Characterization of the hypertonically induced tyrosine phosphorylation of erythrocyte band 3

Unraveling the molecular landscape of kAE1: a narrative review

Kidney anion exchanger 1 (kAE1) is an isoform of the AE1 protein encoded by the SLC4A1 gene. It is a basolateral membrane protein expressed by α-intercalated cells in the connecting tubules and collecting duct of the kidney. Its main function is to exchange bicarbonate and chloride ions between the blood and urine to maintain blood pH at physiological threshold. The kAE1 protein undergoes multiple post-translational modifications such as phosphorylation and ubiquitination and interacts with many different proteins such as claudin-4 and carbonic anhydrase II. Mutations in the gene may lead to the development of distal renal tubular acidosis, characterized by the failure to acidify the urine, which may result in nephrocalcinosis and in more severe cases, renal failure. In this review, we discuss the structure and function of kAE1, its post-translational modifications, and protein-protein interactions. Finally, we discuss insights gained from the study of kAE1 mutations in humans and in mice.

Mechanism of hypoxia-induced damage to the mechanical property in human erythrocytes-band 3 phosphorylation and sulfhydryl oxidation of membrane proteins

Introduction: The integrity of the erythrocyte membrane cytoskeletal network controls the morphology, specific surface area, material exchange, and state of erythrocytes in the blood circulation. The antioxidant properties of resveratrol have been reported, but studies on the effect of resveratrol on the hypoxia-induced mechanical properties of erythrocytes are rare. Methods: In this study, the effects of different concentrations of resveratrol on the protection of red blood cell mor-phology and changes in intracellular redox levels were examined to select an appropriate concentration for further study. The Young's modulus and surface roughness of the red blood cells and blood viscosity were measured via atomic force microsco-py and a blood rheometer, respectively. Flow cytometry, free hemoglobin levels, and membrane lipid peroxidation levels were used to characterize cell membrane damage in the presence and absence of resveratrol after hypoxia. The effects of oxida-tive stress on the erythrocyte membrane proteins band 3 and spectrin were further investigated by immunofluorescent label-ing and Western blotting. Results and discussion: Resveratrol changed the surface roughness and Young's modulus of the erythrocyte mem-brane, reduced the rate of eryptosis in erythrocytes after hypoxia, and stabilized the intracellular redox level. Further data showed that resveratrol protected the erythrocyte membrane proteins band 3 and spectrin. Moreover, resistance to band 3 pro-tein tyrosine phosphorylation and sulfhydryl oxidation can protect the stability of the erythrocyte membrane skeleton net-work, thereby protecting erythrocyte deformability under hypoxia. The results of the present study may provide new insights into the roles of resveratrol in the prevention of hypoxia and as an antioxidant.

Signaling mechanisms in red blood cells: A view through the protein phosphorylation and deformability

Intracellular signaling mechanisms in red blood cells (RBCs) involve various protein kinases and phosphatases and enable rapid adaptive responses to hypoxia, metabolic requirements, oxidative stress, or shear stress by regulating the physiological properties of the cell. Protein phosphorylation is a ubiquitous mechanism for intracellular signal transduction, volume regulation, and cytoskeletal organization in RBCs. Spectrin-based cytoskeleton connects integral membrane proteins, band 3 and glycophorin C to junctional proteins, ankyrin and Protein 4.1. Phosphorylation leads to a conformational change in the protein structure, weakening the interactions between proteins in the cytoskeletal network that confers a more flexible nature for the RBC membrane. The structural organization of the membrane and the cytoskeleton determines RBC deformability that allows cells to change their ability to deform under shear stress to pass through narrow capillaries. The shear stress sensing mechanisms and oxygenation-deoxygenation transitions regulate cell volume and mechanical properties of the membrane through the activation of ion transporters and specific phosphorylation events mediated by signal transduction. In this review, we summarize the roles of Protein kinase C, cAMP-Protein kinase A, cGMP-nitric oxide, RhoGTPase, and MAP/ERK pathways in the modulation of RBC deformability in both healthy and disease states. We emphasize that targeting signaling elements may be a therapeutic strategy for the treatment of hemoglobinopathies or channelopathies. We expect the present review will provide additional insights into RBC responses to shear stress and hypoxia via signaling mechanisms and shed light on the current and novel treatment options for pathophysiological conditions.

Differential dielectroscopic data on the relation of erythrocyte membrane skeleton to erythrocyte deformability and flicker

Two dielectric relaxations, β_sp (1.5 MHz) and γ1_sp (7 MHz), have been detected on spectrin-based membrane skeleton (MS) of red blood cells (RBCs) using the plot of admittance changes at the spectrin denaturation temperature (Ivanov and Paarvanova in Bioelectrochemistry 110: 59-68, 2016, Electrochim Acta 317: 289-300, 2019a). In this study, we treated RBCs and RBC ghost membranes with agents that make membranes rigid and suppress membrane flicker, and studied the effect on β_sp and γ1_sp relaxations. Diamide (diazene dicarboxylic acid bis-(N,N-dimethylamide)) (up to 0.85 mM), taurine mustard (tris(2-chloroethyl)amine) (up to 2 mM), known to specifically cross-link and stiffen spectrin, and glutaraldehyde (up to 0.044%) all inhibited the relaxations in RBC ghost membranes. Similar inhibition was obtained resealing RBC ghost membranes with 2,3-diphosphoglicerate (up to 15 mM), binding WGA (wheat germ agglutinin) (up to 0.025 mg/ml) to exofacial aspect of RBCs, incubating RBCs in hypotonic (200 mOsm) and hypertonic (600-900 mOsm) media and depleting RBCs of ATP. By contrast, concanavalin A (1 mg/ml) and DIDS (4,4'-diiso-thiocyanato stilbene-2,2'-disulfonic acid) (75 μM, pH 8.2), both known to bind specifically band 3 integral protein of RBCs without effect on RBC membrane rigidity, did not affect the relaxations. We conclude there might be a relation between the strength of dielectric relaxations on MS spectrin and the deformability and flicker of RBC membrane.

