Ion Permeation Mechanism in Epithelial Calcium Channel TRVP6

Abstract

Calcium is the most abundant metal in the human body that plays vital roles as a cellular electrolyte as well as the smallest and most frequently used signaling molecule. Calcium uptake in epithelial tissues is mediated by tetrameric calcium-selective transient receptor potential (TRP) channels TRPV6 that are implicated in a variety of human diseases, including numerous forms of cancer. We used TRPV6 crystal structures as templates for molecular dynamics simulations to identify ion binding sites and to study the permeation mechanism of calcium and other ions through TRPV6 channels. We found that at low Ca2+ concentrations, a single calcium ion binds at the selectivity filter narrow constriction formed by aspartates D541 and allows Na+ permeation. In the presence of ions, no water binds to or crosses the pore constriction. At high Ca2+ concentrations, calcium permeates the pore according to the knock-off mechanism that includes formation of a short-lived transition state with three calcium ions bound near D541. For Ba2+, the transition state lives longer and the knock-off permeation occurs slower. Gd3+ binds at D541 tightly, blocks the channel and prevents Na+ from permeating the pore. Our results provide structural foundations for understanding permeation and block in tetrameric calcium-selective ion channels.

Cell-autonomous regulation of epithelial cell quiescence by calcium channel Trpv6

“…1F). D542 in mammalian TRPV6 occupies a critical position in the ion pore region and mutation of this residue abolishes its Ca 2+ permeability (McGoldrick et al, 2018;Sakipov, Sobolevsky, & Kurnikova, 2018). This residue is conserved in zebrafish Trpv6 at position 539 (Fig.…”

Structure of the ancestral TRPY1 channel fromSaccharomyces cerevisiaereveals mechanisms of modulation by lipids and calcium

“…The membrane patch was generated using the Membrane Builder function on the CHARMM-GUI server. The transmembrane region was placed based on hydrophobic regions of the core helices, proximity of tryptophan residues to lipid head groups, and previous simulations performed on TRP channels 70,71 . Then, the solvate plugin in VMD 72 was used to add TIP3P water molecules to the system, and the autoionize plugin was used to neutralize the system and add KCl to a final concentration of 150 mM.…”

Structural mechanisms of transient receptor potential ion channels

“…Ion selectivity of TRP channels is perhaps defined by interaction energy of competing permeable cations with dynamic selectivity filters, which is beyond what can be gleaned from only inspecting the existing static snapshots of TRP channel structures. Further experiments, such as molecular dynamic studies performed for TRPV1 (Jorgensen et al, 2016) and TRPV6 (Sakipov et al, 2018), are needed to fill this gap of knowledge. Nevertheless, as discussed below, the selectivity filters of TRP channels do conform to the same basic design principles that have emerged from structural analyses of K + (Doyle et al, 1998), Na v (Payandeh et al, 2011; Zhang et al, 2012; Shen et al, 2017; Yan et al, 2017; Pan et al, 2018; Jiang et al, 2019), and Ca v channels (Tang et al, 2014; Wu et al, 2016).…”

“…Multiple closely spaced ion binding sites were initially observed in KcsA crystal structures (Doyle et al, 1998), and later in voltage-gated Na + and Ca 2+ as well (Payandeh et al, 2011; Tang et al, 2014), an arrangement that appears to support a “knock-off” mechanism whereby repulsion between adjacent ions would overcome the attractive interactions between ions and the channel protein so as to achieve rapid rates of ion conduction that, in some cases, can approach the diffusion limit (Almers and McCleskey, 1984; Hess and Tsien, 1984; Neyton and Miller, 1988). As might have been expected from their close kinship to VGICs, many TRP channels also likely consist of multiple ion binding sites within the selectivity filter, indicating that such a “knock-off” mechanism of ion permeation may also be pervasive in the TRP channel superfamily (Jorgensen et al, 2016; Saotome et al, 2016; Sakipov et al, 2018). Surprisingly, only a single ion-binding site was identified in the current TRPV4 crystal structures (Deng et al, 2018).…”