Comparison of murine band 3 protein-mediated Cl- transport as measured in mouse red blood cells and in oocytes of Xenopus laevis

Electrogenic sulfate/chloride exchange in Xenopus oocytes mediated by murine AE1 E699Q

Functional evaluation of chemically modified human erythrocytes has led to the proposal that amino acid residue E681 of the band 3 anion exchanger AE1 lies on the anion translocation pathway and is a proton carrier required for H+/SO4(2-) cotransport. We have tested in Xenopus oocytes the functional consequences of mutations in the corresponding residue E699 of mouse AE1. Most mutations tested abolished AE1-mediated Cl- influx and efflux. Only the E699Q mutation increased stilbene disulfonate-sensitive efflux and influx of SO4(2-). E699Q-mediated Cl- influx was activated by elevation of intracellular SO4(2-), but E699Q-mediated Cl- efflux was undetectable. The DNDS (4,4'-dinitrostilbene-2,2'-disulfonic acid) sensitivity of E699Q-mediated SO4(2-) efflux was indistinguishable from that of wt AE1-mediated Cl- efflux. The extracellular anion selectivity of E699Q-mediated SO4(2-) efflux was similar to that of wt AE1-mediated Cl- efflux. The stoichiometry of E699Q-mediated exchange of extracellular Cl- with intracellular SO4(2-) was 1:1. Whereas SO4(2-) injection into oocytes expressing wt AE1 produced little change in membrane potential or resistance, injection of SO4(2-), but not of Cl- or gluconate, into oocytes expression E699Q depolarized the membrane by 17 mV and decreased membrane resistance by 66%. Replacement of bath Cl- with isethionate caused a 28-mV hyperpolarization in SO4(2-)-loaded oocytes expressing E699Q, but had no effect on oocytes expressing wt AE1. Extracellular Cl(-)-dependent depolarization of SO4(2-)-preloaded oocytes was blocked by DNDS. AE1 E699Q-mediated inward current measured in the presence of extracellular Cl- was of magnitude sufficient to account for measured 35SO4(2-) efflux. Thus, AE1 E699Q-mediated SO4(2-)/Cl- exchange operated largely, if not exclusively, as an electrogenic, asymmetric, 1:1 anion exchange. The data confirm the proposal that E699 resides on or contributes to the integrity of the anion translocation pathway of AE1. A single amino acid change in the sequence of AE1 converted electroneutral to electrogenic anion exchange without alteration of SO4(2-)/Cl- exchange stoichiometry.

Resistance to osmotic lysis in BXD-31 mouse erythrocytes: association with upregulated K-Cl cotransport

The decreased osmotic fragility and reduced K+ content of BXD-31 mouse erythrocytes arise from variation at a single genetic locus. We compared ion transport in erythrocytes from BXD-31 mice and the parental strain, DBA/2J. The strains had similar rates for Na-K pump, Na/H exchange, Na-K-2Cl cotransport, Ca2+ activated K+ channel, or AE1-mediated SO4 transport. In contrast, K-Cl cotransport was twice as active in BXD-31 as in DBA/2J cells. Cl- dependent K+ efflux from BXD-31 cells displayed steep activation by acid pH (with maximal transport occurring at pH 6.75), whereas DBA/2J erythrocytes displayed a far less dramatic response to pH. Both strains displayed regulatory volume decrease in response to cell swelling. However, a 62% greater loss of cell K+ via K-Cl cotransport was observed in the BXD-31 strain. Furthermore the decreased osmotic fragility of BXD-31 red blood cells was normalized by treatment with nystatin to achieve normal cell K+ and water content. Thus upregulated K-Cl cotransport induces cell dehydration and K+ deficit in BXD-31 erythrocytes and causes their characteristic resistance to osmotic lysis.

