High-resolution 13C nuclear magnetic resonance studies of small unilamellar vesicles containing glycophorin A

Dynamics of the Glycophorin A Dimer in Membranes of Native-Like Composition Uncovered by Coarse-Grained Molecular Dynamics Simulations

Membranes are central for cells as borders to the environment or intracellular organelle definition. They are composed of and harbor different molecules like various lipid species and sterols, and they are generally crowded with proteins. The membrane system is very dynamic and components show lateral, rotational and translational diffusion. The consequence of the latter is that phase separation can occur in membranes in vivo and in vitro. It was documented that molecular dynamics simulations of an idealized plasma membrane model result in formation of membrane areas where either saturated lipids and cholesterol (liquid-ordered character, L_o) or unsaturated lipids (liquid-disordered character, L_d) were enriched. Furthermore, current discussions favor the idea that proteins are sorted into the liquid-disordered phase of model membranes, but experimental support for the behavior of isolated proteins in native membranes is sparse. To gain insight into the protein behavior we built a model of the red blood cell membrane with integrated glycophorin A dimer. The sorting and the dynamics of the dimer were subsequently explored by coarse-grained molecular dynamics simulations. In addition, we inspected the impact of lipid head groups and the presence of cholesterol within the membrane on the dynamics of the dimer within the membrane. We observed that cholesterol is important for the formation of membrane areas with L_o and L_d character. Moreover, it is an important factor for the reproduction of the dynamic behavior of the protein found in its native environment. The protein dimer was exclusively sorted into the domain of L_d character in the model red blood cell plasma membrane. Therefore, we present structural information on the glycophorin A dimer distribution in the plasma membrane in the absence of other factors like e.g. lipid anchors in a coarse grain resolution.

NMR of molecules interacting with lipids in small unilamellar vesicles

Detailed characterization of protein, peptide or drug interactions with natural membrane is still a challenge. This review focuses on the use of nuclear magnetic resonance (NMR) for the analysis of interaction of molecules with small unilamellar vesicles (SUV). These phospholipid vesicles are often used as model membranes for fluorescence or circular dichroism experiments. The various NMR approaches for studying molecule-lipid association are reviewed. After a brief survey of the SUV characterization, the use of heteronuclear NMR (phosphorous, carbon, fluorine) is described. Applications of proton NMR through transferred nuclear Overhauser effect to perform structural determination of peptide are presented. Special care is finally given to the influence of the kinetic of the interactions for the proton NMR of bound molecules in SUV, which can constitute a good model for the study of dynamical processes at the membrane surface.

Deuterium nuclear magnetic resonance studies on the interaction of glycophorin with 1,2-dimyristoylamido-1,2-deoxyphosphatidylcholine

Membrane dynamics of dimyristoylphosphatidylcholine (DMPC) lipid bilayer which contains glycophorin with an artificial boundary lipid, 1,2-dimyristoylamido-1,2-deoxyphosphatidylcholine (DDPC), was investigated by 2H-NMR technique. For this purpose, both DMPC and DDPC were deuterated at the position of the 8th carbon atom of their acyl chains. Comparing, with DMPC bilayers, DDPC bilayers showed larger deuterium quadrupole splitting (delta nu rho) by approx. 2 kHz. This was explicable in terms of the stabilization of the membrane due to the formation of a strong hydrogen bonding in bilayers. Addition of glycophorin to the DMPC or DDPC single bilayers caused an increase in the delta nu rho value. The delta nu rho value of DMPC/DDPC mixed lipid bilayer was smaller than that of each single lipid bilayer DDPC in the DDPC/DMPC mixed bilayer was not phase-separated but homogeneously distributed. In glycophorin-reconstituted DMPC-d4/DDPC mixed bilayers, the delta nu rho of DMPC-d4 was almost identical to that of the simple DMPC-d4 bilayer. On the other hand, the delta nu rho of DDPC-d4 in the DMPC/DDPC-d4 mixed bilayer increased significantly upon the reconstitution of glycophorin. Judging from these data, we concluded that, in the DDPC/DMPC mixed bilayer which contains glycophorin, DMPC simply behaves as the matrix lipid, while DDPC surrounds glycophorin and certainly plays a role of the boundary lipid.

Dipalmitoylphosphatidylcholine-palmitic acid phase diagram studied by 13C nuclear magnetic resonance

The phase diagram of dipalmitoylphosphatidylcholine (DPPC) and palmitic acid mixtures in excess D2O was studied by 13C-NMR. Phase boundaries were determined from plots of apparent spin-spin relaxation time T2 (for both choline methyl and fatty acid chain carbons) versus temperature. A peritectic transition in the 1-10 mol% region, whose existence has been theoretically inferred from the Gibbs phase rule but which was undetectable by differential thermal analysis (DTA) (S.E. Schullery et al. Biochemistry, 20 (1981) 6818-6824), was located by NMR at 41.6 degrees C. A second, nearby peritectic line at 44 degrees C, which had been shown by DTA to extend from about 3-25 mol% palmitic acid, was seen by NMR only above 10 mol%. The palmitic acid/DPPC complex (2:1), with a sharp melting point at 64 degrees C, reported in earlier studies, was also seen by NMR. A phase diagram including both NMR and DTA results is presented. Important general conclusions from this study are: (i) NMR and scanning thermal analysis are complementary techniques for phase studies; each can see transitions that are invisible to the other. (ii) The case for the applicability of the Gibbs phase rule to lipid bilayer systems has been strengthened by the observance of two predicted, close-spaced boundaries. (iii) Low concentrations of fatty acids and related molecules can not be assumed to disperse as simple ideal solutes in the bilayer matrix.

