Tlie long term goal of this study is to elucidate tfie molecular mechanisms underlying P2X receptor functions by combining structural and functional approaches. P2X receptors are trimeric ATP-gated ion channels that are widely distributed throughout the human body. Activation of these receptors causes cation influx that evokes action potentials or triggers calcium mediated signaling, important functions in signal transmission, inflammation, and pain sensation. While the first crystal structure in its closed state provides a number of novel insights into receptor architecture, mechanisms of P2X receptor functions are largely unknown. To determine the cation selection mechanism and the structural changes during gating, two specific aims are designed as follows;1) to identify the ion selection machinery using patch clamp recording techniques, and 2) to localize ion binding sites and determine the crystal structure in the open state using X-ray crystallography. Completion of these studies is expected to provide the first compelling mechanisms and structural basis for the cation selectivity, which may in turn lead to the design of novel pharmacological agents targeting P2X receptors.