Hydrophobic gating in ion channels

Abstract

Biological ion channels are nanoscale transmembrane pores. When water and ions are enclosed within the narrow confines of a sub-nanometer hydrophobic pore, they exhibit behavior not evident from macroscopic descriptions. At this nanoscopic level, the unfavorable interaction between the lining of a hydrophobic pore and water may lead to stochastic liquid-vapor transitions. These transient vapor states are "dewetted", i.e. effectively devoid of water molecules within all or part of the pore, thus leading to an energetic barrier to ion conduction. This process, termed "hydrophobic gating", was first observed in molecular dynamics simulations of model nanopores, where the principles underlying hydrophobic gating (i.e., changes in diameter, polarity, or transmembrane voltage) have now been extensively validated. Computational, structural, and functional studies now indicate that biological ion channels may also exploit hydrophobic gating to regulate ion flow within their pores. Here we review the evidence for this process and propose that this unusual behavior of water represents an increasingly important element in understanding the relationship between ion channel structure and function.

Keywords: K2P channel; hydrophobic gating; ion channel; nanopore; potassium channel.

Figure 1. Principles of hydrophobic gating

(a) Cartoon representation of a cross-section through a model hydrophobic nanopore. Hydrophobic surfaces are shown in yellow, the membrane in green. In solution, these nanopores can switch stochastically between both wet and dry states via liquid-vapor oscillations within the pore. The dewetted vapor state presents an effective barrier to water and ion permeation. (b) These oscillations occur on the nanosecond timescale, and the stability of the wetted state is highly dependent upon pore diameter. (c) The probability of the pore being in the liquid or wetted state is not only dependent upon diameter, but also the hydrophobicity of atoms lining the pore. This was shown by progressively adding hydrophilic atoms to a model nanopore [3]. A fully hydrophilic pore remains fully occupied by water. However, a hydrophobic pore starts dewetting below 14 Å and becomes completely dewetted below ~8-10 Å. Semi-hydrophobic pores also exhibit similar dewetting below ~ 10 Å (dotted vertical line). (d) The process of hydrophobic gating has now been shown to be influenced by pore diameter, hydrophobicity and also changes in transmembrane voltage. This figure is adapted from results within references [2,3].

Figure 2. Hydrophobic gates and pores in biological ion channels

(a) Longitudinal sections through the centre of the pore lumen for several different ion channels. Carbon and sulphur atoms are colored yellow, and hydrophilic atoms red. The approximate position of the channels within the membrane is marked by dotted lines. The channels shown are: the closed pores of MscS (2OAU), MscL (2OAR) and GLIC (4NPQ). The positions of the hydrophobic gates are circled; in MscS this gate contains Leu105 and Leu109, in MscL Gly22 (Ala20 in 2OAR), and Ile-9’-Ile-16’ in GLIC. These pores are in marked contrast to gramicidin (1MAG) which is hydrophilic throughout the pore. (b) The inner pore of many K⁺ channels is also hydrophobic (circled). Shown are sections of KcsA (1K4C), Kv1.2 (2A79), MthK (3LDC) and TWIK-1/K2P1 (3UKM). The circled region of MthK contains Ala88 [49] whilst TWIK-1 contains Leu146 and Leu261 [61] (see also Fig. 3). Structures are colored and positioned as in (a).

Figure 3. Hydrophobic barrier in a K2P channel pore

(a) MD simulations of the TWIK-1 K2P potassium channel structure (3UKM) demonstrate that dewetting occurs deep within the inner pore thus creating an energetic barrier to ion permeation [61]. Shown are the average water densities within the inner pore during simulations of a wild-type and L146N mutant pore which disrupts this hydrophobic barrier. The transparent cyan surface is contoured at 0.50 of bulk water density, overlaid on a snapshot of the inner-pore at 100 ns. The side chains at position 146 are highlighted. The K+ ions at the S4 position are shown as purple spheres. (b) Averaged whole-cell currents for WT TWIK-1*, and L146N TWIK-1* mutant channels. Disruption of the hydrophobic barrier produces a large increase in channel activity. Hydrophobic gating may therefore contribute to the regulation of channels which do not possess a classical cytoplasmic bundle-crossing gate. This figure is adapted from results within reference [61].