Mutation of a single amino acid converts the human water channel aquaporin 5 into an anion channel

Aquaporin 6 (AQP6) is unique among mammalian AQPs in being an anion channel with negligible water permeability. However, the point mutation Asn60Gly converts AQP6 from an anion channel into a water channel. In the present study of human AQP5, we mutated Leu51 (corresponding to residue 61 in AQP6), the side chain of which faces the central pore. We evaluated function in Xenopus oocytes by two-electrode voltage clamp, video measurements of osmotic H 2 O permeability ( P f ), microelectrode measurements of surface pH (pH S ) to assess CO 2 permeability, and surface biotinylation. We found that AQP5-L51R does not exhibit the H 2 O or CO 2 permeability of the wild-type protein but instead has a novel p-chloromercuribenzene sulfonate (pCMBS)-sensitive current. The double mutant AQP5-L51R/C182S renders the conductance insensitive to pCMBS, demonstrating that the current is intrinsic to AQP5. AQP5-L51R has the anion permeability sequence I − > NO 3 − ≅ NO 2 − > Br − > Cl − > HCO 3 − > gluconate. Of the other L51 mutants, L51T (polar uncharged) and L51V (nonpolar) retain H 2 O and CO 2 permeability and do not exhibit anion conductance. L51D and L51E (negatively charged) have no H 2 O or CO 2 permeability. L51K (positively charged) has an intermediate H 2 O and CO 2 permeability and anion conductance. L51H is unusual in having a relatively low CO 2 permeability and anion conductance, but a moderate P f . Thus, positively charged mutations of L51 can convert AQP5 from a H 2 O/CO 2 channel into an anion channel. However, the paradoxical effect of L51H is consistent with the hypothesis that CO 2 , in part, takes a pathway different from H 2 O through AQP5.