Calorimetric and fourier transform infrared spectroscopic studies on the interaction of glycophorin with phosphatidylserine/dipalmitoylphosphatidylcholine-d62 mixtures

Glycophorin has been isolated in pure form from human erythrocyte membranes and reconstituted into lipid vesicles composed of binary mixtures of bovine brain phosphatidylserine (PS) and acyl-chain perdeuterated dipalmitoylphosphatidylcholine (DPPC-d62). The effect of protein on lipid melting behavior and order has been monitored with differential scanning calorimetry and Fourier transform infrared spectroscopy (FT-IR). The phase diagram for PS/DPPC-d62 is consistent with that previously reported for PS/DPPC (Stewart et al. (1979) Biochim. Biophys. Acta 556, 1-16) and indicates that acyl chain perdeuteration does not greatly alter the lipid mixing characteristics. The use of deuterated lipid allows the examination of lipid order by FT-IR of each lipid component in the binary mixtures as well as in the ternary (lipid/lipid/protein) systems. Addition of glycophorin to a 30:70 PS/DPPC-d62 binary lipid mixture results in a preferential glycophorin/PS interaction leading to bulk lipid enriched in DPPC-d62. This is revealed in two ways: first, through cooperative calorimetric transitions increased in temperature from the binary lipid system and second, through FT-IR melting curves of the DPPC-d62 component which shows transitions increased in both onset and completion temperatures in the presence of protein. In addition, non-cooperative melting events are observed at temperatures below the onset of phase separation. The FT-IR data are used to assign these non-cooperative events to the melting of the PS component. For the 50:50 lipid mixture with protein, two transitions are observed in the DSC experiments. The IR results indicate that both lipid components are involved with the lower temperature event.

Interaction of glycophorin with phosphatidylserine: a Fourier transform infrared investigation

Glycophorin, from the human erythrocyte membrane, has been isolated in pure form and reconstituted into unilamellar vesicles with bovine brain phosphatidylserine (PS). Fourier transform infrared spectroscopy has been used to monitor the protein conformation as well as the effect of protein on lipid order and melting. Glycophorin, at levels of 1 mol %, nearly abolishes the gel to liquid-crystal phase transition seen in pure PS vesicles between 8 and 16 degrees C by inducing significant disorder into the lipid gel phase. A transition of reduced magnitude remains between 14 and 22 degrees C in the lipid/protein complexes. Evidence is presented for specific interaction of glycophorin with the interfacial region of PS. In general, the effects on lipid melting produced by protein at the 1 mol % level are more pronounced than those noted in a previous study of glycophorin/phosphatidylcholine interactions [ Mendelsohn , R., Dluhy , R. A., Taraschi , T., Cameron, D., & Mantsch , H.H. (1981) Biochemistry 20, 6699-6706]. Two bands are observed for the protein amide I (C = O stretching) mode. A main feature at 1653 cm-1 indicates that the bulk of the secondary structure is random coil or alpha-helical. A weaker shoulder at 1675 cm-1 suggests the occurrence of a small proportion of the beta-sheet form. The results confirm circular dichroism studies of Schulte & Marchesi (1979) [ Schulte , T.H., & Marchesi , V.T. (1979) Biochemistry 18, 275-280]. Fourier transform infrared (FT-IR) studies of a ternary complex of PS/dipalmitoyl-phosphatidylcholine- d62 (DPPC- d62 )/glycophorin indicate that the glycophorin preferentially interacts with the PS component.(ABSTRACT TRUNCATED AT 250 WORDS)

31P and 19F NMR studies of glycophorin-reconstituted membranes: preferential interaction of glycophorin with phosphatidylserine

Glycophorin A, a major glycoprotein of the erythrocyte membrane, has been incorporated into small unilamellar vesicles composed of a variety of pure and mixed phospholipids. Nuclear spin labels including 31P and 19F have been used at natural abundance or have been synthetically incorporated in lipids to act as probes of lipid-protein interaction. Interactions produce broadening of resonances in several cases and it can be used to demonstrate preferential interaction of certain lipids with glycophorin. 31P and 19F probes show a strong preferential interaction of glycophorin with phosphatidylserine over phosphatidylcholine. There is some evidence that interactions are more pronounced at the inner surface of the bilayer and these results are rationalized in terms of the asymmetric distribution of protein and lipid.