Nitric oxide in red blood cell adaptation to hypoxia

Nitric oxide (NO) appears to be involved in virtually every aspect of cardiovascular biology. Most attention has been focused on the role of endothelial-derived NO in basal blood flow regulation by relaxing vascular smooth muscle; however, it is now known that NO derived from red blood cells (RBCs) plays a fundamental role in vascular homeostasis by enhancing oxygen (O2) release at the cellular and physiological level. Hypoxia is an often seen problem in diverse conditions; systemic adaptations to hypoxia permit people to adjust to the hypoxic environment at high altitudes and to disease processes. In addition to the cardiopulmonary and hematologic adaptations that support systemic O2 delivery in hypoxia, RBCs assist through newly described NO-based mechanisms, in line with their vital role in O2 transport and delivery. Furthermore, to increase the local blood flow in proportion to metabolic demand, NO regulates membrane mechanical properties thereby modulating RBC deformability and O2 carrying-releasing function. In this review article, we focus on the effect of NO bioactivity on RBC-based mechanisms that regulate blood flow and RBC deformability. RBC adaptations to hypoxia are summarized, with particular attention to NO-dependent S-nitrosylation of membrane proteins and hemoglobin (S-nitrosohemoglobin). The NO/S-nitrosylation/RBC vasoregulatory cascade contributes fundamentally to the molecular understanding of the role of NO in human adaptation to hypoxia and may inform novel therapeutic strategies.

Inhibition of an Erythrocyte Tyrosine Kinase with Imatinib Prevents Plasmodium falciparum Egress and Terminates Parasitemia

With half of the world's population at risk for malaria infection and with drug resistance on the rise, the search for mutation-resistant therapies has intensified. We report here a therapy for Plasmodium falciparum malaria that acts by inhibiting the phosphorylation of erythrocyte membrane band 3 by an erythrocyte tyrosine kinase. Because tyrosine phosphorylation of band 3 causes a destabilization of the erythrocyte membrane required for parasite egress, inhibition of the erythrocyte tyrosine kinase leads to parasite entrapment and termination of the infection. Moreover, because one of the kinase inhibitors to demonstrate antimalarial activity is imatinib, i.e. an FDA-approved drug authorized for use in children, translation of the therapy into the clinic will be facilitated. At a time when drug resistant strains of P. falciparum are emerging, a strategy that targets a host enzyme that cannot be mutated by the parasite should constitute a therapeutic mechanism that will retard evolution of resistance.

Influence of magnesium sulfate on HCO3/Cl transmembrane exchange rate in human erythrocytes

Magnesium sulfate (MgSO4) is widely used in medicine but molecular mechanisms of its protection through influence on erythrocytes are not fully understood and are considerably controversial. Using scanning flow cytometry, in this work for the first time we observed experimentally (both in situ and in vitro) a significant increase of HCO3(-)/Cl(-) transmembrane exchange rate of human erythrocytes in the presence of MgSO4 in blood. For a quantitative analysis of the obtained experimental data, we introduced and verified a molecular kinetic model, which describes activation of major anion exchanger Band 3 (or AE1) by its complexation with free intracellular Mg(2+) (taking into account Mg(2+) membrane transport and intracellular buffering). Fitting the model to our in vitro experimental data, we observed a good correspondence between theoretical and experimental kinetic curves that allowed us to evaluate the model parameters and to estimate for the first time the association constant of Mg(2+) with Band 3 as KB~0.07mM, which is in agreement with known values of the apparent Mg(2+) dissociation constant (from 0.01 to 0.1mM) that reflects experiments on enrichment of Mg(2+) at the inner erythrocyte membrane (Gunther, 2007). Results of this work partly clarify the molecular mechanisms of MgSO4 action in human erythrocytes. The method developed allows one to estimate quantitatively a perspective of MgSO4 treatment for a patient. It should be particularly helpful in prenatal medicine for early detection of pathologies associated with the risk of fetal hypoxia.

Blood or spores? A cautionary note on interpreting cellular debris on human skeletal remains

The identification of red blood cells on both skeletal human remains and decomposed corpses is of remarkable importance in forensic sciences, irrespective of its diagnostic value; their presence is often perplexing and difficult to interpret especially when in the context of decomposition and taphonomical variables. Some clinical research has focused on the morphological changes of red blood cells over time by scanning electron microscopy (SEM), but no research has investigated whether botanical structures can be confused for red blood cells. Since some literature has recently presumed the detection of erythrocyte-like cells on skeletal remains (even ancient) as surely erythrocytes, and most have never taken into consideration the chance of an origin different from blood, such as botanical, the present study aims at verifying the possibility of confusion between erythrocytes and botanical cells by applying SEM analysis and at highlighting the pitfalls in this particular issue through a test submitted to pathologists and natural scientists asked to discriminate between red blood cells and different vegetal structures (60 images obtained by SEM analysis). The results showed that although there are diagnostic features useful in identifying red blood cells from botanical structures, some spores resulted very similar to decaying red blood cells, which calls for attention and great caution when studying decomposed human remains.

Regulation of membrane-cytoskeletal interactions by tyrosine phosphorylation of erythrocyte band 3

The cytoplasmic domain of band 3 serves as a center of erythrocyte membrane organization and constitutes the major substrate of erythrocyte tyrosine kinases. Tyrosine phosphorylation of band 3 is induced by several physiologic stimuli, including malaria parasite invasion, cell shrinkage, normal cell aging, and oxidant stress (thalassemias, sickle cell disease, glucose-6-phosphate dehydrogenase deficiency, etc). In an effort to characterize the biologic sequelae of band 3 tyrosine phosphorylation, we looked for changes in the polypeptide's function that accompany its phosphorylation. We report that tyrosine phosphorylation promotes dissociation of band 3 from the spectrin-actin skeleton as evidenced by: (1) a decrease in ankyrin affinity in direct binding studies, (2) an increase in detergent extractability of band 3 from ghosts, (3) a rise in band 3 cross-linkability by bis-sulfosuccinimidyl-suberate, (4) significant changes in erythrocyte morphology, and (5) elevation of the rate of band 3 diffusion in intact cells. Because release of band 3 from its ankyrin and adducin linkages to the cytoskeleton can facilitate changes in multiple membrane properties, tyrosine phosphorylation of band 3 is argued to enable adaptive changes in erythrocyte biology that permit the cell to respond to the above stresses.

