Lipid gating of BK channels and mechanism of activation by negatively charged lipids

Abstract

BK channels are a class of K+ channels that possess an unusually high conductance and are synergistically gated by intracellular Ca2+ and voltage. Despite the significant array of experimental and computational data, many aspects of their function and dynamics remain unclear - such as how ion permeation is halted in the closed state of the channel. Available CryoEM structures obtained in deactivating conditions capture the channel with a wide, unobstructed pore, in contrast to e.g. a helix bundle crossing observed in some K+ channels. Several hypotheses of BK closure were proposed, including selectivity filter and hydrophobic gating. In this work, we expand on the model of hydrophobic gating and focus on the role of lipids. Using atomistic molecular dynamics simulations with applied voltage, we directly investigate the ability of the full length BK channel in various CryoEM states to permeate ions, and propose lipid entrance into the pore - either with lipid tails or entire lipid molecules - through the membrane-facing fenestrations to be a critical determinant of BK conductivity. Furthermore, we elucidate the mechanism of BK activation by negatively charged lipids via a combination of atomistic & coarse-grained simulations, and suggest that they act by a multi-modal mechanism, which encompasses lipid entry reduction, increase of the K+ occupancy of the pore, and stabilization of the channels open-state structure - in broad agreement with experimental data.