Relative CO2/NH3 selectivities of AQP1, AQP4, AQP5, AmtB, and RhAG
Abstract
The water channel aquaporin 1 (AQP1) and certain Rh-family members are permeable to CO2 and NH3. Here, we use changes in surface pH (pHS) to assess relative CO2 vs. NH3 permeability of Xenopus oocytes expressing members of the AQP or Rh family. Exposed to CO2 or NH3, AQP1 oocytes exhibit a greater maximal magnitude of pHS change (ΔpHS) compared with day-matched controls injected with H2O or with RNA encoding SGLT1, NKCC2, or PepT1. With CO2, AQP1 oocytes also have faster time constants for pHS relaxation (τpHs). Thus, AQP1, but not the other proteins, conduct CO2 and NH3. Oocytes expressing rat AQP4, rat AQP5, human RhAG, or the bacterial Rh homolog AmtB also exhibit greater ΔpHS(CO2) and faster τpHs compared with controls. Oocytes expressing AmtB and RhAG, but not AQP4 or AQP5, exhibit greater ΔpHS(NH3) values. Only AQPs exhibited significant osmotic water permeability (Pf). We computed channel-dependent () ΔpHS or Pf by subtracting values for H2O oocytes from those of channel-expressing oocytes. For the ratio ΔpHS(CO2)*/Pf*, the sequence was AQP5 > AQP1 ≅ AQP4. For ΔpHS(CO2)*/ΔpHS(NH3)*, the sequence was AQP4 ≅ AQP5 > AQP1 > AmtB > RhAG. Thus, each channel exhibits a characteristic ratio for indices of CO2 vs. NH3 permeability, demonstrating that, like ion channels, gas channels can exhibit selectivity.
- Publication:
-
Proceedings of the National Academy of Science
- Pub Date:
- March 2009
- DOI:
- 10.1073/pnas.0813231106
- Bibcode:
- 2009PNAS..106.5406M
- Keywords:
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- gas channel;
- oocyte;
- permeability;
- signal peptide;
- surface pH measurement;
- Biological Sciences:Physiology