Oxidized and poorly glycosylated band 3 is selectively phosphorylated by Syk kinase to form large membrane clusters in normal and G6PD-deficient red blood cells

Oxidative events involving band 3 (Anion Exchanger 1) have been associated with RBC (red blood cell) removal through binding of NAbs (naturally occurring antibodies); however, the underlying mechanism has been only partially characterized. In addition to inducing direct membrane protein oxidative modification, oxidative treatment specifically triggers the phosphorylation of band 3 tyrosine residues. The present study reports that diamide, a thiol group oxidant, induces disulfide cross-linking of poorly glycosylated band 3 and that the oligomerized band 3 fraction is selectively tyrosine phosphorylated both in G6PD (glucose-6-phosphate dehydrogenase)-deficient and control RBCs. This phenomenon is irreversible in G6PD-deficient RBCs, whereas it is temporarily limited in control RBCs. Diamide treatment caused p72 Syk phosphorylation and translocation to the membrane. Diamide also induced p72 Syk co-immunoprecipitation with aggregated band 3. Moreover, following size-exclusion separation of Triton X-100-extracted membrane proteins, Syk was found only in the high-molecular-mass fraction containing oligomerized/phosphorylated band 3. Src family inhibitors efficiently abrogated band 3 tyrosine phosphorylation, band 3 clustering and NAbs binding to the RBC surface, suggesting a causal relationship between these events. Experiments performed with the non-permeant cross-linker BS(3) (bis-sulfosuccinimidyl-suberate) showed that band 3 tyrosine phosphorylation enhances its capability to form large aggregates. The results of the present study suggest that selective tyrosine phosphorylation of oxidized band 3 by Syk may play a role in the recruitment of oxidized band 3 in large membrane aggregates that show a high affinity to NAbs, leading to RBC removal from the circulation.

Human kidney anion exchanger 1 localisation in MDCK cells is controlled by the phosphorylation status of two critical tyrosines

An important question in renal physiology is how the alpha-intercalated cells of the kidney regulate the distribution of the basolateral kidney anion exchanger 1 (kAE1) according to systemic acid-base status. Previous work using a MDCKI model system demonstrated that kAE1 basolateral targeting requires an N-terminal determinant and a critical C-terminal tyrosine (Y904). Here, we show that the N-terminal determinant is residue Y359, because a Y359A substitution mutant was mistargeted to the apical membrane. Further determinants might exist because a range of N-terminal kAE1 truncations that contained Y359 were incorrectly targeted to the TGN. Y359 and Y904 in kAE1 are phosphorylated upon pervanadate treatment and this phosphorylation is sensitive to specific Src kinase family inhibitors. We tested a range of stimuli on this model system and only the application of high nonphysiological concentrations of extracellular bicarbonate, and to a lesser extent hypertonicity or hyperosmolarity, induced tyrosine phosphorylation of kAE1. Treatment with pervanadate caused internalisation of kAE1 from the plasma membrane, but treatment with high concentrations of bicarbonate did not, because of the hypertonicity of the solution. We propose that alpha-intercalated cells control the distribution of kAE1 by reversible phosphorylation of tyrosine residues Y359 and Y904.

Effect of glycyrrhetinic acid on membrane band 3 in human erythrocytes

Glycyrrhetinic acid (GA) is a hydrolytic product of the triterpene glycoside of glycyrrhizic acid, one of the main constituents of licorice root, which has long been studied, due to its several biological and endocrine properties. In this paper, GA was tested on human erythrocytes, and GA-induced alterations were compared with those caused by diamide, a mild oxidant inducing well-characterized cell/membrane alterations, and n-ethylmaleimide (NEM), as alkylating agent. In order to verify the biochemical steps underlying the action of GA, band 3 Tyr-phosphorylation level, enzyme recruitment and band 3 clustering in cells pre-incubated with GA before diamide treatment were all examined. Results show that GA, in a dose-dependent manner, prevents both diamide and NEM-induced band 3 Tyr-phosphorylation, but not GSH decrease caused by both compounds. In addition, diamide-induced band 3 clustering and IgG binding to altered cells were also completely reversed by GA pre-treatment. Also, when membrane sensitivity toward proteolytic digestion was tested, GA-treated cells showed high resistance to proteolysis. In conclusion, in human erythrocytes, GA is proposed to strengthen membrane integrity against both oxidative and proteolytic damage.

Antibiotic effects on SO4(2-) uptake in human erythrocytes

The erythrocyte is a cell highly exposed to oxygen pressure that, in turn, provokes oxidative stress involving loss of SH-groups, cell shrinkage by activation of K(+)-Cl(-) cotransport (KCC) and membrane destabilization which plays an important role in the premature haemolysis of red blood cells (RBCs). Oxidative stress provoked by chemicals frequently occurs in human erythrocytes. The aim of this study was to test whether the antibiotics alter the redox state and investigate their influences on band 3 protein that is involved in the facilitated electro neutral exchange of Cl(-) for HCO(3)(-) across the membrane of mammalian erythrocytes. Normal erythrocytes were treated with some antibiotics and thiol oxidizing agent N-ethylmaleimide (NEM) and tested for sulphate uptake, K(+) efflux and for glutathione (GSH) concentration as an index of oxidative stress. The rate constant of SO(4)(=) uptake measured in erythrocytes treated with antibiotics as well as NEM was decreased with respect to control cells as a result of band 3 SH-groups oxidation or the stress-induced K(+)-Cl(-) symport-mediated cell shrinkage. In fact, this hypothesis was verified by increased K(+) efflux and decreased GSH values measured in treated erythrocytes compared to controls.

Modulation of glyceraldehyde 3 phosphate dehydrogenase activity and tyr-phosphorylation of Band 3 in human erythrocytes treated with ferriprotoporphyrin IX

Erythrocyte glyceraldehyde-3-phosphate dehydrogenase (G3PD), is a glycolytic enzyme normally inhibited upon binding to the anion transporter Band 3 and activated when free in the cytosol. We have previously reported that ferric protoporphyrin IX (FP) enhances G3PD activity in human erythrocytes (RBC). This could be due to two mechanisms considered in this work: Band 3 tyrosine phosphorylation or oxidative damage of specific G3PD binding sites in the membrane. In both cases binding of G3PD to the membrane would be prevented, leading to the enhancement of G3PD activity. Here, we show that FP induces a dose- and time-dependent phosphorylation of tyrosine 8 and 21 of Band 3, as confirmed by the recruitment of SHP2 phosphatase to the membrane. It appears that Band 3 phosphorylation is due to the oxidation of critical sulfydryl groups of a membrane phosphatase (PTP). Data on membrane localization, Mg2+ dependence, sensitivity to thiol oxidizing agents and protection by N-acetylcysteine (NAC) and DTT strongly suggest the involvement of PTP1B, the major PTP of human RBC associated to and acting on Band 3. However, FP activates G3PD even when Band 3 phosphorylation is inhibited, therefore phosphorylation is not the mechanism underlying G3PD activation by FP. The capacity of NAC of counteracting the stimulatory activity of FP, supports the hypothesis that FP might induce the oxidative damage of specific G3PD binding sites in the membrane, causing the displacement of the enzyme into the cytosol and/or the release from its binding site and therefore its activation.

