In silico characterization of the gating and selectivity mechanism of the human TPC2 cation channel

Two pore channels (TPCs) are two-fold symmetric endo-lysosomal cation channels forming important drug targets especially for antiviral drugs. They are activated by calcium, ligand binding, and membrane voltage, and to date, are the only ion channels shown to alter their ion selectivity depending on the type of bound ligand. However, despite their importance, ligand activation of TPCs and the molecular mechanisms underlying their ion selectivity are still poorly understood. Here, we set out to elucidate the mechanistic basis for the ion selectivity of human TPC2 (hTPC2) and the molecular mechanism of ligand-induced channel activation by the lipid PI(3,5)P2. We performed all-atom in silico electrophysiology simulations to study Na+ and Ca2+ permeation across full-length hTPC2 on the timescale of ion conduction, and to investigate the conformational changes induced by the presence or absence of bound PI(3,5)P2. Our findings reveal that hTPC2 adopts distinct structures depending on the presence of PI(3,5)P2 and elucidate the allosteric transition pathways between these structures. Additionally, we examined the permeation mechanism, solvation states, and binding sites of ions during ion permeation through the pore. The results of our simulations explain the experimental observation that hTPC2 is more selective for Na+ over Ca2+ ions in the presence of PI(3,5)P2 via a multi-layer selectivity mechanism. Importantly, mutations in the selectivity filter region of hTPC2 maintain cation conduction but change the ion selectivity of hTPC2 drastically.