Cation dependence of the phosphorylation of specific residues in red cell membrane protein band 3

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.

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.

Kell and Kx, two disulfide-linked proteins of the human erythrocyte membrane are phosphorylated in vivo

Kell and Kx are two quantitatively minor proteins from the human erythrocyte membrane which carry blood groups antigens and are thought to be a metalloprotease and a membrane transporter, respectively. In the red cell membrane, these proteins form a complex stabilized by disulfide bond(s). Phosphorylation status of these proteins was studied, in the presence or absence of effectors of several kinases, either on intact cells incubated with [32P]-orthophosphate or on ghosts incubated with [gamma-32P]ATP. Purification of Kell-Kx complex, by immunochromatography on an immobilized human monoclonal antibody of Kell blood group specificity allowed to establish that (i) neither protein is phosphorylated on tyrosine; (ii) the Kell protein is a putative substrate for Casein Kinase II (CKII) and Casein Kinase I (CKI) but not for protein kinase C (PKC), whereas Kx protein is phosphorylated by CKII and PKC but not by CKI; (iii) Protein Kinase A neither phosphorylates the Kell nor the Kx proteins.

The Lyn-catalyzed Tyr phosphorylation of the transmembrane band-3 protein of human erythrocytes

Band-3 protein (approximately 95 kDa), the major and multifunctional transmembrane protein of human erythrocytes, has been shown to be phosphorylated by endogenous Tyr-protein kinases on different Tyr residues at its N and C cytoplasmic domains. Both the added p36syk (catalytic domain of p72syk) and Lyn kinases are able to phosphorylate the isolated cytoplasmic domain of band 3 (cdb3), yielded by chymotryptic digestion of band 3 in the isolated membranes (ghosts). However, the two Tyr-protein kinases exhibited different phosphorylation behaviours when added to the isolated erythrocyte membranes. More precisely, the added p36syk markedly Tyr phosphorylates the band-3 protein, whereas the added Lyn phosphorylates it very poorly. It is of interest that Lyn can associate with membranes and markedly phosphorylate band 3 when this latter protein has been previously phosphorylated by p36syk, i.e. the p36(syk)-catalyzed phosphorylation is proposed to be a prerequisite for the association of Lyn with the membrane (likely to band 3) and for the Lyn-catalyzed phosphorylation of different band-3 Tyr sites.

Functional correlation between the Ser/Thr-phosphorylation of band-3 and band-3-mediated transmembrane anion transport in human erythrocytes

In human erythrocytes, okadaic acid, a potent inhibitor of certain protein phosphatases, promotes a marked increase of Ser/Thr-phosphorylation of membrane proteins, including band-3 protein. Moreover, okadaic acid also increases the band-3-mediated oxalate transport across the membranes, thus suggesting that this process is regulated by Ser/Thr-phosphorylation of transporter band-3 protein.

Three regions of erythrocyte band 3 protein are phosphorylated on tyrosines: characterization of the phosphorylation sites by solid phase sequencing combined with capillary electrophoresis

The major part of band 3 phosphorylation was recently shown to concern the first tryptic peptide of the protein (Yannoukakos et al. (1991) Biochim. Biophys. Acta 1061, 253-266). Tyrosine 8 is the prevalent site of phosphorylation, but other phosphorylated regions were found which could not be analyzed with certainty. Direct characterization of the phosphorylated residues in all these phosphorylated fragments was made possible due to recent advances in protein chemistry techniques, such as solid phase sequence analysis and capillary electrophoresis. The present report establishes that band 3 phosphorylation occurs predominantly on tyrosines: besides tyrosine 8 already known in the N-terminal region, two other tyrosines are demonstrated to be targets for the tyrosine kinase, tyrosine 359 and tyrosine 904. These residues lie in regions of band 3 exposed to the cytoplasm, the junction of the cytoplasmic and the membrane-spanning domains, and the C-terminal end of the protein which is also cytosolic, respectively.

The isolated cytoplasmic domain of band 3 binds calcium at physiological salt concentration and neutral PH

Calcium is known to be a potent but partial intracellular inhibitor of band 3 anion exchange. Here we test the hypothesis that the cytoplasmic domain of band 3 (CDB3) contains a calcium binding site. Calcium binding to CDB3 was monitored by measuring the formation of the Aresenazo III-calcium complex at various constant CDB3 concentrations. These experiments were performed at physiological salt and neutral pH. The calcium-CDB3 dissociation constant was estimated to be less than or equal to 24 microM. We also found that the Arsenazo III-calcium complex binds to CDB3, while the free dye does not bind. We conclude that CDB3 contains a site which is capable of binding free calcium under physiological conditions. A specific role for this site in inhibition of band 3 anion exchange is suggested, but that role remains to be established.