Augmented erythrocyte band-3 phosphorylation in septic mice

Infection-induced RBC dysfunction has been shown to play a role in the modulation of host response to injury and infection. The underlying biochemical mechanisms are not known. This study investigated alterations in RBC band-3 phosphorylation status and its relationship to anion exchange activity in vitro as well as under in vivo septic conditions induced by cecal ligation and puncture (CLP) in mice. Pervanadate treatment in vitro increased band-3 tyrosine phosphorylation that was accompanied by decreased RBC deformability and anion exchange activity. Following sepsis, band-3 tyrosine phosphorylation in whole RBC ghosts as well as in cytoskeleton-bound or soluble RBC protein fractions were elevated as compared to controls. Although anion exchange activity was similar in RBCs from septic and control animals, band-3 interaction with eosin-5-maleimide (EMA), which binds to band-3 lysine moieties, was increased in cells from septic animals as compared to controls, indicating that sepsis altered band 3 organization within the RBC membrane. Since glucose-6-phosphate dehydrogenase is a major antioxidant enzyme in RBC, in order to assess the potential role of oxidative stress in band-3 tyrosine phosphorylation, sepsis-induced RBC responses were also compared between WT and (G6PD) mutant animals (20% of normal G6PD activity). Band-3 membrane content and EMA staining were elevated in G6PD mutant mice compared to WT under control non-septic conditions. Following sepsis, G6PD mutant animals showed lessened responses in band-3 tyrosine phosphorylation and EMA staining compared to WT. RBC anion exchange activity was similar between mutant and WT animals under all tested conditions. In summary, these studies indicate that sepsis results in elevated band-3 tyrosine phosphorylation and alters band-3 membrane organization without grossly affecting RBC anion exchange activity. The observations also suggest that factors other than oxidative stress are responsible for the sepsis-induced increase in RBC band-3 tyrosine phosphorylation.

SHP-1 Tyrosine Phosphatase in Human Erythrocytes

SHP-1 is a SH2-domain containing protein Tyr-phosphatase expressed in hematopoietic cell lines, which is hypothesized to play a negative role in signal transduction. In human erythrocytes, the phospho-Tyr level of proteins, mainly transmembrane band 3, is closely controlled by the antithetic activity of Tyr-protein kinases and phosphatases, resulting in a dephosphorylated state. Only after particular stimuli, as with oxidizing agents, diamide or pervanadate, or thiol alkylating compound, N-ethyl maleimide (NEM), Tyr-phosphorylation of band 3 can be triggered, inhibiting Tyr-phosphatase action and inducing erythrocyte membrane reorganization. We demonstrate that, in human erythrocytes, SHP-1 is present in membranes from resting cells, but in 5% of the protein amount. Interestingly, this amount increases up to threefold following NEM treatment of intact cells, whereas diamide and pervanadate do not alter the normal protein location. In addition, SHP-1 translocation from cytosol to membrane is not affected by band 3 P-Tyr level, because it is not mediated by the SH2-P-Tyr recruitment mechanism, and localizes into the cytoskeletal compartment. Band 3 is the target of SHP-1, which dephosphorylates Tyr 8, 21, and 904. These findings support the idea that, in human erythrocytes, the normal level of Tyr-phosphorylation of membrane protein, mainly band 3, must be downregulated. We hypothesize that the presence of both SHP-2 and SHP-1 ensures band 3 dephosphorylation in different conditions: SHP-2, through interaction of its SH2 domain/s to P-Tyr protein, is regulated by the band 3 Tyr-phosphorylation level; SHP-1 may be involved by simple membrane rearrangement.

Band 3 tyr-phosphorylation in human erythrocytes from non-pregnant and pregnant women

Pregnancy is associated with changes in circulating red blood cells, mainly involving band 3 protein and membrane lipid peroxidation. Membrane band 3 is a multifunctional protein containing four Tyr-phosphorylatable residues which modulate the physiological status of erythrocytes by regulating glycolysis, cell shape and membrane transport. Erythrocytes from nine pregnant and 12 age-matched non-pregnant healthy women were subjected to oxidative and hyperosmotic stress conditions and the extent of band 3 Tyr-phosphorylation and membrane Syk recruitment as a membrane marker were evaluated. Results indicated that, in pregnancy, red blood cells show a decrease in band 3 Tyr-phosphorylation and a clear-cut rearrangement of band 3 protein within the membrane. In fact, band 3 shows a decrease in high molecular weight aggregates (HMWA), with different subdivision between Triton-soluble and -insoluble compartments, and an increase in proteolytic fragments. In conclusion, it is demonstrated that pregnancy is associated with membrane adjustments which reduce the sensitivity of erythrocytes to both oxidative and osmotic stress. Band 3 Tyr-phosphorylation is proposed as a new parameter in the evaluation of erythrocyte membrane arrangement.

Band 3 tyr-phosphorylation in normal and glucose-6-phospate dehydrogenase-deficient human erythrocytes

Haemolysis is usually episodic in glucose-6-phosphate dehydrogenase (G6PD) deficiency, often triggered by a period of oxidative stress. In the present work, we investigate a possible biochemical mechanism underlying the enhanced susceptibility of G6PD deficient red blood cells (RBC) to oxidative stress. We analysed eight male subjects with Mediterranean glucose-6P-dehydrogenase deficiency (G6PDd), class II, for their ability in phosphorylating erythrocyte membrane band 3 following oxidative and osmotic stress. Our findings show that this sensitivity is connected to an early membrane band 3 Tyr-phosphorylation in the presence of diamide. However, since both Syk, and Lyn kinases, and SHP-2 phosphatase, mostly implicated in the band 3 P-Tyr level regulation, are alike in content and activity in normal and patient erythrocytes, an alteration in the membrane organization is likely the cause of the anomalous response to the oxidant. We report, in fact, that hypertonic-induced morphological change in G6PDd erythrocyte induces a higher membrane band 3 Tyr-phosphorylation, suggesting a pre-existing membrane alteration, likely due to the chronic lowering of the redox systems in patients. We also report that 1-chloro-2,4-dinitrobenzene-pre-treatment of normal red cells can alter the normal protein-protein and protein-membrane interaction under hypertonic rather than oxidative stress, thus partially resembling the response in patients, and that RBC may utilize a wider range of redox defence, under oxidative conditions, including, but not exclusively, NADPH and glutathione. On the whole, these results would encourage a different approach to the evaluation of the effects of pharmacological administration to patients, giving more attention to the possible drug-induced membrane alteration evidenced by the abnormal band 3 Tyr-phosphorylation.

