Entry pathways for cations entering the hTPC2 SF. (A) Top view of the luminal entrance highlighting the acidic residues in each subunit that recruit cations. (B) Horizontal approach pathway for cations toward the SF; the C623/C623 disulfide bond is shown. (C) An overlay of 200 frames from a simulation at −70 mV shows the accumulation of Na⁺ (purple) and Ca²⁺ ions (gray) near residues D660 at the top of the SF; the channel cavity is occupied only by Na⁺ ions. (D) Location of the C623/C623 disulfide bond forming a “roof” above the SF entrance.