Direct knock-on of desolvated ions governs strict ion selectivity in K+ channels

The seeming contradiction that K+ channels conduct K+ ions at maximal throughput rates while not permeating slightly smaller Na+ ions has perplexed scientists for decades. Although numerous models have addressed selective permeation in K+ channels, the combination of conduction efficiency and ion selectivity has not yet been linked through a unified functional model. Here, we investigate the mechanism of ion selectivity through atomistic simulations totalling more than 400 μs in length, which include over 7,000 permeation events. Together with free-energy calculations, our simulations show that both rapid permeation of K+ and ion selectivity are ultimately based on a single principle: the direct knock-on of completely desolvated ions in the channels' selectivity filter. Herein, the strong interactions between multiple 'naked' ions in the four filter binding sites give rise to a natural exclusion of any competing ions. Our results are in excellent agreement with experimental selectivity data, measured ion interaction energies and recent two-dimensional infrared spectra of filter ion configurations

Structural mechanism of voltage-dependent gating in an isolated voltage-sensing domain

The voltage-sensing domain (VSD) is a common scaffold responsible for the transduction of transmembrane electric fields into protein motion. They play an essential role in the generation and propagation of cellular signals driven by voltage gated ion channels, voltage sensitive enzymes and proton channels. All available VSD structures are thought to represent the activated conformation of the sensor due to the overall structural similarities and the mid-point of the voltage dependence of activation curves. Yet, in the absence of a resting state structure, the mechanistic details of voltage sensing remain controversial. The voltage dependence of the VSD from Ci-VSP (Ci-VSD) is dramatically right shifted, so that at 0 mV it presumably populates the putative resting state. We have determined crystal structures of the Ci-VSP voltage sensor in both active (Up) and resting (Down) conformations, between which the S4 undergoes a ∼5 Å displacement along its main axis with an accompanying 55-90o rotation resembling the basic helix-screw mechanism of gating. In the process, the gating charges change position relative to a “hydrophobic gasket” that electrically separates intra and extracellular compartments. This movement is stabilized by an exchange in countercharge partners in helices S1 and S3, for an estimated net charge movement of ∼1 eo. EPR spectroscopic measurements confirm the limited nature of S4 movement in a membrane environment. These results provide an explicit mechanism of voltage sensing in diverse voltage dependent cellular responses

Structural mechanism of voltage-dependent gating in an isolated voltage-sensing domain

The transduction of transmembrane electric fields into protein motion has an essential role in the generation and propagation of cellular signals. Voltage-sensing domains (VSDs) carry out these functions through reorientations of positive charges in the S4 helix. Here, we determined crystal structures of the Ciona intestinalis VSD (Ci-VSD) in putatively active and resting conformations. S4 undergoes an ~5-Å displacement along its main axis, accompanied by an ~60° rotation. This movement is stabilized by an exchange in countercharge partners in helices S1 and S3 that generates an estimated net charge transfer of ~1 eo. Gating charges move relative to a \u27\u27hydrophobic gasket\u27 that electrically divides intra- and extracellular compartments. EPR spectroscopy confirms the limited nature of S4 movement in a membrane environment. These results provide an explicit mechanism for voltage sensing and set the basis for electromechanical coupling in voltage-dependent enzymes and ion channels