Hydrophobic gating in BK channels

Abstract

The gating mechanism of transmembrane ion channels is crucial for understanding how these proteins control ion flow across membranes in various physiological processes. Big potassium (BK) channels are particularly interesting with large single-channel conductance and dual regulation by membrane voltage and intracellular Ca2+. Recent atomistic structures of BK channels failed to identify structural features that could physically block the ion flow in the closed state. Here, we show that gating of BK channels does not seem to require a physical gate. Instead, changes in the pore shape and surface hydrophobicity in the Ca2+-free state allow the channel to readily undergo hydrophobic dewetting transitions, giving rise to a large free energy barrier for K+ permeation. Importantly, the dry pore remains physically open and is readily accessible to quaternary ammonium channel blockers. The hydrophobic gating mechanism is also consistent with scanning mutagenesis studies showing that modulation of pore hydrophobicity is correlated with activation properties.

Structure-based identification and characterisation of novel inhibitors of KNa1.1 potassium channels, a stratified target for KCNT1-related epilepsy

“…Whilst the cryo-EM structures indicate that activation by sodium involves an expansion of the intracellular pore vestibule (Hite & MacKinnon, 2017), functional experiments with this and the closely-related KNa1.2 and KCa1.1 (BKCa) channels point to the selectivity filter and proximal hydrophobic residues, rather than an S6 helix bundle, as the location of the channel gate (Garg et al, 2013;Suzuki et al, 2016;Giese et al, 2017;Jia et al, 2018). This means that the inhibitors described here block at the channel gate and this should be a mode of inhibition that is efficacious with virtually all clinical gain-of-function mutations, independent of the mechanism by which increased open probability if achieved, rather than an inhibitor that binds to modulatory sites.…”

Nonspecific membrane interactions can modulate BK channel activation

“…We analyzed the optimal and suboptimal pathways of coupling between I323 in the S6 helices, where substantial conformational change occurs during the gating event (14), and critical metal binding residues, including D895 and R514 in the RCK2 and RCK1 Ca 2+ binding site, respectively, and E374 in the Mg 2+ binding site residing in the CTD. The results reveal that the main pathways of communication from Ca 2+ and Mg 2+ binding sites to the PGD, go through the C-linker for all four chains ( Figure 2c-d and S9).…”

“…Interestingly though, in the metal-free state the main path from the RCK2 Ca 2+ binding site to PGD goes from the neighboring monomer in every other chain ( Figure 2c-d). This can be attributed to much tighter S6 helix packing in the metal-free state (14), and, combined with the domain swapped arrangement of BK tetramers, may help enforce cooperative gating response upon metal binding.…”

“…Atomistic simulations later revealed that, due to the movement of pore-lining S6 helices (Figure 1b), the BK pore cavity become narrower, more elongated, and crucially, more hydrophobic in the metal-free state (i.e., without bound Ca 2+ and Mg 2+ ; presumably the closed state) (14). As such, the pore can readily undergo hydrophobic dewetting transition to give rise to a vapor barrier that prevents ion permeation (14). Recognition of hydrophobic gating in BK channels provides a mechanistic basis for further understanding how the C-linker may mediate the sensor-pore coupling.…”

CHAP: A versatile tool for the structural and functional annotation of ion channel pores

“…The first example of a hydrophobic gate in an ion channel was found in an early cryo-EM structure of the nicotinic acetylcholine receptor, which represents a functionally closed state without being sterically occluded (Beckstein and Sansom, 2006) . Further examples of hydrophobic gates and barriers include the MscL mechanosensitive channel (Spronk et al, 2006) , the cation-selective bacterial channel GLIC (Zhu and Hummer, 2012) , the BK and TWIK-1 potassium channels (Jia et al, 2018;Aryal et al, 2014) , the BEST1 chloride channel (Rao et al, 2017) , and the CorA magnesium channel (Neale et al, 2015) . Thus hydrophobic gating clearly represents an important mechanism for regulating ion channel permeation and needs to be taken into consideration when assessing the functional status of any new ion channel structure.…”