Reconstitution of the erythrocyte anion channel

An immunological study of band 3, the anion transport protein of the human red blood cell membrane

Band 3, the predominant membrane-spanning polypeptide and purported anion transport protein of human red cells, was isolated by a new procedure which utilized selective solubilization and anion exchange chromatography on Affi-Gel 102 in 0.5% and Triton X-100/0.03% sodium dodecyl sulfate. Rabbit anti-serum prepared against the purified protein reacted with human and monkey band 3 but gave no immunoprecipitate with membrane proteins from several non-primate species. The antiserum was directed solely towards a portion of the cytoplasmic pole of the band 3 polypeptide contained within a 23,000 dalton amino-terminal fragment, as shown by agglutination, absorption, double diffusion and immunoprecipitation techniques. Saturation of both surfaces of resealed erythrocyte ghosts with the anti-band 3 antiserum had no significant effect on chloride transport. Our data define the topographically-limited immunogenicity of human band 3 in rabbits, demonstrate a lack of immunological cross-reactivity of band 3 between primates and non-primates, and support the hypothesis that the cytoplasmic domain of band 3 is not intimately involved in anion transport.

HCO3-/Cl- exchange across the human erythrocyte membrane: effects of pH and temperature

Changes in extracellular pH (PH0) in red cell suspensions were monitored in a stopped-flow rapid reaction apparatus under conditions where dpH0/dt was determined by the rate of HCO3-/Cl- exchange across the membrane. Experiments were performed at 5 degrees C less than T less than 40 degrees C using either untreated cells or cells exposed to 0.11 mM SITS (4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid). Although SITS exposure reduced the rate of exchange by 90%, both untreated and SITS-treated cells are similarly affected by changes in pH0 and temperature. The rate of HCO3-/Cl- exchange exhibits a minimum at about pH0 5 and a maximum at about pH0 7.4 at all temperatures. A transition temperature of 17 degrees C was observed in the Arrhenius relationship for all pH0. The activation energies (Ea) in kcal/mol are 19.6 below and 11.7 above 17 degrees C for 5 less than pH0 less than 8. These findings, similar to those reported for Cl- self-exchange, suggest that: (i) a change in the rate-limiting step for HCO3-/Cl- exchange occurs at 17 degrees C, possibly due to an altered interaction between the transport pathway and membrane lipids; (ii) the carrier system can be titrated by either H+ or SITS from the outside of the membrane, but the untitrated sites continue to transport normally; (iii) the pH0 dependence of the rate of exchange is consistent with the titratable carrier having its most alkaline pK in the range expected for amino groups; and (iv) below pH0 5, the nature of the exchange is markedly altered.

Fluorescence labeling of the human erythrocyte anion transport system

The anion transport system of human red cells was isolated in vesicles containing the original membrane lipids and the 95 000 dalton polypeptides (band 3) by the method of Wolosin et al. (J. Biol. Chem. (1977) 252, 2419--2427). The vesicles have a functional anion transprot system since they display sulfate transport that is inhibited by the fluorescent probe 8-anilinonaphthalene 1-sulfonate (ANS) with similar potency as in red cells. The vesicles were labeled with the SH-specific probe fluorescein mercuric acetate (FMA). Labeling lowers FMA fluorescence, and is prevented or reversed by dithiothreitol, suggesting that the reaction is with a thiol group on the protein. Fluorescnece titrations show a maximum labeling stoichiometry of 1.3 +/- 0.4 mol FMA/mol 95 000 dalton polypeptide. The polarization of bound FMA fluorescence is high indicating that the probe is highly immobilized. Pretreatment with Cu2+ + o-phenanthroline under conditions that crosslink band 3 in ghosts decreases FMA labeling 50%. Differences in kinetics of FMA labeling in sealed and leaky vesicles suggest that the reactive SH group is located in the intravesicular portion of the protein (corresponding to the cytoplasmic surface of the red cell) and that FMA can cross the membrane. Inhibitors of anion transport have no effect on FMA labeling kinetics suggesting it is not transported via the anion transport system. Sulfate transport in the labeled vesicles remains fully functional. We detected self-energy transfer between bound FMA molecules by fluorescence depolarization. With excitation at 450--50 nm P decreases from 0.4, when less than half of the proteins are labeled, to 0.1 at saturation. This depolarization is not observed with red edge excitation (510--530 nm). Addition of 0.1% sodium dodecyl sulfate (SDS) changes P to 0.32, regardless of the excitation wavelength or degree of saturation with FMA. These results indicate that the band 3 proteins are close enough to allow energy transfer between fluorophores(Ro = 37.4 A), which does not occur upon red edge excitation or when the proteins are separated by SDS. We conclude that the functional anion transport system exists as a dimer or higher oligomer of band 3 proteins in these membranes, confirming previous suggestions derived using other methods. Future applications are discussed.