Phosphotyrosine phosphatases acting on band 3 in human erythrocytes of different age: PTP1B processing during cell ageing

The phosphorylation of tyrosine residues of human red blood cell (RBC) band 3 is regulated in vivo by constitutively active tyrosine-kinases (PTKs) and phosphotyrosine-phosphatases (PTPs), identified so far as, respectively, p72(syk) and p56/53(lyn), and PTP1B and SHPTP-2. Tyr-phosphorylation of band 3 increases upon reduction of cell volume as in hypertonic media or during Ca(2+)-induced membrane vesiculation. We show here that old RBCs display higher Tyr-phosphorylation levels of band 3 than younger cells under hypertonic conditions, at least in part due to the reduced cell volume of old RBCs, a condition of lowered threshold for activation of volume-sensitive PTKs. We have also analysed the membrane-bound PTP activity and the relative abundance of PTP1B (as the main membrane-associated PTP) in RBCs of different age. Immunodetection of PTP1B in purified ghost membranes revealed that the catalytic, N-terminal domain of the PTP is partially cleaved, in an age-dependent manner, from the membrane-bound domain, and it is lost during the preparation of ghost membranes. This suggests that erythrocytes may undergo in vivo activation of the Ca(2+)-dependent calpain system that proteolytically regulates PTP1B activity, as already documented for other cell types. On the other hand, the assay of the PTP activity that remains associated with the membranes of RBCs of different age indicated that the PTP undergoes oxidative inactivation that can be further differentiated into reversible and irreversible components.

Effector-induced Syk-mediated phosphorylation in human erythrocytes

Band 3 (AE1), the most prominent polypeptide of the human erythrocyte membrane, becomes heavily tyrosine phosphorylated following treatment of intact cells with protein tyrosine phosphatase inhibitors such as diamide, pervanadate, vanadate, or N-ethylmaleimide (NEM). The mechanism underlying this tyrosine phosphorylation is thought to involve the sequential action of two protein tyrosine kinases, Syk (p72syk) and Lyn (p53/56lyn). While Lyn catalysed phosphorylation appears to be strictly dependent on prior phosphorylation of Tyr8 and 21 of band 3 by Syk, little is known about the mechanism of induction of Syk phosphorylation. Data presented here show that both the fraction of Syk that associates with the membrane and the extent of phosphorylation of band 3 differ in response to the above inhibitors. While diamide and NEM stimulate syk translocation to the membrane during their induction of band 3 tyrosine phosphorylation, pervanadate and vanadate induce no change in kinase distribution. Moreover, diamide and NEM-induced Syk recruitment to the membrane are phosphotyrosine independent and involve their preferential association with Triton X-100-insoluble membrane skeletons. Together these data reveal a complex process controlling the association and catalytic activity of protein tyrosine kinases syk and lyn with the human erythrocyte membrane.

Chemical and pathological oxidative influences on band 3 protein anion-exchanger

BACKGROUND/AIMS

The erythrocyte is a cell exposed to a high level of oxygen pressure and to oxidative chemical agents. This stress involves SH-groups oxidation, cell shrinkage by activation of K-Cl co-transport (KCC) and elevation of the band 3 tyrosine phosphorylation level. The aim of our study was to test whether oxidative stress could influence band 3-mediated anion transport in human red blood cells.

METHODS

To evaluate this hypothesis, normal and pathological (glucose 6 phosphate dehydrogenase (G6PDH) defficient) erythrocytes were treated with known sulphydryl-blocking or thiol-oxidizing agents, such as N-ethylmaleimide (NEM), azodicarboxylic acid bis[dimethylamide] (diamide), orthovanadate, Mg2+ and tested for sulphate (SO4-) uptake, K+ efflux, G6PDH activity and glutathione (GSH) concentration.

RESULTS

In normal red blood cells, the rate constants of SO4- uptake decreased by about 28 % when cells were incubated with NEM, diamide and orthovanadate. In G6PDH-deficient red blood cells, in which oxidative stress occurs naturally, the rate constant of sulphate uptake was decreased by about 40% that of normal red cells. Addition of oxidizing and phosphatase inhibitor agents to pathological erythrocytes further decreased anion transport. In contrast, G6PDH activity was increased under oxidative stress in normal as well as in pathological cells and was lower in the presence of exogenous Mg2+ in parallel to a significant increase in sulphate transport. In both cells, the oxidizing agents increased K+ efflux with depletion of GSH.

CONCLUSION

The data are discussed in light of the possible opposite effects exerted by oxidative agents and Mg2+ on KCC and on the protein tyrosine kinase (PTK)-protein tyrosine phosphatase (PTP) equilibrium. The decreased sulphate uptake observed in the experimental and pathological conditions could be due to band 3 SH-groups oxidation or to oxidative stress-induced K-Cl symport-mediated cell shrinkage with concomitant band 3 tyrosine phosphorylation.

Differential sorting of tyrosine kinases and phosphotyrosine phosphatases acting on band 3 during vesiculation of human erythrocytes

One of the most intensively studied post-translational modifications of erythrocyte proteins is the phosphorylation of tyrosine residues of band 3, which is strictly regulated in vivo by PTKs (protein-tyrosine kinases) and PTPs (protein-phosphotyrosine phosphatases). Two PTKs (p72(syk) and p56/53(lyn)) and two PTP activities (PTP1B and SHPTP-2) have been immunologically identified so far in mature human erythrocytes. We have shown previously that band 3 undergoes tyrosine phosphorylation upon a decrease in cell volume, as occurs when erythrocytes treated with Ca(2+)/Ca(2+) ionophore (A23187) lose KCl and release microvesicles. Similar levels of band 3 tyrosine phosphorylation in vesicles and in the parent cells are induced by this treatment. However, we have found that tyrosine phosphorylation of band 3 in vesicles is more stable than in whole erythrocytes. Examination of how the identified PTPs and PTKs are partitioned between the vesicles and the remnant cells during vesiculation reveals that PTP1B, unlike the PTKs, is retained entirely in the parent cell compartment. Since a tight association between PTP1B and band 3 has been documented previously, we have investigated the partitioning of PTP1B and band 3 between the membrane and the membrane-skeletal fractions prepared from resting or Ca(2+)/A23187-treated cells. Our results rule out the possibility that the preferential retention of PTP1B within the cell was due to an increase in the amount of membrane-skeleton-associated band 3 (and of PTP1B) during the release of spectrin-free vesicles, suggesting a more complex modality of interaction of PTP1B with band 3 in the erythrocyte membrane. Analysis of erythrocytes of different cell ages revealed that PTP1B, unlike the other enzymes examined, was quantitatively conserved during erythrocyte aging. This suggests important roles for the down-regulation of tyrosine phosphorylation of band 3 in erythrocyte physiology, and for vesiculation as a mechanism of human erythrocyte senescence.

