terminal alpha-helix folding is completely disrupted and forms relatively larger and unstable discoidal complexes, indicating that perturbation of the C-terminal alpha-helical structure by the DeltaE235 mutation leads to reduced lipid binding. Supporting this, apoA-I 209-241/DeltaE235 peptide showed significantly decreased ability to form alpha-helix both in the lipid-free and lipid-bound states, and reduced efficiency to solubilize vesicles. In addition, both apoA-I Nichinan and its C-terminal peptide exhibited reduced activity in ABCA1-mediated cellular cholesterol efflux. Thus, the disruption of the ability of the C-terminal region to form alpha-helix caused by the E235 deletion appears to be the important determinant of impaired lipid binding and cholesterol efflux ability and, consequently, the low plasma HDL levels of apoA-I Nichinan probands.

Aromatic residues in the C-terminal helix of human apoC-I mediate phospholipid interactions and particle morphology

Human apolipoprotein C-I (apoC-I) is an exchangeable apolipoprotein that binds to lipoprotein particles in vivo. In this study, we employed a LC-MS/MS assay to demonstrate that residues 38-51 of apoC-I are significantly protected from proteolysis in the presence of 1,2-dimyristoyl-3-sn-glycero-phosphocholine (DMPC). This suggests that the key lipid-binding determinants of apoC-I are located in the C-terminal region, which includes F42 and F46. To test this, we generated site-directed mutants substituting F42 and F46 for glycine or alanine. In contrast to wild-type apoC-I (WT), which binds DMPC vesicles with an apparent Kd [Kd(app)] of 0.89 microM, apoC-I(F42A) and apoC-I(F46A) possess 2-fold weaker affinities for DMPC with Kd(app) of 1.52 microM and 1.58 microM, respectively. However, apoC-I(F46G), apoC-I(F42A/F46A), apoC-I(F42G), and apoC-I(F42G/F46G) bind significantly weaker to DMPC with Kd(app) of 2.24 microM, 3.07 microM, 4.24 microM, and 10.1 microM, respectively. Sedimentation velocity studies subsequently show that the protein/DMPC complexes formed by these apoC-I mutants sediment at 6.5S, 6.7S, 6.5S, and 8.0S, respectively. This is compared with 5.0S for WT apoC-I, suggesting the shape of the particles was different. Transmission electron microscopy confirmed this assertion, demonstrating that WT forms discoidal complexes with a length-to-width ratio of 2.57, compared with 1.92, 2.01, 2.16, and 1.75 for apoC-I(F42G), apoC-I(F46G), apoC-I(F42A/F46A), and apoC-I(F42G/F46G), respectively. Our study demonstrates that the C-terminal amphipathic alpha-helix of human apoC-I contains the major lipid-binding determinants, including important aromatic residues F42 and F46, which we show play a critical role in stabilizing the structure of apoC-I, mediating phospholipid interactions, and promoting discoidal particle morphology.

Correlation of structural stability with functional remodeling of high-density lipoproteins: the importance of being disordered

High-density lipoproteins (HDLs) are protein-lipid assemblies that remove excess cell cholesterol and prevent atherosclerosis. HDLs are stabilized by kinetic barriers that decelerate protein dissociation and lipoprotein fusion. We propose that similar barriers modulate metabolic remodeling of plasma HDLs; hence, changes in particle composition that destabilize HDLs and accelerate their denaturation may accelerate their metabolic remodeling. To test this notion, we correlate existing reports on HDL-mediated cell cholesterol efflux and esterification, which are obligatory early steps in cholesterol removal, with our kinetic studies of HDL stability. The results support our hypothesis and show that factors accelerating cholesterol efflux and esterification in model discoidal lipoproteins (including reduced protein size, reduced fatty acyl chain length, and/or increased level of cis unsaturation) destabilize lipoproteins and accelerate their fusion and apolipoprotein dissociation. Oxidation studies of plasma spherical HDLs show a similar trend: mild oxidation by Cu(2+) or OCl(-) accelerates cell cholesterol efflux, protein dissociation, and HDL fusion, while extensive oxidation inhibits these reactions. Consequently, moderate destabilization may be beneficial for HDL functions by facilitating insertion of cholesterol and lipophilic enzymes, promoting dissociation of lipid-poor apolipoproteins, which are primary acceptors of cell cholesterol, and thereby accelerating HDL metabolism. Therefore, HDL stability must be delicately balanced to maintain the structural integrity of the lipoprotein assembly and ensure structural specificity necessary for interactions of HDL with its metabolic partners, while facilitating rapid HDL remodeling and turnover at key junctures of cholesterol transport. The inverse correlation between HDL stability and remodeling illustrates the functional importance of structural disorder in macromolecular assemblies stabilized by kinetic barriers.

Effects of acyl chain length, unsaturation, and pH on thermal stability of model discoidal HDLs

HDLs prevent atherosclerosis by removing excess cell cholesterol. Lipid composition affects HDL functions in cholesterol removal, yet its effects on the disk stability remain unclear. We hypothesize that reduced length or increased cis-unsaturation of phosphatidylcholine acyl chains destabilize discoidal HDL and promote protein dissociation and lipoprotein fusion. To test this hypothesis, we determined thermal stability of binary complexes reconstituted from apoC-I and diacyl PCs containing 12-18 carbons with 0-2 cis-double bonds. Kinetic analysis using circular dichroism shows that, for fully saturated PCs, chain length increase by two carbons stabilizes lipoprotein by deltaDeltaG* (37 degrees C) congruent with 1.4 kcal/mol, suggesting that hydrophobic interactions dominate the disk stability; distinct effects of pH and salt indicate contribution of electrostatic interactions. Similarly, apoA-I-containing disks show increased stability with increasing chain length. Acyl chain unsaturation reduces disk stability. In summary, stability of discoidal HDL correlates directly with fatty acyl chain length and saturation: the longer and more fully saturated are the chains, the more extensive are the stabilizing lipid-protein and lipid-lipid interactions and the higher is the free energy barrier for protein dissociation and lipoprotein fusion. This sheds new light on the existing data of cholesterol efflux to discoidal HDL and suggests that moderate lipoprotein destabilization facilitates cholesterol insertion.