Kv channel S6 helix as a molecular switch: simulation studies

Ion channels form pores of nanoscopic dimensions in biological membranes and play a key role in the physiology of cells. The majority of ion channels are gated, i.e. they contain a molecular switch that allows a transition between a closed (functionally 'off') and open (functionally 'on') state. Comparison of crystal structures of potassium channels suggest that the gating mechanism of voltage-gated potassium (Kv) channels involves a key role for the pore-lining S6 helix. There is a conserved PVP sequence motif in the S6 helix. Molecular dynamics simulations are used here to explore the conformational dynamics of the S6 helix hinge in models of fragments of a Kv channel, namely an S5-P-S6 monomer and an (S5-P-S6)4 tetramer. The latter is a model of the complete pore-forming domain of a Kv channel. All models were simulated embedded in an octane slab (a simple membrane mimetic). The results of these simulations indicate that the PVP motif may form a molecular hinge, even when the S6 helix forms part of a more complex model. The conformational dynamics of S6 are modulated by the remainder of protein, but it remains flexible. These simulation results are compatible with a channel gating model in which S6 bends in the vicinity of the PVP motif in addition to the region around the conserved glycine (G466) that is N-terminal to the PVP motif. This model is supported by comparison of the Kv S6 models with the S6 helix of the bacterial KvAP channel crystal structure. Thus, K channel gating may depend on a complex nanoswitch with three rigid helical sections linked by two molecular hinges.