Abstract
Mammalian glycosylated rhesus (Rh) proteins include the erythroid RhAG and the nonerythroid RhBG and RhCG. RhBG and RhCG are expressed in multiple tissues, including hepatocytes and the collecting duct (CD) of the kidney. Here, we expressed human RhAG, RhBG and RhCG in Xenopus oocytes (vs. H2O-injected control oocytes) and used microelectrodes to monitor the maximum transient change in surface pH (ΔpHS) caused by exposing the same oocyte to 5 % CO2/33 mM HCO3 − (an increase) or 0.5 mM NH3/NH4 + (a decrease). Subtracting the respective values for day-matched, H2O-injected control oocytes yielded channel-specific values (*). \(({\Updelta {\text{pH}}_{\text{S}}^{*} })_{{{\text{CO}}_{ 2} }}\) and \(({ - \Updelta {\text{pH}}_{\text{S}}^{*} })_{{{\text{NH}}_{ 3} }}\) were each significantly >0 for all channels, indicating that RhBG and RhCG—like RhAG—can carry CO2 and NH3. We also investigated the role of a conserved aspartate residue, which was reported to inhibit NH3 transport. However, surface biotinylation experiments indicate the mutants RhBGD178N and RhCGD177N have at most a very low abundance in the oocyte plasma membrane. We demonstrate for the first time that RhBG and RhCG—like RhAG—have significant CO2 permeability, and we confirm that RhAG, RhBG and RhCG all have significant NH3 permeability. However, as evidenced by \(({\Updelta {\text{pH}}_{\text{S}}^{*} })_{{{\text{CO}}_{ 2} }} /({ - \Updelta {\text{pH}}_{\text{S}}^{*} })_{{{\text{NH}}_{ 3} }}\) values, we could not distinguish among the CO2/NH3 permeability ratios for RhAG, RhBG and RhCG. Finally, we propose a mechanism whereby RhBG and RhCG contribute to acid secretion in the CD by enhancing the transport of not only NH3 but also CO2 across the membranes of CD cells.
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Acknowledgments
We thank Dale Huffman for computer support, Dr. Alice Brown (University of Bristol) for plasmid cloning and Dr. Nancy Amaral Rebouças (University of São Paulo) and Dr. Seong-Ki Lee (Case Western Reserve University) for helpful discussions. R. R. G. was supported by postdoctoral fellowship N00014-09-1-0246 from the Office of Naval Research. R. M.-A. was supported by Fundação de Amparo a Pesquisa do Estado de São Paulo (FAPESP, 08/128663). This work was supported by Office of Naval Research grant N00014-11-1-0889 and NIH grant DK81567 to W. F. B. A. M. T. received funding from Kidney Research UK and NHS Blood and Transplant R&D.
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Geyer, R.R., Parker, M.D., Toye, A.M. et al. Relative CO2/NH3 Permeabilities of Human RhAG, RhBG and RhCG. J Membrane Biol 246, 915–926 (2013). https://doi.org/10.1007/s00232-013-9593-0
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DOI: https://doi.org/10.1007/s00232-013-9593-0