Structure and Dynamics of the Mthk K+ Channel Selectivity Filter during Gating

Potassium channel inactivation is essential for regulating the duration and frequency of neuronal action potentials, yet the associated structural changes involving the selectivity filter are not well understood. We have investigated the influence of ionic conditions on the structure and dynamics of the MthK K+ channel pore using X-ray crystallography and molecular dynamics (MD) simulations. Crystals of the MthK pore were grown in a range of [K+]. Electron density maps showed that as [K+] is lowered, ion binding to the S2 site within the selectivity filter is eliminated without altering the conductive conformation. In order to investigate how ion binding may influence the behavior of the selectivity filter in the absence of crystal lattice constraints, we performed MD simulations of the MthK pore in different ionic conditions. We found that K+ binding exerts a significant influence on the structural dynamics of the selectivity filter. With two K+ ions bound at sites S2/S4, the filter carbonyl groups remained directed towards the conduction pathway with only occasional, unstable outward rotations. Binding of two water molecules behind the selectivity filter, corresponding to those observed in the crystal structures, further stabilized the conductive carbonyl positions. In contrast, with no K+ ions bound, carbonyl rotations, perturbed protein interactions behind the filter, and a constricted pore were observed, similar to the collapsed low [K+] KcsA crystal structure. This likely non-conductive MthK conformation is of similar energetic stability with the conductive conformation during these simulations, which may explain why crystallization of the conductive conformation is possible in low [K+]. From these results we conclude that closure of the MthK selectivity filter is a viable gating mechanism and that interactions between protein, ions, and water govern the dynamic behavior of the filter.