Transmembrane mobility and distribution of phospholipids in the membrane of mouse beta-thalassaemic red blood cells

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.

Transmembrane topography of plasma membrane constituents in mung bean (Vigna radiata L.) hypocotyl cells. I. Transmembrane distribution of phospholipids

The transmembrane distribution of phospholipids (PLs) in the plasma membrane (PM) of mung bean (Vigna radiata L.) hypocotyl cells was investigated using annexin V-fluorescein isothiocyanate, porcine pancreas phospholipase A(2), and (31)P-nuclear magnetic resonance (NMR) spectroscopy. Phosphatidylserine was not located on the cell surface of mung bean protoplasts. However, phosphatidylcholine, phosphatidylethanolamine and phosphatidic acid were found to be almost symmetrically distributed across right-side-out PM vesicles obtained by aqueous two-phase partitioning by porcine pancreas phospholipase A(2) assay. (31)P-NMR assay showed that the amount of PLs is about equal in the outer and the inner leaflets of the right-side-out PM vesicles. These results suggest that the topography of PM PLs might not contribute to well-known asymmetrical properties of the outer and inner surfaces of higher plant PMs. It is also indicated that inside-out PM vesicles created by Brij 58-treatment do not retain the native PL topography on dithionate reduction of 7-nitro-2,1,3-benzoxadiazol-4-yl-labeled PLs incorporated in the PM vesicles.

AE2 anion exchanger polypeptide is a homooligomer in pig gastric membranes: a chemical cross-linking study

Although considerable information is available on the oligomeric states of the AE1 (band 3) anion exchanger, little is known about the physiological state of the polypeptides encoded by the nonerythroid AE genes, AE2 and AE3. We have previously characterized the proteolytic susceptibility of native pig gastric AE2. In the course of studies in which pig gastric membranes were treated with the AE2 transport antagonist, DIDS, we noted evidence for cross-linking of AE2 proteolytic fragments to higher-order oligomeric forms. We have characterized the ability of DIDS and of selected N-hydroxysuccinimide cross-linking agents to increase the proportion of SDS-resistant oligomers of pig gastric AE2 and its proteolytic fragments. Cross-linking exhibited time and concentration dependence. N-Terminal protein sequencing proved that DIDS treatment created AE2 homodimers. Putative homotetramers were also observed. Protomers were cross-linked via residues within the C-terminal 40 kDa of AE2. Prior proteolytic cleavage of AE2 in membranes resulted in decreased yield of subsequently cross-linked products. AE2 cross-linking could not be detected in membranes pretreated by hypotonic wash and freeze-thaw. The results are interpreted in light of the deduced amino acid sequence of the transmembrane domain of pig AE2.

Altered membrane phospholipid organization and erythrophagocytosis in E beta-thalassemia

The underlying cause behind the accelerated destruction of erythrocytes in the bone marrow and in the peripheral circulation, accompanying the beta-thalassemic syndromes is still not clearly understood. The present investigation demonstrates that increased phagocytosis of erythrocytes in E beta-thalassemia is inhibited by the presence of phosphatidylserine (PS) vesicles, suggesting a PS-'PS-receptor' type of interaction in premature recognition of these erythrocytes by macrophages. Increased exposure of both aminophospholipids phosphatidylethanolamine (PE) and PS was demonstrated by fluorescamine labeling and annexin binding, respectively. The slower rate of translocation of PS across the bilayer suggested that this contributed towards the increased exposure of PS in E beta-thalassemic erythrocytes.

Phospholipid composition and organization in model beta-thalassemic erythrocytes

The membrane phospholipid organization in human red blood cells (RBC) is rigidly maintained by a complex system of enzymes. However, several elements of this system are sensitive to oxidative damage. An important component in the destruction of beta-thalassemic RBC is the generation of reactive oxygen species and the release of redox-active iron by the unpaired alpha-hemoglobin chains. Consequently, we hypothesized that the presence of this oxidative stress to the RBC membrane could lead to alterations in membrane lipid organization. Model beta thalassemic RBC, prepared by the introduction of excess alpha-globin in the cell, have previously been shown to exhibit structural and functional changes almost identical to those observed in beta-thalassemic cells. After 24 hr at 37 degrees C, the model beta thalassemic cells exhibited a significant loss of deformability, as measured by ektacytometric analysis, indicative of extensive membrane damage. However, a normal steadystate distribution of endogenous phospholipids was found, as evidenced by the accessibility of membrane phospholipids to hydrolysis by phospholipases. Similarly, the kinetics of transbilayer movement of spin-labeled phosphatidylserine (PS) and phosphatidylethanolamine (PE) in all samples was in the normal range and was not affected by the presence of excess alpha-globin chains. In contrast, a faster rate of spin-labeled phosphatidylcholine (PC) transbilayer movement was observed in these cells. While control RBC exhibited a complete loss of their initial (2 mol%) lysophosphatidylcholine (LPC) levels following 24 hr of incubation at 37 degrees C, 1.5 mol% LPC was still present in model beta-thalassemic cells, suggesting an altered phospholipid molecular species turnover, possibly as a result of an increased repair of oxidatively damaged phospholipids.