Regulation of K-Cl cotransport by Syk and Src protein tyrosine kinases in deoxygenated sickle cells

Protein tyrosine kinases (PTK) of the Src family are thought to suppress K-Cl cotransport (KCC) activity via negative regulation of protein phosphatases. However, some PTK inhibitors reduce KCC activity, suggesting opposite regulation by different PTK families. We have reported previously that deoxygenation of sickle cells stimulates KCC and activates Syk (a Syk family PTK), but not Lyn (an Src family PTK). In this study the same results were obtained when PTK activities were measured under the conditions used to measure KCC activity and which prevent any change in intracellular [Mg(2+)]. Methyl-2,5-dihydroxycinnamate (DHC), a PTK inhibitor, was more selective for Syk than Lyn, while staurosporine (ST), a broad-specificity protein kinase inhibitor, inhibited Lyn more than Syk. Deoxygenation or 4-amino-5-(4-chlorophenyl)-7-( t-butyl)pyrazolo[3,4- d] pyrimidine (pp2, a specific Src inhibitor) stimulated KCC independently. These effects were not additive and were inhibited by DHC. In contrast, ST-induced KCC activation was resistant to DHC, suggesting a different pathway of activation. Overall, these data indicate that Syk activity is required for KCC activation, either induced by deoxygenation of sickle cells, or mediated by Src inhibition in oxygenated cells, and that Syk and Src PTKs exert opposing and interconnected regulatory effects on the activity of the transporter.

Ca2+ promotes erythrocyte band 3 tyrosine phosphorylation via dissociation of phosphotyrosine phosphatase from band 3

The anion-exchange band 3 protein is the main erythrocyte protein that is phosphorylated by protein tyrosine kinase (PTK). We have previously identified a band 3-associated phosphotyrosine phosphatase (PTP) that is normally highly active and prevents the accumulation of band 3 phosphotyrosine. Band 3 tyrosine phosphorylation can be induced by inhibition of PTP (vanadate, thiol oxidation), activation of PTK (hypertonic NaCl) or intracellular increased Ca(2+) (mechanism unknown). We now show that there is inhibition of dephosphorylation of band 3 in Ca(2+)/ionophore-treated erythrocytes and in membranes isolated from the treated cells. These membranes exhibit phosphatase activity upon the addition of exogenous substrate. Dephosphorylation of the endogenous substrate (band 3) can be activated in these membranes by the addition of Mg(2+). Thus the inability of PTP to dephosphorylate the band 3 phosphotyrosine is not due to inhibition of the enzyme itself. Ca(2+) rise in the erythrocyte causes dissociation of PTP from band 3, thus leaving the kinase unopposed. This is shown by a significant diminution in band 3/PTP co-precipitation. Addition of Mg(2+) to these membranes leads to reassociation of band 3 with PTP. The Ca(2+)-induced inhibition of band 3 dephosphorylation may be due to Ca(2+)-dependent alterations in membrane components and structure, affecting the interaction of band 3 with PTP. The Ca(2+)-induced tyrosine phosphorylation, involving an apparent PTP inhibition via dissociation from the substrate, may play a role in signal transduction pathways and in certain pathological disorders associated with increased cell Ca(2+).

Band 3 is an anchor protein and a target for SHP-2 tyrosine phosphatase in human erythrocytes

Tyr phosphorylation of the multifunctional transmembrane protein band 3 has been implicated in several erythrocyte functions and disorders. We previously demonstrated that pervanadate treatment of human erythrocytes induces band-3 Tyr phosphorylation, which is catalyzed by the sequential action of tyrosine kinase Syk and tyrosine kinase(s) belonging to the Src family. In this study, we show that Tyr phosphorylation of band 3, elicited by pervanadate, N-ethylmaleimide, or diamide, greatly increases band-3 interaction with the tyrosine phosphatase SHP-2 in parallel with the translocation of SHP-2 to erythrocyte membranes. These events seem to be mediated by Src-like catalyzed phosphorylation of band 3 because both SHP-2 translocation to cellular membranes and its interaction with Tyr-phosphorylated protein are greatly counteracted by PP2, a specific inhibitor of Src kinases. Binding-competition experiments demonstrate that SHP-2 recruitment to band 3 occurs via its SH2 domain(s). In particular, our data support the view that SHP-2 docks specifically with P-Y359 of band 3. Experiments performed with intact erythrocytes in the presence of the SHP-2 inhibitor calpeptin suggest that, once recruited to Tyr-phosphorylated band 3, the tyrosine phosphatase dephosphorylates the protein. P-Y8, 21, and 904 are the residues affected by SHP-2, as judged by (32)P-peptide mapping of band 3 digested with trypsin. These results indicate that in treated erythrocytes, recruitment of cytosolic SHP-2 to band 3 is a prerequisite for the subsequent dephosphorylation of the transmembrane protein.

Protein kinases activities in erythrocyte membranes of asphyxiated newborns

OBJECTIVES

Perinatal asphyxia represents a major cause of acute brain impairment and mortality in neonates. To develop the effective therapies able to reduce post-asphyxial damages, the understanding of biochemical processes accompanying asphyxia appears to be of the great relevance.

DESIGN AND METHODS

The activities of protein kinases A and C, and tyrosine kinases in erythrocyte membranes of healthy and asphyxiated neonatals were compared. Using monoclonal antibodies the band 3 presence and its phosphotyrosine levels were assayed.

RESULTS

In asphyxiated erythrocyte membranes the activities of PKA and tyrosine kinases increased, whereas the activity of PKC was reduced in relation to healthy newborns. Under asphyxia the band 3 has been overphosphorylated; however, its amount decreased.

