Principles of conduction and hydrophobic gating in K ⁺ channels

Abstract

We present the first atomic-resolution observations of permeation and gating in a K þ channel, based on molecular dynamics simulations of the Kv1.2 pore domain. Analysis of hundreds of simulated permeation events revealed a detailed conduction mechanism, resembling the Hodgkin-Keynes "knock-on" model, in which translocation of two selectivity filter-bound ions is driven by a third ion; formation of this knock-on intermediate is rate determining. In addition, at reverse or zero voltages, we observed pore closure by a novel "hydrophobic gating" mechanism: A dewetting transition of the hydrophobic pore cavity-fastest when K þ was not bound in selectivity filter sites nearest the cavity-caused the open, conducting pore to collapse into a closed, nonconducting conformation. Such pore closure corroborates the idea that voltage sensors can act to prevent pore collapse into the intrinsically more stable, closed conformation, and it further suggests that molecularscale dewetting facilitates a specific biological function: K þ channel gating. Existing experimental data support our hypothesis that hydrophobic gating may be a fundamental principle underlying the gating of voltage-sensitive K þ channels. We suggest that hydrophobic gating explains, in part, why diverse ion channels conserve hydrophobic pore cavities, and we speculate that modulation of cavity hydration could enable structural determination of both open and closed channels.ion channel | ion permeation | membrane | electrophysiology | dewetting E lectrophysiological, structural, and computational studies have provided a wealth of insight into the mechanisms underlying conductance and gating-how ionic current is switched on and off-by ion channels, most notably potassium (K þ ) channels. Key questions nonetheless remain about how channels perform these functions (1-8). X-ray structures and experimental measurements of ion binding and conduction (3,(8)(9)(10), as well as computer simulations (6, 7), have suggested that K þ ions and copermeating water molecules move in concert, single file, through the ion-selective region of K þ channels-the so-called selectivity filter (SF; Fig. 1A). Gating involves significant protein conformational changes as the pore domain opens and closes (5, 11).The widely accepted view of permeation across K þ channels follows Hodgkin and Keynes's knock-on model (12), in which conduction is driven by an incoming ion that "knocks on" ion(s) already bound within the SF (7,8,12). Attempts to define further the mechanistic details of K þ ion permeation from ionic current measurements have confronted a major challenge, however, in that recording the positions of individual ions as they permeate the channel has not been possible. Thus, a conduction mechanism derived from the dynamics of the individual ions as they permeate a selective ion channel has remained elusive.Gating of voltage-sensitive channels, i.e., pore opening and closing, involves voltage-sensing domain motions that trigger conformational changes of the S4-S5 linker and the S5 an...

Grafting voltage and pharmacological sensitivity in potassium channels

“…Interestingly, the studies in some Kv channels suggest that the intrinsically most stable state of the Kv pore is the closed state [1,52,53]. The VSDs thus do not need to actively push the S4-S5 linker down to close the pore because the pore cavity is highly hydrophobic [52]. Consistently, a molecular simulation further showed that the limited motion of a single VSD towards the resting state is sufficient to close the pore [1].…”

“…Some previous studies supported that the Kv pore is intrinsically more stable when closed, which distinguishes it from constitutively open K2P channels [52,53]. Notably, the study on KvLm channels revealed that pore only module can populate at open state, suggesting that the pore of Kv channels may also reside in between closed-activated and open state [16].…”

“…In prokaryotic KvLm channels, four VSDs were required for full closure [16]. Interestingly, the studies in some Kv channels suggest that the intrinsically most stable state of the Kv pore is the closed state [1,52,53]. The VSDs thus do not need to actively push the S4-S5 linker down to close the pore because the pore cavity is highly hydrophobic [52].…”

Counting Ion and Water Molecules in a Streaming File through the Open-Filter Structure of the K Channel

“…Among these studies, the "knock-on" mechanism still dominates as an explanatory account of the permeation processes (Bernèche and Roux, 2001;Jensen et al, 2010). One may define the knock-on mechanism as an electrostatic repulsion taking place among the ions permeating in and out of the pore in a manner that drives ion flux efficiently (Hille, 2001).…”

“…Recently, computer simulations have suggested other arrays of ions and water molecules in the selectivity filter (Khalili-Araghi et al, 2006;Furini and Domene, 2009;Jensen et al, 2010;Nelson, 2011). Two issues were raised: may ions be located adjacently, and whether there are five sites rather than four sites in the selectivity filter.…”