Dissecting activation steps in P2X7 receptors.

P2X7 receptors are trimeric ion channels activated by extracellular ATP. Upon activation, they trigger cytolysis and apoptosis but also control cell proliferation. To shed more light on channel gating and the underlying function of the individual subunits, receptors of concatenated subunits were built containing a defined number of functional binding sites. The currents evoked by ATP were obtained in the outside-out configuration of the patch-clamp technique, and steady-state activation, as well as time courses, were analyzed. Our results show that each occupied binding site contributes to channel activation. While the occupation of a single binding site can already activate the channels, three bound ligands maximally stabilize the open state. Hence, P2X7 receptors can be described by a stepwise activation process.