Roles of Histidine 752 and Glutamate 699 in the pH Dependence of Mouse Band 3 Protein-Mediated Anion Transport

In the accompanying paper we have shown that four different histidine residues are involved in the maintenance of mouse band 3 in a state in which it is able to execute its anion transport function. Here we focus on the functional significance of His 752 and demonstrate that this residue, together with Glu 699, plays a key role in the control of pH dependence of Cl^- transport. Mouse band 3-encoding cRNA was expressed in Xenopus oocytes, and band 3-mediated Cl^- transport was measured at zero membrane potential over the pH range 6.0-9.2. Transport decreased with increasing H⁺ concentration and was governed by a single pK of 5.8. After correction for temperature differences, this result agrees well with measurements in erythrocyte ghosts of Cl^- flux by Funder and Wieth [Funder, J., & Wieth, J. O. (1976) J. Physiol. 262, 679-698] and our own determinations by ³⁵C1 NMR spectroscopy of Cl^- exchange between the substrate binding site and the medium. After mutation of either Glu 699 to Asp or of His 752 to Ser, the maximal rate of transport is reduced and the rate of anion exchange is now governed by a single pK of about 6.8-6.9. This suggests that the formation of a hydrogen bond between His 752 and Glu 699 is essential for the decrease of band 3-mediated Cl^- transport at low pH. We suggest that in the wild type band 3 both the decrease of the chloride exchange between the medium and the substrate binding site and the inhibition of chloride translocation across the membrane are dominated by a common rate-limiting step and that this step involves hydrogen bond formation between Glu 699 and His 752.