Domain Of Anion Exchanger Research Articles

Role of Circulating S -Nitrosothiols in Control of Blood Pressure

The biological effects of nitric oxide (NO) are in large part mediated by S -nitrosylation of peptides and proteins to produce bioactive S -nitrosothiols (SNOs).1–3 The observation of abnormal SNO levels in numerous pathophysiological states2 suggests that dysregulation of SNO homeostasis may contribute to disease pathogenesis. For example, the hypotension of human sepsis is accompanied by increases in circulating levels of vasodilatory SNOs.3 Although such altered SNO levels may simply mirror NO production (eg, induction of inducible NO synthase in sepsis), they may also reflect changes specific to SNO biosynthesis and metabolism. Indeed, mice lacking a SNO-metabolizing enzyme are profoundly hypotensive under anesthesia.3 Thus, blood pressure is evidently regulated by both synthesis and turnover of SNOs. In this issue of Hypertension , Gandley et al4 extend this paradigm by proposing that a defect in SNO turnover contributes to the hypertension of preeclampsia. In the blood, S- nitrosoalbumin (SNO-albumin) and S -nitrosohemoglobin (SNO-Hb) constitute the major conduits for circulating NO bioactivity. Although both SNOs may influence blood pressure, they operate within distinct signaling circuits. SNO-Hb can be viewed as a principal regulator of SNO homeostasis, adaptively modulating NO chemistry to control NO bioactivity. SNO-Hb is formed by transfer of NO from heme-iron to Cysβ93 thiol on T to R structural transition (oxygenation) of the hemoglobin tetramer.5 SNO-Hb associates with the red blood cell (RBC) membrane via an interaction with the cytoplasmic domain of anion-exchanger 1 protein (CDAE1, Band 3); on deoxygenation (R→T) transfer of the NO group from SNO-Hb to a cysteine thiol within CDAE1 supports export of RBC vasodilatory activity.6 SNO-Hb thus serves as an O2 sensor and O2-dependent transducer of NO bioactivity. In contrast, it appears that rather than transducing a specific signal, albumin operates as a buffer to …

Caspase 3-mediated Proteolysis of the N-terminal Cytoplasmic Domain of the Human Erythroid Anion Exchanger 1 (Band 3)

The N-terminal cytoplasmic domain of the anion exchanger 1 (AE1 or band 3) of the human erythrocyte associates with peripheral membrane proteins to regulate membrane-cytoskeleton interactions, with glycolytic enzymes such as glyceraldehyde-3-phosphate dehydrogenase and aldolase, with the protein-tyrosine kinase p72syk, with hemoglobin and with hemichromes. We have demonstrated that the N-terminal cytoplasmic domain of band 3 (CDB3) is a substrate of the apoptosis executioner caspase 3 (1). CDB3 has two non-conventional caspase 3 cleavage sites, TATD45 and EQGD205 (2). In vitro treatment of recombinant CDB3 with caspase 3 generated two fragments, which could be blocked by pretreatment with the caspase 3 inhibitor Z-DEVD-fmk (3). Recombinant CDB3 in which the caspase 3 cleavage sites Asp45 and Asp205 were mutated, was resistant to proteolysis (4). Proteolytically derived fragments crossreactive with polyclonal anti-band 3 antibody appeared with simultaneous cleavage of poly (ADP-ribose) polymerase and procaspase 3 in staurosporine (STS)-treated HEK293 cells transiently transfected with CDB3 (5). In vivo cleavage of CDB3 could be blocked by pretreatment of cells with Z-DEVD-fmk or in cells transfected with mutant CDB3 (D45A, D205A) (6). Co-transfection experiments showed that STS-mediated cleavage of CDB3 diminished its interaction with the N-terminal domain of protein 4.2, confirming that such cleavage interferes with the interaction of CDB3 with cytoskeletal proteins (7). Active caspase 3 was observed in aged red cells but not in young cells. This red cell caspase 3 could cleave band 3 present in inside-out vesicles prepared from young erythrocytes arguing in favor of a physiological role of caspase 3 in aged erythrocytes.

A Cluster of Cytoplasmic Histidine Residues Specifies pH Dependence of the AE2 Plasma Membrane Anion Exchanger

Intracellular pH is maintained by a dynamic equilibrium balancing the opposing forces of proton loading and proton extrusion. By providing an efflux pathway for base, anion exchangers constitute a key component of the plasma membrane proton-loading machinery. The data in this paper identify a histidine-rich sequence within the cytoplasmic domain of the nonerythroid anion exchanger, AE2, that serves as an intracellular pH “sensor” that modulates anion exchange activity within the physiological range of cytoplasmic pH. These data reveal an interaction between the two major domains of the anion exchanger and suggest a novel substrate feedback mechanism by which intracellular protons directly control the activity of an acid-loading plasma membrane ion transporter.