Towards the localization of the essential arginine residues in the band 3 protein of human red blood cell membranes

Cell Physiology and Molecular Mechanism of Anion Transport by Erythrocyte Band 3/AE1

The major transmembrane protein of the red blood cell, known as band 3, AE1, and SLC4A1, has two main functions: 1) catalysis of Cl^-/HCO₃^- exchange, one of the steps in CO₂ excretion; 2) anchoring the membrane skeleton. This review summarizes the 150 year history of research on red cell anion transport and band 3 as an experimental system for studying membrane protein structure and ion transport mechanisms. Important early findings were that red cell Cl^- transport is a tightly coupled 1:1 exchange and band 3 is labeled by stilbenesulfonate derivatives that inhibit anion transport. Biochemical studies showed that the protein is dimeric or tetrameric (paired dimers) and that there is one stilbenedisulfonate binding site per subunit of the dimer. Transport kinetics and inhibitor characteristics supported the idea that the transporter acts by an alternating access mechanism with intrinsic asymmetry. The sequence of band 3 cDNA provided a framework for detailed study of protein topology and amino acid residues important for transport. The identification of genetic variants produced insights into the roles of band 3 in red cell abnormalities and distal renal tubular acidosis. The publication of the membrane domain crystal structure made it possible to propose concrete molecular models of transport. Future research directions include improving our understanding of the transport mechanism at the molecular level and of the integrative relationships among band 3, hemoglobin, carbonic anhydrase, and gradients (both transmembrane and subcellular) of HCO₃^-, Cl^-, O₂, CO₂, pH, and NO metabolites during pulmonary and systemic capillary gas exchange.

Band 3, the human red cell chloride/bicarbonate anion exchanger (AE1, SLC4A1), in a structural context

The crystal structure of the dimeric membrane domain of human Band 3(1), the red cell chloride/bicarbonate anion exchanger 1 (AE1, SLC4A1), provides a structural context for over four decades of studies into this historic and important membrane glycoprotein. In this review, we highlight the key structural features responsible for anion binding and translocation and have integrated the following topological markers within the Band 3 structure: blood group antigens, N-glycosylation site, protease cleavage sites, inhibitor and chemical labeling sites, and the results of scanning cysteine and N-glycosylation mutagenesis. Locations of mutations linked to human disease, including those responsible for Southeast Asian ovalocytosis, hereditary stomatocytosis, hereditary spherocytosis, and distal renal tubular acidosis, provide molecular insights into their effect on Band 3 folding. Finally, molecular dynamics simulations of phosphatidylcholine self-assembled around Band 3 provide a view of this membrane protein within a lipid bilayer.

Ligation of Glycophorin A Generates Reactive Oxygen Species Leading to Decreased Red Blood Cell Function

Acute, inflammatory conditions associated with dysregulated complement activation are characterized by significant increases in blood concentration of reactive oxygen species (ROS) and ATP. The mechanisms by which these molecules arise are not fully understood. In this study, using luminometric- and fluorescence-based methods, we show that ligation of glycophorin A (GPA) on human red blood cells (RBCs) results in a 2.1-fold, NADPH-oxidase-dependent increase in intracellular ROS that, in turn, trigger multiple downstream cascades leading to caspase-3 activation, ATP release, and increased band 3 phosphorylation. Functionally, using 2D microchannels to assess membrane deformability, GPS-ligated RBCs travel 33% slower than control RBCs, and lipid mobility was hindered by 10% using fluorescence recovery after photobleaching (FRAP). These outcomes were preventable by pretreating RBCs with cell-permeable ROS scavenger glutathione monoethyl ester (GSH-ME). Our results obtained in vitro using anti-GPA antibodies were validated using complement-altered RBCs isolated from control and septic patients. Our results suggest that during inflammatory conditions, circulating RBCs significantly contribute to capillary flow dysfunctions, and constitute an important but overlooked source of intravascular ROS and ATP, both critical mediators responsible for endothelial cell activation, microcirculation impairment, platelet activation, as well as long-term dysregulated adaptive and innate immune responses.

Structural model of the anion exchanger 1 (SLC4A1) and identification of transmembrane segments forming the transport site

The anion exchanger 1 (AE1), a member of bicarbonate transporter family SLC4, mediates an electroneutral chloride/bicarbonate exchange in physiological conditions. However, some point mutations in AE1 membrane-spanning domain convert the electroneutral anion exchanger into a Na(+) and K(+) conductance or induce a cation leak in a still functional anion exchanger. The molecular determinants that govern ion movement through this transporter are still unknown. The present study was intended to identify the ion translocation pathway within AE1. In the absence of a resolutive three-dimensional structure of AE1 membrane-spanning domain, in silico modeling combined with site-directed mutagenesis experiments was done. A structural model of AE1 membrane-spanning domain is proposed, and this model is based on the structure of a uracil-proton symporter. This model was used to design cysteine-scanning mutagenesis on transmembrane (TM) segments 3 and 5. By measuring AE1 anion exchange activity or cation leak, it is proposed that there is a unique transport site comprising TM3-5 and TM8 that should function as an anion exchanger and a cation leak.

