The Inactivated Bacterial Potassium Channel Conducts Water

Abstract

The potassium channel KcsA was reported to conduct water at an extraordinarily high rate, which implied a 20-fold increase in mobility of water molecules inside the channel as compared to bulk water [1]. A conformational state of the channel that is permeable to water, but closed to ions would have resulted in an underestimation of channel number, and thus, in an overestimation of water mobility. Here we tested whether the recently described C-type inactivated state of KcsA may represent such a conformation. Therefore, we reconstituted a KcsA mutant in which the inactivation was suppressed with an E71A mutation in the pore helix. KcsA labeling with a fluorescent dye allowed determination of the number of reconstituted channels per vesicle by fluorescence correlation spectroscopy. It was equal to the number of open channels, as revealed by fusing part of these vesicles with planar lipid bilayers. We subjected the remaining part of these vesicles to stopped flow experiments. This approach allowed unitary water permeability calculation of the non-inactivating KcsA variant. It is roughly an order of magnitude smaller than the single channel permeability of the wild type channel, suggesting that the open but inactivated potassium channel is a selective water channel.[1] Saparov, S. M., and P. Pohl. 2004. Beyond the diffusion limit: Water flow through the empty bacterial potassium channel. Proc. Natl. Acad. Sci. U. S. A. 101: 4805-4809.