Filter gate closure inhibits ion but not water transport through potassium channels

Full text: Conformational changes of the selectivity filter may block ion flow through K+-channels. Here we show that, after entering such an electrically silent conformation, K+ channels remain permeable to water. Using fluorescently labeled, constitutively open but either C-type inactivating or non-inactivating mutants of the bacterial K+ channel KcsA, we determined the number of reconstituted channels per vesicle and their unitary water permeability by fluorescence correlation spectroscopy and stopped-flow experiments, respectively. Water flux inhibition by K+ indicates a dissociation constant of ∼300 mM for singly occupied channels. Thus, transport does not require electrostatic destabilization by entry of a second ion into the filter. (author)