Functional characterization and modified rescue of novel AE1 mutation R730C associated with overhydrated cation leak stomatocytosis

We report the novel, heterozygous AE1 mutation R730C associated with dominant, overhydrated, cation leak stomatocytosis and well-compensated anemia. Parallel elevations of red blood cell cation leak and ouabain-sensitive Na(+) efflux (pump activity) were apparently unaccompanied by increased erythroid cation channel-like activity, and defined ouabain-insensitive Na(+) efflux pathways of nystatin-treated cells were reduced. Epitope-tagged AE1 R730C at the Xenopus laevis oocyte surface exhibited severely reduced Cl(-) transport insensitive to rescue by glycophorin A (GPA) coexpression or by methanethiosulfonate (MTS) treatment. AE1 mutant R730K preserved Cl(-) transport activity, but R730 substitution with I, E, or H inactivated Cl(-) transport. AE1 R730C expression substantially increased endogenous oocyte Na(+)-K(+)-ATPase-mediated (86)Rb(+) influx, but ouabain-insensitive flux was minimally increased and GPA-insensitive. The reduced AE1 R730C-mediated sulfate influx did not exhibit the wild-type pattern of stimulation by acidic extracellular pH (pH(o)) and, unexpectedly, was partially rescued by exposure to sodium 2-sulfonatoethyl methanethiosulfonate (MTSES) but not to 2-aminoethyl methanethiosulfonate hydrobromide (MTSEA) or 2-(trimethylammonium)ethyl methanethiosulfonate bromide (MTSET). AE1 R730E correspondingly exhibited acid pH(o)-stimulated sulfate uptake at rates exceeding those of wild-type AE1 and AE1 R730K, whereas mutants R730I and R730H were inactive and pH(o) insensitive. MTSES-treated oocytes expressing AE1 R730C and untreated oocytes expressing AE1 R730E also exhibited unprecedented stimulation of Cl(-) influx by acid pH(o). Thus recombinant cation-leak stomatocytosis mutant AE1 R730C exhibits severely reduced anion transport unaccompanied by increased Rb(+) and Li(+) influxes. Selective rescue of acid pH(o)-stimulated sulfate uptake and conferral of acid pH(o)-stimulated Cl(-) influx, by AE1 R730E and MTSES-treated R730C, define residue R730 as critical to selectivity and regulation of anion transport by AE1.

Inhibition by phenylglyoxal of nitrate transport in Paracoccus denitrificans: a comparison with the effect of a protonophorous uncoupler

The amino acid modifier phenylglyoxal (PG) gradually inactivated the methyl viologen-coupled nitrate reductase activity of the anoxically grown whole cells of Paracoccus denitrificans. A double log plot of the pseudo-first-order inactivation rate constant versus PG concentration was linear with a mean slope of 1.4 (0.1M sodium phosphate) or 0.87 (0.1M sodium borate). Phenylglyoxalation of cells lowered the limiting velocity (V), while hardly affecting the apparent half-saturation concentration (K(m)) of nitrate. Nitrate afforded no protection against inactivation. The inhibition by PG could be removed by the detergent Triton X-100 or by the lipid-soluble tetraphenylphosphonium countercation, suggesting that PG exerts its effect at the level of nitrate transport. Based on studies with membrane potential- and pH-sensitive fluorescent probes, the inhibition was shown not to be due to changes in the electrochemical gradient of hydrogen ions. Both K(m) and V values for nitrate uptake increased in a hyperbolic fashion in response to exogenously added nitrite. Nitrite promoted a bypass of the inhibition caused by low concentrations of the proton-conducting agent carbonyl cyanide m-chlorophenylhydrazone (CCCP), but was almost ineffective in the case of the PG block. These results are rationalized in terms of two nitrate import pathways that are comparably inhibited by PG and differ in their sensitivities to CCCP. A simplified kinetic model for phenylglyoxalation is proposed to account for the observed nonintegral reaction orders.

Haemolysis of human and sheep red blood cells in glycerol media: the effect of pH and the role of band 3

Haemolysis of red blood cells (RBC) in glycerol media may be measured spectrophotometrically. The haemolytic process in a rapid phase obeys a first order rate law. The rate constant expresses the rate of haemolysis. To gain a better understanding of the mechanism of haemolysis in glycerol media, the effects of pH and band 3 inhibitors on the rate of haemolysis in human and sheep RBC were observed. Over the pH range used (pH 5.8-10.0), the rate of haemolysis decreased with increase in pH in sheep RBC. By contrast, the rate of haemolysis increased from pH 5.8 to 6.4 and decreased above pH 6.4 in human RBC. The different effects of pH on the rate of haemolysis are due to inhibition of glycerol permeability by H(+) in human RBC but not in sheep RBC. This is supported by the different effects of temperature and Cu(2+) on the rate of haemolysis in human and sheep RBC. We did not observe complete inhibition of haemolysis by the classical band 3 inhibitor, 4, 4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS). Another band 3 inhibitor 4,4'-dinitrostilbene-2,2'-disulfonic acid (DNDS) showed only weak inhibition. Phenylgloxal (PG), another band 3 inhibitor, had no effect whatsoever on the rate of haemolysis. These results indicate that the anion pathway of band 3 is not the preferred route of transport of glycerol in mammalian RBC.