Two-pore channels (TPCs) are eukaryotic intracellular voltage-gated and ligand gated cation selective ion channels ^[1]. There are two known paralogs in the human genome, TPC1s and TPC2s ^[2]. Two-pore channels (TPCs) belong to the voltage-gated ion channel superfamily. Both, TPC1 and TPC2, are voltage- and ligand gated cation channels that are permeable to Na⁺, Ca²⁺, and H^{+ [3]}. At the mouth of the TPC pore, there are four amino acid residues with negative charges that can interact with ions that pass through. This site is too wide to select ions. Below the group of negative charges is the selectivity filter which is largely hydrophobic. There are two non-identical Shaker-like pore forming subunits. Subunit 1 consists of voltage sensing domain 1 (VSD1) and subunit 2 consists of the voltage sensing domain 2 (VSD2). The two subunit domains are separated by an EF-hand domain that has a calcium ion binding motif. This binding motif can facilitate channel activation by cytosolic calcium ions. Each of the two subunits are built from 12 transmembrane helices. The two central pore domains are combined from the voltage sensing domains, VSD1 and VSD2. Both the N-terminal domain (NTD) and C-terminal domain (CTD) extend out on the cytosolic side, along with the EF-hand domain in the center that extends into the cytoplasm. The EF-hand domain extends into the cytosol, positioned between VSD1 and VSD2, where it can be activated by cytosolic calcium ^[1]. The VSD2 domain is voltage sensitive active and can be inhibited by calcium in the lumen ^[4]. This is a conformation change from the activation state to the inactive state. Two rings of hydrophobic residues seal the pore cavity from the cytoplasm; this results in forming the pore gate. Voltage sensors, selectivity filter, and the gate work together in a coordinated manner to open and close TPCs for regulation of ion conductance. TPC3 is also found on the plasma membrane and forms a voltage-activated, non-inactivating Na⁺ channel ^[5].

Residue Asp181 (DIII) (Red Sphere in the 3D structure viewer) forms two H-bonds with the guanidine group of TTX in the bound complex at equilibrium of TPC1 and TPC2 ^[6].

Verapamil inhibited outward and inward IhTPC2 with different dose-dependencies under luminal and cytoplasmic symmetric Na⁺ (luminal pH 7.4) ^[7].

TPCs are in fact sodium-selective channels activated by PI(3,5)P2 ^[7].