Role of lipids in hydrophobic gating and blocker affinity in BK channels.

Abstract

The large-conductance, calcium-activated potassium (BK) channel lacks the classic intracellular bundle-crossing gate observed in most other ion channels of the 6TM family. This observation, initially inferred from closed-pore accessibility experiments and recently corroborated by a cryo-EM structure of the non-conductive state, raises naturally a puzzling question: how can gating occur in absence of steric hindrance? To answer this question, we combined electrophysiology and molecular simulations and investigated the kinetics BK inhibition by two pore-blockers. The crux of our strategy was to leverage the state-dependent affinity of the binders to probe the physical properties of the pore. We thus combined kinetic modeling with a series of accurate free energy calculations to obtain a microscopic picture of the sequence of events taking place during the open-to-closed transition and giving rise to a nonconductive state. Our results highlight an unexpected role for annular lipids, which turn out to be an integral part of the gating machinery. Due to the presence of fenestrations, the closed-state pore is transiently occupied by some of the methyl groups from the lipid alkyl chains. This dynamic and intermittent occupancy triggers liquid-vapor oscillations and thus dewetting of the pore. Importantly, this lipid-mediated hydrophobic gating rationalizes several seemingly problematic experimental observations, including the state-dependent pore accessibility of blockers.