Characterization of band 3-ankyrin-Protein 4.2 complex by biochemical and mass spectrometry approaches

The protein organization of a red blood cell

Red blood cells (RBCs) (erythrocytes) are the simplest primary human cells, lacking nuclei and major organelles and instead employing about a thousand proteins to dynamically control cellular function and morphology in response to physiological cues. In this study, we define a canonical RBC proteome and interactome using quantitative mass spectrometry and machine learning. Our data reveal an RBC interactome dominated by protein homeostasis, redox biology, cytoskeletal dynamics, and carbon metabolism. We validate protein complexes through electron microscopy and chemical crosslinking and, with these data, build 3D structural models of the ankyrin/Band 3/Band 4.2 complex that bridges the spectrin cytoskeleton to the RBC membrane. The model suggests spring-like compression of ankyrin may contribute to the characteristic RBC cell shape and flexibility. Taken together, our study provides an in-depth view of the global protein organization of human RBCs and serves as a comprehensive resource for future research.

Structure, dynamics and assembly of the ankyrin complex on human red blood cell membrane

The cytoskeleton of a red blood cell (RBC) is anchored to the cell membrane by the ankyrin complex. This complex is assembled during RBC genesis and comprises primarily band 3, protein 4.2 and ankyrin, whose mutations contribute to numerous human inherited diseases. High-resolution structures of the ankyrin complex have been long sought-after to understand its assembly and disease-causing mutations. Here, we analyzed native complexes on the human RBC membrane by stepwise fractionation. Cryo-electron microscopy structures of nine band-3-associated complexes reveal that protein 4.2 stabilizes the cytoplasmic domain of band 3 dimer. In turn, the superhelix-shaped ankyrin binds to this protein 4.2 via ankyrin repeats (ARs) 6-13 and to another band 3 dimer via ARs 17-20, bridging two band 3 dimers in the ankyrin complex. Integration of these structures with both prior data and our biochemical data supports a model of ankyrin complex assembly during erythropoiesis and identifies interactions essential for the mechanical stability of RBC.

Different Involvement of Band 3 in Red Cell Deformability and Osmotic Fragility-A Comparative GP.Mur Erythrocyte Study

GP.Mur is a clinically important red blood cell (RBC) phenotype in Southeast Asia. The molecular entity of GP.Mur is glycophorin B-A-B hybrid protein that promotes band 3 expression and band 3–AQP1 interaction, and alters the organization of band 3 complexes with Rh/RhAG complexes. GP.Mur+ RBCs are more resistant to osmotic stress. To explore whether GP.Mur+ RBCs could be structurally more resilient, we compared deformability and osmotic fragility of fresh RBCs from 145 adults without major illness (47% GP.Mur). We also evaluated potential impacts of cellular and lipid factors on RBC deformability and osmotic resistivity. Contrary to our anticipation, these two physical properties were independent from each other based on multivariate regression analyses. GP.Mur+ RBCs were less deformable than non-GP.Mur RBCs. We also unexpectedly found 25% microcytosis in GP.Mur+ female subjects (10/40). Both microcytosis and membrane cholesterol reduced deformability, but the latter was only observed in non-GP.Mur and not GP.Mur+ normocytes. The osmotic fragility of erythrocytes was not affected by microcytosis; instead, larger mean corpuscular volume (MCV) increased the chances of hypotonic burst. From comparison with GP.Mur+ RBCs, higher band 3 expression strengthened the structure of RBC membrane and submembranous cytoskeletal networks and thereby reduced cell deformability; stronger band 3–AQP1 interaction additionally supported osmotic resistance. Thus, red cell deformability and osmotic resistivity involve distinct structural–functional roles of band 3.

Ion Transport in Eryptosis, the Suicidal Death of Erythrocytes

Erythrocytes are among the most abundant cells in mammals and are perfectly adapted to their main functions, i.e., the transport of O₂ to peripheral tissues and the contribution to CO₂ transport to the lungs. In contrast to other cells, they are fully devoid of organelles. Similar to apoptosis of nucleated cells erythrocytes may enter suicidal death, eryptosis, which is characterized by the presentation of membrane phosphatidylserine on the cell surface and cell shrinkage, hallmarks that are also typical of apoptosis. Eryptosis may be triggered by an increase in the cytosolic Ca²⁺ concentration, which may be due to Ca²⁺ influx via non-selective cation channels of the TRPC family. Eryptosis is further induced by ceramide, which sensitizes erythrocytes to the eryptotic effect of Ca²⁺. Signaling regulating eryptosis further involves a variety of kinases including AMPK, PAK2, cGKI, JAK3, CK1α, CDK4, MSK1/2 and casein kinase. Eryptosis-dependent shrinkage is induced by K⁺ efflux through Ca²⁺-activated K⁺ channel K_Ca3.1, the Gardos channel. Eryptotic cells are phagocytosed and may adhere to endothelial cells. Eryptosis may help prevent hemolysis since defective erythrocytes usually undergo eryptosis followed by rapid clearance from circulating blood. Excessive eryptosis stimulated by various diseases and xenobiotics may result in anemia and/or impaired microcirculation. This review focuses on the significance and mechanisms of eryptosis as well as on the ion fluxes involved. Moreover, a short summary of further ion transport mechanisms of the erythrocyte membrane is provided.