Phosphorylation sites in human erythrocyte band 3 protein

The human red cell anion-exchanger, band 3 protein, is one of the main phosphorylated proteins of the erythrocyte membrane. Previous studies from this laboratory have shown that ATP-depletion of the red blood cell decreased the anion-exchange rate, suggesting that band 3 protein phosphorylation could be involved in the regulation of anion transport function (Bursaux et al. (1984) Biochim. Biophys. Acta 777, 253-260). Phosphorylation occurs mainly on the cytoplasmic domain of the protein and the major site of phosphorylation was assigned to tyrosine-8 (Dekowski et al. (1983) J. Biol. Chem. 258, 2750-2753). This site being very far from the integral, anion-exchanger domain, the aim of the present study was to determine whether phosphorylation sites exist in the integral domain. The phosphorylation reaction was carried out on isolated membranes in the presence of [gamma-32P]ATP and phosphorylated band 3 protein was then isolated. Both the cytoplasmic and the membrane spanning domains were purified. The predominant phosphorylation sites were found on the cytoplasmic domain. RP-HPLC analyses of the tryptic peptides of whole band 3 protein, and of the isolated cytoplasmic and membrane-spanning domains allowed for the precise localization of the phosphorylated residues. 80% of the label was found in the N-terminal tryptic peptide (T-1), (residues 1-56). In this region, all the residues susceptible to phosphorylation were labeled but in varying proportion. Under our conditions, the most active membrane kinase was a tyrosine kinase, activated preferentially by Mn2+ but also by Mg2+. Tyrosine-8 was the main phosphate acceptor residue (50-70%) of the protein, tyrosine-21 and tyrosine-46 residues were also phosphorylated but to a much lesser extent. The main targets of membrane casein kinase, preferentially activated by Mg2+, were serine-29, serine-50, and threonine(s)-39, -42, -44, -48, -49, -54 residue(s) located in the T-1 peptide. A tyrosine phosphatase activity was copurified with whole band 3 protein which dephosphorylates specifically P-Tyr-8, indicating a highly exchangeable phosphate. The membrane-spanning fragment was only faintly labeled.

Metabolic depletion effect on serine/threonine- and tyrosine-phosphorylations of membrane proteins in human erythrocytes

The response of serine/threonine-phosphorylation of the major transmembrane protein (band 3) in human erythrocytes to the metabolic state of the cells is different from that exhibited by the tyrosine-phosphorylation of the same protein. Precisely, both serine- and tyrosine-phosphorylation are decreased during metabolic depletion of the erythrocytes. However, the depletion-induced tyrosine-phosphorylation decrease of band 3 is not reversed by the subsequent metabolic repletion of the depleted cells, being accompanied by an irreversible inactivation of both membrane-bound and cytosolic tyrosine-protein kinase(s). By contrast, the depletion-induced phosphoserine-dephosphorylation is reversed by the following repletion, being accompanied by a reversible translocation of casein kinase(s) between cytosolic and membrane compartments. A possible functional correlation between the serine-phosphorylation state of band 3 protein and the band 3-mediated anion transport across the membrane is discussed.

Tyrosine phosphorylation of cytosolic proteins in human erythrocytes

Some cytosolic proteins of human erythrocytes can be phosphorylated on tyrosine residues by endogenous Tyr-protein kinase(s). Their phosphorylation is enhanced by addition of Tyr-protein kinase, purified from human erythrocyte cytosol. The most phosphorylatable is a 19 kDa protein. Its phosphorylation is more activated by Mn2+ than by Mg2+. It is inhibited by NaC1, 2,3-bisphosphoglycerate and by heparin. Similar response to the above effectors is exhibited by the phosphorylation of the other protein bands. However, the phosphorylation of a 73 kDa double band, which is negligible in the absence of added NaC1, is stimulated by this salt.

Partial purification and characterization of cytosolic Tyr-protein kinase(s) from human erythrocytes

Tyrosine-protein kinase, phosphorylating tyrosine residues of transmembrane band 3 protein, has been partially purified from human erythrocyte cytosol by DEAE-Sepharose chromatography followed by heparin-Sepharose chromatography. Such a Tyr-protein kinase (36 kDa), as distinct from the Ser/Thre-protein kinases (casein kinase S and TS), appears to display a broader site specificity than does the previously described human erythrocyte P-Tyr-protein phosphatase, dephosphorylating band 3 protein. That is, it is able to phosphorylate not only the highly acidic copolymer poly(Glu-Tyr) but also angiotensin II, lacking an acidic amino acid sequence around the target Tyr residue. Moreover, the phosphorylation of these two substrates exhibits a different pH dependence and a different response to NaCl and 2,3-bisphosphoglycerate. These results suggest that in intact erythrocytes the cytosolic Tyr-protein kinase might phosphorylate band 3 not only on Tyr-8, surrounded by several acidic side-chains (as demonstrated preferentially to occur in isolated ghosts), but also on other Tyr residues surrounded by other amino acid sequences.