Gating of BK Channels: Roles of the C-LINKER and a Potential Hydrophobic Gate

Abstract

Large conductance Ca2+-activated K+ channels (BK channel) are synergistically and independently activated by Ca2+, Mg2+ and membrane depolarization. However, the structure basis of the gating of the channel is still unclear. A “passive spring model” was previously proposed, which suggests that pore opening is induced by mechanical pulling of the pore domain by the Ca2+ sensing domain (RCK domain) via the linker between RCK and the pore (C-linker). However, recently solved EM structures of BK channel reveal that the C-linker has similar structure and position between metal-bound and metal-free states and is thus unlikely the spring. Here, we will report our recent results from atomistic simulation and experiment that together support a more active role of the C-linker in pore-gate coupling of BK channels. In addition, we will also discuss emerging evidence that supports the existence of a hydrophobic gate in the BK-channel. Specifically, the relative small rotation of S6 helix and narrowing of the cytosolic opening as observed in the metal-free EM structure likely represent initial steps towards gate closing. Atomistic simulations reveal that rotation of S6 helix exposes the hydrophobic face and allows further collapsing of the pore. The subsequent dewetting transition near F315-P320 gives rise to a significant barrier for K+ diffusion. Furthermore, F315 side chains can rotate in and out of the pore region, providing transient access to largely hydrophobic blockers such as MTSEA even in the close state of BK channel.