Anion transport activity in the human erythrocyte membrane modulated by proteolytic digestion of the 38,000-dalton fragment in Band 3

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.

A 40-kDa polypeptide from papain digestion of the rabbit intestinal Na+/phosphate cotransporter retains Na+ and phosphate cotransport

The rabbit intestinal brush border membrane Na+/phosphate cotransporter was digested with a variety of proteolytic enzymes. Limited papain digestion generated a 40-kDa polypeptide (P40) which retained putative substrate site markers, fluorescein isothiocyanatophenyl glyoxal and eosin n-acetyl imidazole. P40 retained Na+- and phosphate-selective tryptophan fluorescence quenching, pH sensitivity of ion-induced conformational changes, and tight Na+ and H(2)PO(4)(-) binding. Reconstituted into proteoliposomes, P40 catalyzed Na+-dependent phosphate uptake. The N-terminus of P40 was blocked. An internal sequence of P40 demonstrated that it was derived from NaPi II b. These results suggest that P40 may be a useful model system for studies of the molecular mechanism of Na+-dependent phosphate cotransport and a starting point for structural studies.

Topology of the anion exchange protein AE1: the controversial sidedness of lysine 743

The topology of the band 3 (AE1) polypeptide of the erythrocyte membrane is not fully established despite extensive study. Residues near lysine 743 (K743) have been reported to be extracellular in some studies and cytoplasmic in others. In the work presented here, we have attempted to establish the sidedness of K743 using in situ proteolysis. Trypsin, papain, and proteinase K do not cleave band 3 at or near K743 in intact red cells, even under conditions that cause cleavage on the C-terminal side of the glycosylation site (N642) in extracellular loop 4. In contrast, trypsin sealed inside red cell ghosts cleaves at K743, as does trypsin treatment of inside-out vesicles (IOVs). The transport inhibitor 4,4'-diisothiocyanatodihydrostilbene-2,2'-disulfonate (H(2)DIDS), acting from the extracellular side, blocks trypsin cleavage at K743 in unsealed membranes by inducing a protease-resistant conformation. H(2)DIDS added to IOVs does not prevent cleavage at K743; therefore, trypsin cleavage at K743 in IOVs is not a consequence of cleavage of right-side-out or leaky vesicles. Finally, microsomes were prepared from HEK293 cells expressing the membrane domain of AE1 lacking the normal glycosylation site. This polypeptide does not traffic to the surface membrane; trypsin treatment of microsomes containing this polypeptide produces the 20 kDa fragment, providing further evidence that K743 is exposed at the cytoplasmic surface. Therefore, the actions of trypsin on intact cells, resealed ghosts, unsealed ghosts, inside-out vesicles, and microsomes from HEK293 cells all indicate that K743 is cytoplasmic and not extracellular.

Pathways for flip-flop of mono- and di-anionic phospholipids in the erythrocyte membrane

The inward translocations (flip), from the outer to the inner membrane leaflet of human erythrocytes, of di-anionic NBD-labeled phospholipids containing as a head group phosphate esters of glycolate, butyrate and hydroxyethanesulfonate are slow processes (k = 0.005-0.008 h-1, 37 degrees C) at pH 7.4. A decrease of pH highly stimulates the flip. A major role of the anion exchanger (AE1), band 3, in this flip is indicated by (a) the strong inhibition of the flip (55-85%) by stilbene disulfonates and other inhibitors of anion transport, (b) the stimulation and loss of pH dependence of the flip after modification of band 3 by Woodward's reagent K and NaBH4, and (c) the stimulation of the flip after proteolytic cleavage of band 3 by papain. The flip of mono-anionic NBD-phospholipids with phosphate esters of glycerol, glycol, methanol, butanol and benzyl alcohol is much faster than that of their dianionic analogs (k = 0.04 to > 3.0 h-1, 37 degrees C). It is inhibited by stilbene disulfonates to a decreasing extent (35 to 0%) and is not affected by several reversible inhibitors of anion exchange. This indicates a minor component of band-3-mediated flip and a major component of nonmediated flip. The outward translocations (flop), from the inner to outer membrane leaflet, of both mono- and di-anionic phospholipids are very fast (1.0-5.9 h-1), ATP-dependent and inhibitable by vanadate, fluoride, SH-reagents or Mg(2+)-depletion of cells and thereby likely to be largely mediated by a 'floppase'. The stationary distributions of the NBD-labeled anionic phospholipids are asymmetric to an extent (outer to inner leaflet ratio 2-9) correlating with the ratio of the rates of the outward and the inward translocation. Thus, asymmetry is largely abolished by blockage of the floppase-mediated translocation.

Band 3, the anion exchanger of the erythrocyte membrane, is also a flippase

The transbilayer reorientation (flip-flop) of the long-chain amphiphilic anion DENSA (5-(N-decyl)aminonaphthalene-2-sulfonic acid) in the erythrocyte membrane was studied by fluorescence spectroscopy. DENSA intercalates into the membrane at a high membrane/water partition coefficient (3.2.10(5)) and rapidly reorients from the outer to the inner layer in a first order process (k = 0.11 min-1, 37 degrees C, pH 7.4) leading to a steady-state distribution inner:outer layer of about 30:70. The activation energy of the fully reversible and symmetric flip process is about 110 kJ/mol. DIDS and various other established covalent and non-covalent inhibitors of anion transport via the erythrocyte anion exchanger, band 3 (AE 1), suppress the flip to a minimum of about 30-35% of the control. The flip is also inhibited by Cl- with a half maximal inhibitory concentration equal to that required for the inhibition of the exchange flux of ordinary anions via band 3. These findings indicate the involvement of a band 3 mediated (DIDS-sensitive) component of the flip and a DIDS-insensitive one, possibly involving, at least to some extent, simple transbilayer 'diffusion'. This latter component is stimulated by diamide, an SH oxidant known to increase the permeability of the membrane lipid domain of the erythrocyte. Alcohols (butanol, hexanol) accelerate both flip components. Papain treatment, known to inhibit 'ordinary' anion exchange, accelerates both flip and flop. The results suggest that band 3 protein, besides being a conventional transporter of anions, can act as a flippase translocating anionic, membrane-intercalated amphiphiles approaching the transporter from the lipid domain. The flippase mode of operation of band 3 must, however, differ in its mechanism from the conventional exchange mode.

