The structure of the bacterial potassium channel, KcsA, corresponds to the channel in a closed state. Two lines of evidence suggest that the channel must widen its intracellular mouth when in an open state: 1) internal block by a series of tetraalkylammonium ions and 2) spin labeling experiments. Thus it is known that the protein moves in this region, but it is unclear by how much and the mechanisms that are involved. To address this issue we have applied a novel approach to generate plausibl...