Transport of water and glycerol in aquaporin 3 is gated by H+

Aquaporins (AQPs) were expressed in Xenopus laevis oocytes in order to stud y the effects of external pH and solute structure on permeabilities. For AQ P3 the osmotic water permeability, L-p, was abolished at acid pH values wit h a pK of 6.4 and a Hill coefficient of 3. The L-p values of AQP0, AQP1, AQ P2, AQP4, and AQP5 were independent of pH, For AQP3 the glycerol permeabili ty P-Gl, obtained from [C-14]glycerol uptake, was abolished at acid pH valu es with a pK of 6.1 and a Hill coefficient of 6. Consequently, AQP3 acts as a glycerol and water channel at physiological pH, but predominantly as a g lycerol channel at pH values around 6.1. The pH effects were reversible. Th e interactions between fluxes of water and straight chain polyols were infe rred from reflection coefficients to). For AQP3, water and glycerol interac ted by competing for titratable site(s): sigma(Gl) was 0.15 at neutral pH b ut doubled at pH 6.4. The sigma values were smaller for polyols in which th e -OH groups were free to form hydrogen bonds. The activation energy for th e transport processes was around 5 kcal mol(-1). We suggest that water and polyols permeate AQP3 by forming successive hydrogen bonds with titratable sites.