CONCLUSION

These findings may provide some evidence for a potential role of asphyxia in disturbance of phosphorylation processes in erythrocytes, as reflected by altered protein kinases activities. The diminished band 3 presence may be partially responsible for the impairment of erythrocyte function.

A transport metabolon. Functional interaction of carbonic anhydrase II and chloride/bicarbonate exchangers

The cytoplasmic carboxyl-terminal domain of AE1, the plasma membrane chloride/bicarbonate exchanger of erythrocytes, contains a binding site for carbonic anhydrase II (CAII). To examine the physiological role of the AE1/CAII interaction, anion exchange activity of transfected HEK293 cells was monitored by following the changes in intracellular pH associated with AE1-mediated bicarbonate transport. AE1-mediated chloride/bicarbonate exchange was reduced 50-60% by inhibition of endogenous carbonic anhydrase with acetazolamide, which indicates that CAII activity is required for full anion transport activity. AE1 mutants, unable to bind CAII, had significantly lower transport activity than wild-type AE1 (10% of wild-type activity), suggesting that a direct interaction was required. To determine the effect of displacement of endogenous wild-type CAII from its binding site on AE1, AE1-transfected HEK293 cells were co-transfected with cDNA for a functionally inactive CAII mutant, V143Y. AE1 activity was maximally inhibited 61 +/- 4% in the presence of V143Y CAII. A similar effect of V143Y CAII was found for AE2 and AE3cardiac anion exchanger isoforms. We conclude that the binding of CAII to the AE1 carboxyl-terminus potentiates anion transport activity and allows for maximal transport. The interaction of CAII with AE1 forms a transport metabolon, a membrane protein complex involved in regulation of bicarbonate metabolism and transport.

Deoxygenation of sickle cells stimulates Syk tyrosine kinase and inhibits a membrane tyrosine phosphatase

Polymerization of hemoglobin S in sickle red cells, in deoxygenated conditions, is associated with K+ loss and cellular dehydration. It was previously reported that deoxygenation of sickle cells increases protein tyrosine kinase (PTK) activity and band 3 tyrosine phosphorylation and that PTK inhibitors reduce cell dehydration. Here, the study investigates which PTKs are involved and the mechanism of their activation. Deoxygenation of sickle cells induced a 2-fold increase in Syk activity, measured by autophosphorylation in immune complex assays, but had no effect on Lyn. Syk was not stimulated by deoxygenation of normal red cells, and stimulation was partly reversible on reoxygenation of sickle cells. Syk activation was independent of the increase in intracellular Ca++ and Mg2+ associated with deoxygenation. Lectins that promote glycophorin or band 3 aggregation did not activate Syk. In parallel to Syk stimulation, deoxygenation of sickle cells, but not of normal red cells, decreased the activity of both membrane-associated protein tyrosine phosphatase (PTPs) and membrane protein thiol content. In vitro pretreatment of Syk immune complexes with membrane PTP inhibited Syk autophosphorylation. It is suggested that Syk activation in vivo could be mediated by PTP inhibition, itself resulting from thiol oxidation, as PTPs are known to be inhibited by oxidants. Altogether these data indicate that Syk could be involved in the mechanisms leading to sickle cell dehydration.

Anion permeability and erythrocyte swelling

Permeability of cell membranes to cations may increase as a result of membrane oxidation or in certain pathologies. We studied the effects of nonselective increases in cell membrane permeability to univalent cations on the volume of erythrocytes incubated in phosphate-buffered saline (PBS) using amphotericin B (5-10 mg/l suspension) or gramicidin D (10-100 microg/l suspension) as the membrane permeabilizing agents. Both antibiotics caused K+ to leak, Na+ to accumulate intracellularly, and cell volume to increase. The interval needed to reach the equilibrium between the intracellular and extracellular ion concentrations ranged from 30 min to several hours, depending on the antibiotic concentration. In spite of a rapid disappearance of cation transmembrane gradients, cell volume increased relatively slow. Even 24 h after the membrane permeability was changed, the volume of most erythrocytes did not increase to the lytic values (about 1.6 times the normal volume). The slow increase in erythrocyte volume was accounted for by slow changes in the transmembrane Cl- gradient. 4,4'-Diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS), a specific inhibitor of anion transport, while producing no effect on the transmembrane Na+ and K+ fluxes induced by the antibiotics, significantly inhibited the decrease in the transmembrane Cl- gradient and the increase in erythrocyte volume. Analysis of these data by means of mathematical modeling showed that it failed to satisfactorily describe the experimental kinetics of erythrocyte swelling in response to increases in the membrane permeability to univalent cations if its permeability to Cl was set to be constant. The satisfactory description of this kinetics could be achieved by assuming that the membrane permeability to anions decreased with increasing erythrocyte volume. The results obtained demonstrate that transmembrane anion transport may be considered to be a component of the mechanism responsible for the erythrocyte volume stabilization, because a significant decrease in the swelling rate allows the erythrocytes with damaged membranes to activate a relatively slow (metabolic) mechanisms of cell volume stabilization and/or repair their damaged membranes.

Sequential phosphorylation of protein band 3 by Syk and Lyn tyrosine kinases in intact human erythrocytes: identification of primary and secondary phosphorylation sites

Treatment of intact human erythrocytes with pervanadate induces Tyr (Y)-phosphorylation of the transmembrane protein band 3; in parallel, the activity of the immunoprecipitated tyrosine kinases Syk and Lyn is increased. When erythrocytes are incubated with pervanadate together with PP1, a specific inhibitor of Src kinases, including Lyn, the Y-phosphorylation of band 3 is only partially reduced. Indeed, the PP1-resistant phosphorylation of band 3 precedes and is a prerequisite for its coimmunoprecipitation with Lyn, which interacts with the phosphoprotein via the SH2 domain of the enzyme, as proven by binding competition experiments. Upon recruitment to primarily phosphorylated band 3, Lyn catalyzes the secondary phosphorylation of the transmembrane protein. These data are consistent with the view that band 3 is phosphorylated in intact erythrocytes by both PP1-resistant (most likely Syk) and PP1-inhibited (most likely Lyn) tyrosine kinases according to a sequential phosphorylation process. Similar radiolabeled peptide maps are obtained by tryptic digestion of32P-band 3 isolated from either pervanadate-treated erythrocytes or red cell membranes incubated with exogenous Syk and Lyn. It has also been demonstrated by means of mass spectrometry that the primary phosphorylation of band 3 occurs at Y8 and Y21, while the secondary phosphorylation affects Y359 and Y904.