The MNS glycophorin variant GP.Mur affects differential erythroid expression of Rh/RhAG transcripts

BACKGROUND

The band 3 macrocomplex (also known as the ankyrin-associated complex) on the red cell membrane comprises two interacting subcomplexes: a band 3/glycophorin A subcomplex, and a Rh/RhAG subcomplex. Glycophorin B (GPB) is a component of the Rh/RhAG subcomplex that is also structurally associated with glycophorin A (GPA). Expression of glycophorin B-A-B hybrid GP.Mur enhances band 3 expression and is associated with lower levels of Rh-associated glycoprotein (RhAG) and Rh polypeptides. The goal of this study was to determine whether GP.Mur influenced erythroid Rh/RhAG expression at the transcript level.

MATERIALS AND METHODS

GP.Mur was serologically determined in healthy participants from Taitung County, Taiwan. RNA was extracted from the reticulocyte-enriched fraction of peripheral blood, followed by reverse transcription and quantitative PCR for RhAG, RhD and RhCcEe.

RESULTS

Quantification by real-time PCR revealed significantly fewer RhAG and RhCcEe transcripts in the reticulocytes from subjects with homozygous GYP*Mur. Independent from GYP.Mur, both RhAG and RhD transcript levels were threefold or higher than that of RhCcEe. Also, in GYP.Mur and the control samples alike, direct quantitative associations were observed between the transcript levels of RhAG and RhD, but not between that of RhAG and RhCcEe.

CONCLUSION

Erythroid RhD and RhCcEe were differentially expressed at the transcript levels, which could be related to their different degrees of interaction or sensitivity to RhAG. Further, the reduction or absence of glycophorin B in GYP.Mur erythroid cells affected transcript expressions of RhAG and RhCcEe. Thus, GPB and GP.Mur differentially influenced Rh/RhAG expressions prior to protein translation.

Fullerenol C₆₀(OH)₃₆ could associate to band 3 protein of human erythrocyte membranes

The present study was aimed at investigating the effect of fullerenol C60(OH)36 on chosen parameters of the human erythrocyte membrane and the preliminary estimation of the properties of fullerenol as a potential linking agent transferring the compounds (e.g., anticancer drugs) into the membrane of erythrocytes. The results obtained in this study confirm the impact of fullerenol on erythrocyte cytoskeletal transmembrane proteins, particularly on the band 3 protein. The presence of fullerenol in each of the concentrations used prevented degradation of the band 3 protein. The results show that changes in the morphology of red blood cells caused by high concentrations of fullerenol (up to 150mg/L) did not lead to increased red blood cell hemolysis or the leakage of potassium. Moreover, fullerenol slightly prevented hemolysis and potassium efflux. The protective effect of fullerenol at the concentration of 150mg/L was 20.3%, and similar results were obtained for the efflux of potassium. The study shows that fullerenol slightly changed the morphology of the cells and, therefore, altered the intracellular organization of erythrocytes through the association with cytoskeletal proteins.

Identification of contact sites between ankyrin and band 3 in the human erythrocyte membrane

The red cell membrane is stabilized by a spectrin/actin-based cortical cytoskeleton connected to the phospholipid bilayer via multiple protein bridges. By virtue of its interaction with ankyrin and adducin, the anion transporter, band 3 (AE1), contributes prominently to these bridges. In a previous study, we demonstrated that an exposed loop comprising residues 175-185 of the cytoplasmic domain of band 3 (cdB3) constitutes a critical docking site for ankyrin on band 3. In this paper, we demonstrate that an adjacent loop, comprising residues 63-73 of cdB3, is also essential for ankyrin binding. Data that support this hypothesis include the following. (1) Deletion or mutation of residues within the latter loop abrogates ankyrin binding without affecting cdB3 structure or its other functions. (2) Association of cdB3 with ankyrin is inhibited by competition with the loop peptide. (3) Resealing of the loop peptide into erythrocyte ghosts alters membrane morphology and stability. To characterize cdB3-ankyrin interaction further, we identified their interfacial contact sites using molecular docking software and the crystal structures of D(3)D(4)-ankyrin and cdB3. The best fit for the interaction reveals multiple salt bridges and hydrophobic contacts between the two proteins. The most important ion pair interactions are (i) cdB3 K69-ankyrin E645, (ii) cdB3 E72-ankyrin K611, and (iii) cdB3 D183-ankyrin N601 and Q634. Mutation of these four residues on ankyrin yielded an ankyrin with a native CD spectrum but little or no affinity for cdB3. These data define the docking interface between cdB3 and ankyrin in greater detail.

Refined views of multi-protein complexes in the erythrocyte membrane

The erythrocyte membrane has been extensively studied, both as a model membrane system and to investigate its role in gas exchange and transport. Much is now known about the protein components of the membrane, how they are organised into large multi-protein complexes and how they interact with each other within these complexes. Many links between the membrane and the cytoskeleton have also been delineated and have been demonstrated to be crucial for maintaining the deformability and integrity of the erythrocyte. In this study we have refined previous, highly speculative molecular models of these complexes by including the available data pertaining to known protein-protein interactions. While the refined models remain highly speculative, they provide an evolving framework for visualisation of these important cellular structures at the atomic level.