ATP-gated P2X receptors display ion permeability increases within seconds of receptor activation as the channels enter the I(2) state, which is permeable to organic cations and dye molecules. The mechanisms underlying this important behavior are not completely understood. In one model, the I(2) state is thought to be due to opening of Pannexin-1 (Panx-1) channels, and, in the second, it is thought to be an intrinsic P2X property. We tested both models by measuring ion and dye permeability and used a patch-clamp coordinated spectroscopy approach to measure conformational changes. Our data show that Panx-1 channels make no detectable contribution to the P2X(2) receptor I(2) state. However, P2X(2) receptors display permeability dynamics, which are correlated with conformational changes in the cytosolic domain remote from the selectivity filter itself. Finally, the data illustrate the utility of a new approach, using tetracysteine tags and biarsenical fluorophores to measure site-specific conformational changes in membrane proteins.