Sequential phosphorylation of protein band 3 by Syk and Lyn tyrosine kinases in intact human erythrocytes: identification of primary and secondary phosphorylation sites

Treatment of intact human erythrocytes with pervanadate induces Tyr (Y)-phosphorylation of the transmembrane protein band 3; in parallel, the activity of the immunoprecipitated tyrosine kinases Syk and Lyn is increased. When erythrocytes are incubated with pervanadate together with PP1, a specific inhibitor of Src kinases, including Lyn, the Y-phosphorylation of band 3 is only partially reduced. Indeed, the PP1-resistant phosphorylation of band 3 precedes and is a prerequisite for its coimmunoprecipitation with Lyn, which interacts with the phosphoprotein via the SH2 domain of the enzyme, as proven by binding competition experiments. Upon recruitment to primarily phosphorylated band 3, Lyn catalyzes the secondary phosphorylation of the transmembrane protein. These data are consistent with the view that band 3 is phosphorylated in intact erythrocytes by both PP1-resistant (most likely Syk) and PP1-inhibited (most likely Lyn) tyrosine kinases according to a sequential phosphorylation process. Similar radiolabeled peptide maps are obtained by tryptic digestion of32P-band 3 isolated from either pervanadate-treated erythrocytes or red cell membranes incubated with exogenous Syk and Lyn. It has also been demonstrated by means of mass spectrometry that the primary phosphorylation of band 3 occurs at Y8 and Y21, while the secondary phosphorylation affects Y359 and Y904.

The curvature and cholesterol content of phospholipid bilayers alter the transbilayer distribution of specific molecular species of phosphatidylethanolamine

The curvature, cholesterol content, and transbilayer distribution of phospholipids significantly influence the functional properties of cellular membranes, yet little is known of how these parameters interact. In this study, the transbilayer distribution of phosphatidylethanolamine (PE) is determined in vesicles with large (98 nm) and small (19 nm) radii of curvature and with different proportions of PE, phosphatidylcholine, and cholesterol. It was found that the mean diameters of both types of vesicles were not influenced by their lipid composition, and that the amino-reactive compound 2,4,6-trinitrobenzenesulphonic acid (TNBS) was unable to cross the bilayer of either type of vesicle. When large vesicles were treated with TNBS, approximately 40% of the total membrane PE was derivatized; in the small vesicles 55% reacted. These values are interpreted as representing the percentage of total membrane PE residing in the outer leaflet of the vesicle bilayer. The large vesicles likely contained approximately 20% of the total membrane lipid as internal membranes. Therefore, in both types of vesicles, PE as a phospholipid class was randomly distributed between the inner and outer leaflets of the bilayer. The proportion of total PE residing in the outer leaflet was unaffected by changes in either the cholesterol or PE content of the vesicles. However, the transbilayer distributions of individual molecular species of PE were not random, and were significantly influenced by radius of curvature, membrane cholesterol content, or both. For example, palmitate- and docosahexaenoate-containing species of PE were preferentially located in the outer leaflet of the bilayer. Membrane cholesterol content affected the transbilayer distributions of stearate-, oleate-, and linoleate-containing species. The transbilayer distributions of palmitate-, docosahexaenoate-, and stearate-containing species were significantly influenced by membrane curvature, but only in the presence of high levels of cholesterol. Thus, differences in membrane curvature and cholesterol content alter the array of PE molecules present on the surfaces of phospholipid bilayers. In cells and organelles, these differences could have profound effects on a number of critical membrane functions and processes.

Deoxygenation and elevation of intracellular magnesium induce tyrosine phosphorylation of band 3 in human erythrocytes

Deoxygenation increases the level of tyrosine phosphorylation of band 3 by approximately 25% in human red blood cells (RBCs), as determined by Western blotting. The effect is much more pronounced in osmotically shrunken RBCs or in the presence of vanadate. When the rise in intracellular free Mg2+ concentration in deoxygenated RBCs is simulated via clamping of the intracellular magnesium in oxygenated RBCs by ionomycin, band 3 phosphorylation is elevated by up to 10-fold. Phosphorylated band 3 is preferentially retained by RBC skeletons, after mild extraction with Triton X-100. Elevation of intracellular free Mg2+ leads to band 3 phosphorylation and is accompanied by rigidification of the membrane skeleton as determined by analysis of RBC membrane mechanical fluctuations. These findings suggest that the visco-elastic properties of human erythrocytes may be regulated by band 3 tyrosine phosphorylation.

Volume expansion stimulates p72(syk) and p56(lyn) in skate erythrocytes

Hypotonic volume expansion of skate erythrocytes rapidly stimulates the tyrosine phosphorylation of band 3, the membrane protein thought to mediate the osmotically sensitive taurine efflux. Skate erythrocytes possess numerous tyrosine kinases including p59fyn, p56lyn, pp60(src), and p72(syk), demonstrated by immune complex assays measuring autocatalytic kinase activity. Inclusion of the cytoplasmic domain of band 3 in this assay showed that only Syk and Lyn can directly phosphorylate the cytoplasmic domain of band 3. Upon cell volume expansion, Syk activity was increased as assessed by three different assays (immune complex assay measuring autophosphorylation, assay of the level of phosphotyrosine of the immunoprecipitated kinase, and assay of level of 32P in the kinase immunoprecipitated from cells prelabeled with 32PO4 and then volume-expanded). The tyrosine kinase Lyn was also stimulated by volume expansion, most notably when analyzed by the latter two methods. Volume expansion stimulated a large increase in the ability of Syk to phosphorylate band 3 at times that coincide with the stimulation of taurine flux. The stilbene piceatannol inhibited Syk preferentially over Lyn and other tyrosine kinases and inhibited volume-stimulated taurine efflux in a concentration-dependent manner similar to that for the inhibition of Syk. Two major phosphorylation peaks were detected in tryptic digests of cdb3 separated by reverse phase HPLC. Edman degradation demonstrated a phosphotyrosine in a YXXL motif. In conclusion, p72(syk) appears to be a strong candidate as a pivotal signal-transducing step in the volume-activated taurine efflux in skate red cells. The level of band-3 phosphorylation may be regulated, in addition, by a protein-tyrosine phosphatase of the 1B variety.