Band 3, an accidental flippase for anionic phospholipids?

The inward translocation of the monovalent anionic phospholipid 1-palmitoyl-sn-glycero-3-phosphomethanol in the membrane of human erythrocytes is a fast process (t/2 = 11 min, 37 degrees C). Translocation of the protonated uncharged phospholipid is not responsible for the fast flip rate, and mediation of translocation by the aminophospholipid flippase could be excluded. Involvement of the anion exchanger band 3 in this process was derived from its inhibition (40-70%) by several established inhibitors of band 3-mediated anion exchange and its acceleration after proteolysis of band 3 by external papain. The translocation of the dianionic NBD-labeled phosphatidic acid is 5-fold slower, but also affected by the inhibitors. Thus, the anion exchanger can act as a flippase, defined as a transporter accepting substrates from the lipid bilayer.

Transport domain of the erythrocyte anion exchange protein

The anion transport domain of the anion exchange protein (AEP) of human erythrocyte membranes (band 3, 95 kD mol wt) was probed with the substrate and affinity label pyridoxal-5'-phosphate (PLP). Acting from outside, this probe labels two chymotryptic fragments of 65 and 35 kD of AEP but only the 35-kD fragment is protected from labeling by reversibly acting disulfonic stilbenes (DS). It is shown here by functional studies and by immunoblotting with anti-PLP antibodies that transmembrane gradients of anions determine the availability of a 35-kD fragment lys residue to surface labeling by PLP, in analogy with their effects on labeling of 65-kD fragment by DS. On this basis, it is suggested that both fragments contribute to the formation of the transport domain. However, unlike DS, PLP blocks transport when reacted from within released membranes, indicating that the 35-kD fragment might contain components of the mobile unit of the AEP. Using impermeant fluorescence quenchers of PLP of both complexation type (anti-PLP antibodies) or collisional type (acrylamide) as topological probes for PLP-labeled sites, it is deduced that the 65-kD PLP-labeled and the 35-kD PLP-labeled lys groups are inaccessible to macromolecules from either surface, but the 65-kD PLP-lys is accessible to low molecular weight molecules from without while the 35-kD PLP-labeled lys shows accessibility primarily from within the cell surface. The studies indicate that the accommodation of a wide class of anions by AEP might be associated with the flexibility of the transport domain of the protein and its capacity to undergo transport-related conformational changes.

Conformational change of band 3 protein induced by diethyl pyrocarbonate modification in human erythrocyte ghosts

Diethyl pyrocarbonate inhibited the phosphate exchange across the human erythrocyte membrane. The exchange rate was inhibited only when the membranes were modified with the reagent from the cytosolic surface of resealed ghosts. The intracellular modification by diethyl pyrocarbonate inhibited the extracellular binding of [3H]dihydro-4,4'-diisothiocyanostilbene-2,2'-disulfonic acid to band 3 protein. Furthermore, the extracellular 4,4'-dinitrostilbene-2,2'-disulfonic acid protected the membranes from the intracellular modification by diethyl pyrocarbonate. These results suggest that the extracellular binding of 4,4'-dinitrostilbene-2,2'-disulfonic acid to band 3 protein induces the conformational change of the intracellular counterpart of band 3 protein and the diethyl pyrocarbonate susceptible residue(s) is (are) hidden from the cytosolic surface of the cell membrane in connection with the conformational change. Conversely, under the conditions where the diethyl pyrocarbonate modification is confined to the intracellular side of the membrane, the extracellular binding site of [3H]dihydro-4,4'-diisothiocyanostilbene-2,2'-disulfonic acid is hidden from the cell surface.

Evidence for multisite allosteric interactions on the band 3 monomer

Pyridoxal-5'-phosphate is known to label the two integral, chymotryptic domains (CH17 and CH35) of the erythrocyte anion exchange protein known as band 3. The CH35 sites are mutually exclusive with stilbene disulfonate binding, while the CH17 sites are not. Selective, irreversible pyridoxal-5'-phosphate labeling of CH17, reduces the transport inhibitory potency due to reversible stilbene disulfonate binding to vacant, nonoverlapping CH35 sites. We conclude that multisite allosteric interactions can occur on one band 3 monomer.

Modification of the erythrocyte anion carrier by chromate

It was confirmed that chromate is taken up by human erythrocytes via the general anion carrier. The chromate flux is unidirectional and chromium is accumulated within red cells presumably due to intracellular reduction of Cr(VI) to Cr(III). The analysis of the initial rates of uptake of chromate revealed two distinct uptake mechanisms at low (0.001-0.01 mM) and at high (0.05-1.0 mM) chromate concentrations. After prolonged incubation with 1 mM chromate, the subsequent rate of uptake of chromate was decreased. It is suggested that the decreased uptake is due to a modification of the anion-transport protein by chromate.