Structural insights into the voltage and phospholipid activation of mammalian TPC1 channel

Organellar two-pore channels (TPCs) function as a homodimer with each subunit containing two homologous Shaker-like 6-TM repeats(1). They belong to the voltage-gated ion channel superfamily(2) and are ubiquitously expressed in animals and plants(3,4). Mammalian TPC1 and TPC2 are localized to the endolysosomal membrane and play critical roles in regulating the physiological functions of these acidic organelles(5–7). Here we present the cryo-EM structures of mouse TPC1 (MmTPC1), a voltage-dependent, phosphatidylinositol 3,5-bisphosphate (PtdIns(3,5)P(2)) activated Na(+) selective channel, in both the apo closed and ligand-bound open states which, combined with functional analysis, provide comprehensive structural insights into the selectivity and gating mechanisms of mammalian TPC channels. The channel has a coin slot-shaped ion pathway in the filter that defines the selectivity of mammalian TPCs. Only the voltage sensing domain from the second 6-TM domain confers voltage dependence to MmTPC1. Endolysosome-specific PtdIns(3,5)P(2) binds to the first 6-TM domain and activates the channel under depolarizing membrane potential. Structural comparison between the apo and PtdIns(3,5)P(2)-bound structures elucidates the interplay between voltage and ligand in channel activation. In light of the emerging importance of phosphoinositide regulation of ion channels, the MmTPC1 structures exemplify the lipid binding and regulation in a 6-TM voltage-gated channel.