Structural basis for activity of TRIC counter-ion channels in calcium release

Trimeric intracellular cation (TRIC) channels are thought to provide counter-ion currents that facilitate the active release of Ca ²⁺ from intracellular stores. TRIC activity is controlled by voltage and Ca ² + modulation, but underlying mechanisms have remained unknown. Here we describe high-resolution crystal structures of vertebrate TRIC-A and TRIC-B channels, both in Ca ²⁺ -bound and Ca ²⁺ -free states, and we analyze conductance properties in structure-inspired mutagenesis experiments. The TRIC channels are symmetric trimers, wherein we find a pore in each protomer that is gated by a highly conserved lysine residue. In the resting state, Ca ² + binding at the luminal surface of TRIC-A, on its threefold axis, stabilizes lysine blockage of the pores. During active Ca ²⁺ release, luminal Ca ²⁺ depletion removes inhibition to permit the lysine-bearing and voltage-sensing helix to move in response to consequent membrane hyperpolarization. Diacylglycerol is found at interprotomer interfaces, suggesting a role in metabolic control.