A cooperative knock-on mechanism underpins Ca(2+)-selective cation permeation in TRPV channels

The selective exchange of ions across cellular membranes is a vital biological process. Ca(2+)-mediated signaling is implicated in a broad array of physiological processes in cells, while elevated intracellular concentrations of Ca(2+) are cytotoxic. Due to the significance of this cation, strict Ca(2+) concentration gradients are maintained across the plasma and organelle membranes. Therefore, Ca(2+) signaling relies on permeation through selective ion channels that control the flux of Ca(2+) ions. A key family of Ca(2+)-permeable membrane channels is the polymodal signal-detecting transient receptor potential (TRP) ion channels. TRP channels are activated by a wide variety of cues including temperature, small molecules, transmembrane voltage, and mechanical stimuli. While most members of this family permeate a broad range of cations non-selectively, TRPV5 and TRPV6 are unique due to their strong Ca(2+) selectivity. Here, we address the question of how some members of the TRPV subfamily show a high degree of Ca(2+) selectivity while others conduct a wider spectrum of cations. We present results from all-atom molecular dynamics simulations of ion permeation through two Ca(2+)-selective and two non-selective TRPV channels. Using a new method to quantify permeation cooperativity based on mutual information, we show that Ca(2+)-selective TRPV channel permeation occurs by a three-binding site knock-on mechanism, whereas a two-binding site knock-on mechanism is observed in non-selective TRPV channels. Each of the ion binding sites involved displayed greater affinity for Ca(2+) over Na(+). As such, our results suggest that coupling to an extra binding site in the Ca(2+)-selective TRPV channels underpins their increased selectivity for Ca(2+) over Na(+) ions. Furthermore, analysis of all available TRPV channel structures shows that the selectivity filter entrance region is wider for the non-selective TRPV channels, slightly destabilizing ion binding at this site, which is likely to underlie mechanistic decoupling.