Hydrophobic gating in bundle-crossing ion channels: A case study of TRPV4.

Abstract

Many transmembrane (TM) ion channels regulate ion permeation by forming bundle crossing of the pore-lining TM helices when deactivated. The resulting physical constriction is generally believed to serve as the “gate” and impose the major free energy barrier to ion permeation. On the other hand, many ion channels have been observed contain inner pores that are highly hydrophobic and can undergo spontaneous dewetting transitions. The resulting “vapor” region can block ion flow even in the absence of physical constriction. However, it is not clear whether such hydrophobic gating mechanism could be involved in channels with bundle-crossing in the deactivated state. Using atomistic simulations, we show that the human TRPV4 (hTRPV4) channel in the bundle-crossing closed state can readily undergoes hydrophobic dewetting transition and forms a vapor region in the lower pore region. Importantly, free energy analysis reveals that the major barrier for ion permeation is not located at the most constricted site, but within the vapor region. Furthermore, a single hydrophilic mutation I715N in the lower pore region significantly increases pore hydration without affecting the physical opening near bundle crossing; yet it largely removes the free energy barrier of ion permeation in hTRPV4, consistent with a previous experimental study showing that I715N leads to a constitutively conductive channel. Therefore, hTRPV4 is a hydrophobic gating channel despite the presence of bundle-crossing in the deactivated state. We anticipate that hydrophobic gating may play a key role in other bundle-crossing ion channels with hydrophobic inner pores.