A single‐amino acid mutation converts human aquaporin 5 to an anion channel

Abstract

Previous studies showed that one amino acid substitution N60G converts AQP6 from an anion to a water channel. Reverse mutation of G to N in AQP0, AQP1 and AQP2 reduced osmotic water permeability (Pf) but did not induce current. In the present study, we made single‐point mutations of AQP5 at L51, a residue that faces the central pore of AQP5, and one residue downstream from the N60 equivalent in AQP6. We evaluated function by two‐electrode voltage clamp, measurements of Pf, surface biotinylation, and extracellular surface pH (to assess CO2 permeability). We found that, whereas L51R has no H2O or CO2 permeability, it produces a current that can be blocked by the mercury derivative pCMBS. Double mutant L51R/C182S has currents that are about half those of L51R, but is now insensitive to pCMBS. L51R showed an anion permeability sequence of I− > NO3− > NO2− > Br− > Cl−. Furthermore, we made several other L51 mutants, and found that L51T (polar uncharged) and L51V (nonpolar) retain H2O and CO2 permeability and do not exhibit anion conductance; L51D and L51E (negatively charged) have no H2O or CO2 permeability, L51K and L51H (positively charged) have decreased H2O and CO2 permeability but an anion conductance. All the mutations are expressed on the oocyte plasma membrane. In conclusion, mutations of L51 can convert AQP5 to an anion channel or reduce Pf or reduce CO2 permeability. Funded by AHA 11POST7670014 & ONR N00014‐08‐10532.