The two-pore channels (TPCs) are voltage-gated cation channels consisting of single polypeptides with two repeats of a canonical 6-transmembrane unit. TPCs are known to be regulated by various physiological signals such as membrane voltage and phosphoinositide (PI). The fourth helix in the second repeat (second S4) plays a major role in detecting membrane voltage, whereas the first repeat contains a PI binding site. Therefore, each of these stimuli is detected by a unique repeat to regulate the gating of the TPC central pore. How these various stimuli regulate the dynamic structural rearrangement of the TPC molecule remain unknown. Here, we found that PI binding to the first repeat in TPC3 regulates the movement of the distally located second S4 helix, showing that the PI-binding signal is not confined to the pore gate but also transmitted to the voltage sensor. Using voltage clamp fluorometry, measurement of gating charges, and Cys-accessibility analysis, we observed that PI binding significantly potentiates the voltage dependence of the movement of the second S4 helix. Notably, voltage clamp fluorometry analysis revealed that the voltage-dependent movement of the second S4 helix occurred in two phases, of which the second phase corresponds to the transfer of the gating charges. This movement was observed in the voltage range where gate-opening occurs and was potentiated by PI. In conclusion, this regulation of the second S4 helix by PI indicates a tight inter-repeat coupling within TPC3, a feature which might be conserved among TPC family members to integrate various physiological signals.
Keywords: TPC; dynamic structure; electrophysiology; gating; ligand-binding protein; phosphoinositide; sodium channel; two-pore channel; voltage sensor; voltage-gated cation channel.
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