Fast osmotic water flow through the bacterial potassium channel (KcsA) occurs only when all ions are rinsed out of the filter. This situation is illustrated in the figure where all binding sites are occupied by water molecules (red). The potassium ion (yellow) has been placed into the cavity which in contrast to the selectivity filter is wide enough to allow different molecules to pass each other (Fig.3 from Biol. Chem. 2004, 385 : 921–926).
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