Understanding polymodal gating of a lysosomal ion channel

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Selective ion channels open to allow the flow of a single type of ion whereas non-selective ion channels allow multiple ions through. Ion selectivity is a defining feature of a given ion channel and is generally considered a fixed entity. But the lysosomal ion channel TPC2 has been shown recently to defy dogma displaying completely different ion selectivity profiles when activated by its endogenous activators, NAADP, a calcium mobilising messenger and PI(3,5)P2, an endo-lysosomal lipid. This highly unusual behaviour needs to be explained mechanistically as it contradicts the text-book view.

Armed with new NAADP and PI(3,5)P2 mimetics, our pilot high-resolution single channel recordings show that ion selection and gating profiles are linked in an agonist-specific manner. PI(3,5)P2 binds TPC2 directly but NAADP is thought to bind to associated NAADP-binding proteins that have only very recently been identified. Despite such segregated action, further pilot studies identify a key region of TPC2 that appears to be responsible for activation by both endogenous cues. This leads us to hypothesize gating and ion selection by TPC2 are coupled through distinct but converging agonist-specific mechanisms.

Our aims are to i) establish the behavior of single agonist-activated TPC2 channels and define ii) the dynamics of agonist activation and iii) the coupling of TPC2 channels to NAADP receptors. We will do so through an interdisciplinary approach comprising electrophysiology, molecular dynamics simulations and molecular cell biology brought about by the collaboration.

Successful outcome of this project will provide fundamental insight into how an ion channel morphs on demand.