The Electrogenicity of the Sodium/Bicarbonate Cotransporter NBCe1 is Confined in the Conserved Transmembrane Domains

To identify the domains important for electrogenic ion translocation of the Na/HCO 3 cotransporter NBC, we constructed a series of chimeric proteins with electrogenic NBCe1 and electroneutral NBCn1 and measured the HCO 3 − currents mediated by those chimeras in Xenopus oocytes. In voltage clamp, the electrogenic HCO 3 − current was recorded in oocytes expressing a chimera that swapped the N‐terminal domain of NBCe1 with the homologous domain of NBCn1. The conductance for the HCO 3 − currents was 6.0 μS (n=7), higher than 3.1 μS in the absence of HCO 3 − current was −99.3 3. The reversal potential (E HCO3 ) for the HCO 3 mV (n=7), more negative than the resting potential (Vm, −43.1 mV) in the absence of HCO 3 − . Similarly, in oocytes expressing the chimeras that replaced either the C‐terminal domain, the extracellular domain between transmembrane segments (TM) 5 and 6, or all three domains, the E HCO3 was more negative (−77.8, −79.4, or −67.1 mV, respectively, n=7–8 for each) than the V m in the absence of HCO 3 − . In contrast, no measurable HCO 3 − currents were made by chimeras that replaced individually or together the three domains of NBCn1 with those of NBCe1. These chimeras had electroneutral Na/HCO 3 cotransport. Two other chimeras, which replaced either amino acids 1–821 (the N‐terminus and TM1‐5) or 822–1035 (TM6‐13 and the C‐terminus) of NBCe1 with the corresponding region of NBCn1, also had electroneutral Na/HCO 3 transport. We conclude that the electrogenicity of NBCe1 requires the simultaneous presence of both TM1‐5 and TM6‐13 of the protein. Supported by NKF (I.C) and NIH (W.B).