Control of P2X₂ Channel Permeability by the Cytosolic Domain

ATP-gated P2X channels are the simplest of the three families of transmitter-gated ion channels. Some P2X channels display a time- and activation-dependent change in permeability as they undergo the transition from the relatively Na⁺-selective I₁ state to the I₂ state, which is also permeable to organic cations. We report that the previously reported permeability change of rat P2X₂ (rP2X₂) channels does not occur at mouse P2X₂ (mP2X₂) channels expressed in oocytes. Domain swaps, species chimeras, and point mutations were employed to determine that two specific amino acid residues in the cytosolic tail domain govern this difference in behavior between the two orthologous channels. The change in pore diameter was characterized using reversal potential measurements and excluded field theory for several organic ions; both rP2X₂ and mP2X₂ channels have a pore diameter of ∼11 Å in the I₁ state, but the transition to the I₂ state increases the rP2X₂ diameter by at least 3 Å. The I₁ to I₂ transition occurs with a rate constant of ∼0.5 s⁻¹. The data focus attention on specific residues of P2X₂ channel cytoplasmic domains as determinants of permeation in a state-specific manner.