Interaction of rabbit C-reactive protein with phospholipid monolayers studied by microfluorescence film balance with an externally applied electric field

Calcium-independent binding of human C-reactive protein to lysophosphatidylcholine in supported planar phospholipid monolayers

Details describing the molecular dynamics of inflammation biomarker human C-reactive protein (CRP) on plasma membranes containing bioactive lipid lysophosphatidylcholine (LPC) remain elusive. Here, we measured the binding kinetics of CRP to supported phospholipid monolayers deposited on an alkanethiol self-assembled monolayer on a planar gold substrate using surface plasmon resonance. Surprisingly, CRP binding to supported 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC)/LPC monolayers was calcium-independent although CRP binding to supported POPC monolayers was calcium-dependent. Binding inhibition assays indicate a specific interaction between CRP and the glycerophosphate group in LPC in the absence of calcium ions. Binding experiments on supported POPC/1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-(1'-rac-glycerol) monolayers further validated calcium-independent binding of CRP through the glycerophosphate moiety. Docking analysis predicted a new binding site for LPC in the absence of calcium ions, which is located on the opposite side of the known binding site for PC of cyclic pentameric CRP. These results using model plasma membranes should aid our understanding of the activation dynamics of CRP in altered local microenvironments of inflammation and infection.

STATEMENT OF SIGNIFICANCE

C-reactive protein (CRP), a major acute-phase pentraxin, binds to plasma membranes through the multivalent contacts with zwitterionic phosphorylcholine groups for activating classical complement systems. However, the interaction of CRP with phosphorylcholine-based biomaterials is unknown due to the lack of our understanding on the activation mechanism of CRP in altered local microenvironments. This paper reports the novel calcium-independent interaction of CRP to bioactive phospholipid lysophosphatidylcholine (LPC) in supported phospholipids monolayers as determined using SPR. Binding inhibition experiments indicate exposure of glycerophosphate moiety of LPC is responsible for the calcium-free interaction. Our study may explode the established concept that CRP requires calcium for binding to LPC on damaged cell membranes or biomaterials.

Electrostatic field effects on membrane domain segregation and on lateral diffusion

Natural membranes are organized structures of neutral and charged molecules bearing dipole moments which generate local non-homogeneous electric fields. When subjected to such fields, the molecules experience net forces that can modify the lipid and protein organization, thus modulating cell activities and influencing (or even dominating) the biological functions. The energetics of electrostatic interactions in membranes is a long-range effect which can vary over distance within r^-1 to r^-3. In the case of a dipole interacting with a plane of dipoles, e.g. a protein interacting with a lipid domain, the interaction is stronger than two punctual dipoles and depends on the size of the domain. In this article, we review several contributions on how electrostatic interactions in the membrane plane can modulate the phase behavior, surface topography and mechanical properties in monolayers and bilayers.

Disparate effects of two phosphatidylcholine binding proteins, C-reactive protein and surfactant protein A, on pulmonary surfactant structure and function

C-reactive protein (CRP) and surfactant protein A (SP-A) are phosphatidylcholine (PC) binding proteins that function in the innate host defense system. We examined the effects of CRP and SP-A on the surface activity of bovine lipid extract surfactant (BLES), a clinically applied modified natural surfactant. CRP inhibited BLES adsorption to form a surface-active film and the film's ability to lower surface tension (γ) to low values near 0 mN/m during surface area reduction. The inhibitory effects of CRP were reversed by phosphorylcholine, a water-soluble CRP ligand. SP-A enhanced BLES adsorption and its ability to lower γ to low values. Small amounts of SP-A blocked the inhibitory effects of CRP. Electron microscopy showed CRP has little effect on the lipid structure of BLES. SP-A altered BLES multilamellar vesicular structure by generating large, loose bilayer structures that were separated by a fuzzy amorphous material, likely SP-A. These studies indicate that although SP-A and CRP both bind PC, there is a difference in the manner in which they interact with surface films.

Human apolipoprotein H may have various orientations when attached to lipid layer

Apolipoprotein H (ApoH), also known as beta(2)-glycoprotein I, is a plasma glycoprotein with its in vivo physiological and pathogenic roles being closely related to its interaction with negatively charged membranes. Although the three-dimensional crystal structure of ApoH has been recently solved, direct evidence about the spatial state of ApoH on the membrane is still lacking. In this work, the interactions of ApoH with the lipid layer are studied by a combination of lipid monolayer approach and surface concentration determination. The spatial state of the orientation of ApoH on the lipid layer is investigated by analyzing the process of membrane-attached ApoH molecules being extruded out from the phospholipid monolayer by compression. The results show that on neutral lipid layer ApoH has an upright orientation, which is not sensitive to the phase state of the lipid layer. However, on acidic lipid layer, ApoH may have two forms of orientation. One is an upright orientation in the liquid phase region, and the other is flat orientation on the condensed domain region. The variation of the spatial state of ApoH on the lipid layer may relate to a variety of its physiological functions.

