Fig. 2.
Gating (A) and block (B) of water transport. (A) The osmotic water permeability (46 μm/s) was determined (13) from spatially resolved measurements of Mg2+ dilution in response to a 1 M urea gradient at pH 4. Augmenting pH to 7 closed the KcsA channels, as indicated by the increase of the electrical membrane resistance (measured at a frequency of 1 kHz), R, from 8 × 105 to 108 Ω. Simultaneously, channel closing decreased Pf to 25 μm/s. (B) Channel block by 15 mM Na+ changed R and Pf from 5 × 106 to 5 × 108 Ω and from 35 to 24 μm/s, respectively.