Design of mP2X4 receptors carrying pHluorin in the extracellular domain. (A) Cartoons illustrate the basic layout of a P2X receptor subunit and how three identical subunits are assembled into a trimeric ion channel. The location of the PM is shown approximately with black lines. (B) Sequence alignment for parts of the extracellular loop that were targeted for insertion of pHluorin. The pHluorin insertion sites are shown with a dashed line and a cartoon of GFP. Nine constructs were made and tested using approaches described in detail in the Results. They are identified in the figure.