Ion permeation pathway within the internal pore of P2X receptor channels.

Abstract

P2X receptor channels are trimeric ATP-activated ion channels expressed in neuronal and non-neuronal cells that are attractive therapeutic targets for human disorders. Each P2X receptor channel subunit consists of two transmembrane helices, a large extracellular domain that contains the ATP binding sites at subunit interfaces, and intracellular N-and C-termini. P2X1-4 and P2X7 receptor channels are cation permeable, whereas P2X5 has been reported to have a significant anion permeability. Recent structures of ATP-bound P2X receptors with the activation gate open reveal the unanticipated presence of a cytoplasmic cap over the central ion permeation pathway, leaving lateral fenestrations that may be largely buried within the membrane as potential pathways for ions to permeate at the intracellular end of the pore. We identified a critical residue within the intracellular lateral fenestrations that is readily accessible to thiol reactive compounds from both sides of the membrane and where substitutions influence the relative permeability of the channel to cations and anions. Taken together, our results demonstrate that ions can enter or exit the internal pore through lateral fenestrations and that these structural elements play a critical role in determining the ion selectivity of P2X receptor channels.