Incorporation of Blood-Clotting Proteins into Phospholipid Langmuir Monolayers: A Fluorescence Microscopy Study

Phospholipid monolayers adsorbed at an air-water interface are model cell membranes and have been used in this work to study interactions with blood-clotting proteins. Factor I (non-membrane binding) was used as a control protein, and its association with L-alpha-dipalmitoylphosphatidylcholine Langmuir monolayers was compared to factor VII, a membrane-binding protein. Fluorescence micrographs indicated that factor I penetration of the lipid monolayers in the phase transition region occurred extensively, causing condensation of the lipid film. The association of factor I with phospholipid monolayers was deemed nonspecific. Factor VII was shown to associate with the periphery of lipid domains in the absence of calcium ions, causing flattening of domain edges. In the presence of calcium, factor VII induced expansion of the lipid monolayer. This effect is a specific interaction attributed to exposure of hydrophobic residues upon calcium binding, followed by protein association with lipid hydrocarbon chains. Copyright 2001 Academic Press.

Pentameric two-dimensional crystallization of rabbit C-reactive protein on lipid monolayers

As a member of the pentraxin family, C-reactive protein plays various roles in the nonspecific immunity of animals. Though soluble, C-reactive protein always functions on membranes. In order to study the structure of the membrane-bound protein and the reaction between protein and membranes, two-dimensional (2D) crystallization of rabbit C-reactive protein on lipid monolayers was performed. The 2D crystals composed of pentameric proteins were obtained on lipid monolayers by specific adsorption for the first time. The projection map at 26-A resolution is presented, which exhibits P2 symmetry with lattice parameters a = 158(+/-3) A, b = 92(+/-1) A, and gamma = 107(+/-1) degrees. The current work may give a basis for the further study on the structure of complexes made up of C-reactive protein with its functional binding molecules on membranes.

Effect of hydrophobic surfactant peptides SP-B and SP-C on binary phospholipid monolayers. I. Fluorescence and dark-field microscopy

The influence of the hydrophobic proteins SP-B and SP-C, isolated from pulmonary surfactant, on the morphology of binary monomolecular lipid films containing phosphocholine and phosphoglycerol (DPPC and DPPG) at the air-water interface has been studied using epifluorescence and dark-field microscopy. In contrast to previously published studies, the monolayer experiments used the entire hydrophobic surfactant protein fraction (containing both the SP-B and SP-C peptides) at physiologically relevant concentrations (approximately 1 wt %). Even at such low levels, the SP-B/C peptides induce the formation of a new phase in the surface monolayer that is of lower intrinsic order than the liquid condensed (LC) phase that forms in the pure lipid mixture. This presumably leads to a higher structural flexibility of the surface monolayer at high lateral pressure. Variation of the subphase pH indicates that electrostatic interaction dominates the association of the SP-B/C peptides with the lipid monolayer. As evidenced from dark-field microscopy, monolayer material is excluded from the DPPC/DPPG surface film on compression and forms three-dimensional, surface-associated structures of micron dimensions. Such exclusion bodies formed only with SP-B/C peptides. This observation provides the first direct optical evidence for the squeeze-out of pulmonary surfactant material in situ at the air-water interface upon increasing monolayer surface pressures.

Calcium-dependent binding of rabbit C-reactive protein to supported lipid monolayers containing exposed phosphorylcholine group

The interaction of rabbit C-reactive protein (rCRP) with a supported monolayer containing a phosphorylcholine moiety was studied. Three types of phospholipids were synthesized, each containing a insertion spacer of eight, six, or three atoms between the phosphorylcholine group and hydrophobic tail. By varying the length of the insertion spacer, we can vary the extension of the phosphorylcholine group from the membrane surface. By varying the monolayer composition, we can control the lateral distance between the exposed phosphorylcholine groups. Using the surface plasmon resonance technique (SPR), we demonstrated that the calcium-dependent binding of rCRP to the model membrane is governed not only by the ability of the ligand to access the binding pocket fully (spacer length), but also by lateral hindrance within the two-dimensional plane of the membrane. The value of the apparent binding constant was estimated by theoretical analysis, which is obviously dependent on the composition of the lipid mixture, and a maximum of (9.9 +/- 1.5) x 10(6) M-1 was obtained.