Ectodomain Movements of an ATP-gated Ion Channel ( P2X2 Receptor) Probed by Disulfide Locking*

Background: The ATP-gated channel (P2X2) receptor ectodomain is formed by -strands from three subunits. Results: ATP-induced channel opening was prevented by disulfide locking between pairs of substituted cysteines in different subunits. Conclusion: The turret region, the lower body wall, and the outer ends of transmembrane domains move significantly in channel opening. Significance: Large ectodomain movements couple ATP binding to channel opening in P2X receptors. The ectodomain of the P2X receptor is formed mainly from two- or three-stranded -sheets provided symmetrically by each of the three subunits. These enclose a central cavity that is closed off furthest from the plasma membrane (the turret) and that joins with the transmembrane helices to form the ion permeation pathway. Comparison of closed and open crystal structures indicates that ATP binds in a pocket positioned between strands provided by different subunits and that this flexes the -sheets of the lower body and enlarges the central cavity: this pulls apart the outer ends of the transmembrane helices and thereby opens an aperture, or gate, where they intersect within the membrane bilayer. In the present work, we examined this opening model by introducing pairs of cysteines into the rat P2X2 receptor that might form disulfide bonds within or between subunits. Receptors were expressed in human embryonic kidney cells, and disulfide formation was assessed by observing the effect of dithiothreitol on currents evoked by ATP. Substitutions in the turret (P90C, P89C/S97C), body wall (S65C/S190C, S65C/D315C) and the transmembrane domains (V48C/I328C, V51C/I328C, S54C/I328C) strongly inhibited ATP-evoked currents prior to reduction with dithiothreitol. Western blotting showed that these channels also formed predominately as dimers and/or trimers rather than monomers. The results strongly support the channel opening mechanism proposed on the basis of available crystal structures.