Cation transport in mouse erythrocytes: role of K(+)-Cl- cotransport in regulatory volume decrease

We investigated cation transport and cell volume regulation in erythrocytes of CD1 and C57/B6 mice. Swelling of cells from either strain stimulated K+ efflux that was insensitive to ouabain, bumetanide, and clotrimazole. Seventy-five percent of swelling-induced K+ efflux was Cl- dependent (inhibited by sulfamate or methanesulfonate, partially by NO3-, but not by SCN-) and was inhibited by okadaic acid (OA; 50% inhibitory concentration = 18 +/- 6 nM in CD1 and 10 +/- 4 nM in C57/B6). In both strains, K+ efflux into isotonic medium was stimulated by staurosporine or by N-ethylmaleimide, and the latter was partially blocked by pretreatment of cells with OA. When cells of either strain were incubated in hypotonic medium or preswollen isosmotically with nystatin, OA-sensitive regulatory volume decrease (RVD) and K+ loss were observed. RVD produced by hypotonic swelling was prevented by Cl- replacement with sulfamate or methanesulfonate. These properties suggest the presence in outbred and inbred mouse erythrocytes of RVD mediated by K(+)-Cl- cotransport.

Functional characterization and regulation by pH of murine AE2 anion exchanger expressed in Xenopus oocytes

cRNA encoding the murine band 3-related protein AE2 was expressed in Xenopus oocytes. AE2-mediated transport function and regulation were analyzed by unidirectional 36Cl- influx and efflux studies. AE2 cRNA-injected oocytes took up 36Cl- as much as 40-fold faster than did water-injected oocytes. AE2-mediated 36Cl- uptake increased as a function of increasing uptake time, number of days after cRNA injection, and amount of injected cRNA. Among the functional properties of AE2 evaluated were transport mechanism and substrate specificity, inhibitor pharmacology, and regulation by pH. The apparent Km for external Cl- was 5.6 mM. AE2 was defined as a Cl-/anion exchanger by two criteria: 1) 36Cl- efflux from AE2-expressing oocytes was maximally stimulated by extracellular Cl- or nitrate; AE2-associated 36Cl- efflux was supported by substitution of extracellular Cl- with other anions in the rank order bromide > isethionate > or = gluconate > iodide and 2) prolonged preincubation of AE2 cRNA-injected oocytes in Cl(-)-free media containing isethionate, gluconate, or glutamate decreased subsequent AE2-associated 36Cl- uptake from Cl- media in rough proportion to the degree of intracellular Cl- depletion, whereas preincubation in nitrate medium had no effect. AE2-associated 36Cl- uptake was inhibited by 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid at half-maximally inhibitory concentrations between 0.5 and 19 microM, depending on extracellular Cl- concentration, and progressed to irreversibility at 20 degrees C with a half-time of 20-30 min. Many additional inhibitors showed lower potency for AE2 than previously reported for AE1. Although AE2 expression did not change oocyte resting intracellular pH, AE2-associated 36Cl- influx and efflux were each decreased in acid incubation medium and increased in alkaline medium.

Role of N-glycosylation in the expression of human band 3-mediated anion transport

The human erythrocyte anion transporter (band 3; AE1) has a single N-linked glycosylation site at amino residue Asn-642. To investigate the functional role of the N-glycan in band 3 (b3) we have constructed mutant b3 cDNAs in which this residue has been replaced by Gly, Ser or Thr, and the expression of these mutants was examined in Xenopus oocytes. Chymotrypsin treatment of intact oocytes was used to assess surface b3. Similar amounts of cleavage were observed with both glycosylated and unglycosylated b3. Greater cleavage of b3 was obtained when human red cell glycophorin A (GPA) was co-expressed with either glycosylated or unglycosylated b3. The co-expression of GPA with either glycosylated or unglycosylated b3 increased the stilbene disulphonate-sensitive chloride transport into oocytes at low cRNA concentrations. In both the presence or absence of GPA, a higher b3-mediated chloride influx into oocytes was observed on expression of glycosylated b3 cRNA compared with similar amounts of unglycosylated b3 cRNA. We suggest that glycosylation is not essential for the expression of functional b3 in oocytes, but may play a role in enabling the protein to acquire its correct folding with the highest anion transport activity.