[Structual and functional dynamics of the ATP receptor channel P2X(2)]

ATP is known to function as a neurotransmitter. There are 2 major families of ATP receptors-the ion channel-type P2X receptors and metabotropic P2Y receptors. P2X receptors are known to possess unique properties of pore dilation that depends on the time lapse after ATP application; further, they exert their functions by directly interacting with nicotinic ACh receptors. These properties suggest the flexibility of the pore formed by these receptors. We studied the biophysical properties of P2X(2) receptor by using in vitro expression systems and focused on various dynamic regulations and structural rearrangements. Firstly, the pore property clearly depended on the expression levels of the P2X(2) receptors on the membrane. When the expression level was high, inward rectification was weak, and pore dilation was clearly observed. We also clarified that the key feature of the pore property is not the number of channels expressed but the number of open channels on the membrane. Secondly, we focused on the regulation of these channels by phosphoinositides (PIP(ns)). PIP(ns) are known to regulate the activity of various ion channels, but in the case of P2X(2), we observed that PIP(ns) regulate not only the activity but also the processes of pore dilation and desensitization. The binding of PIP(ns) to P2X(2) in the pore dilated state was observed to be less stable. Application of a reagent, which decreases the levels of PIP(ns), and mutation of the binding site facilitated desensitization of P2X(2) in the pore dilated state. Thirdly, we analyzed the voltage-dependent gating of these channels. Although P2X(2) lacks a canonical voltage-sensor domain, it undergoes voltage-dependent activation upon hyperpolarization. Further, we observed that the voltage-dependent gating depends on ATP concentration; conductance-voltage relationship curve shifted toward depolarization potential with increase in ATP concentration. We found that the ATP-binding site and the extracellular sideus of the transmembrane region were critical for the voltage-dependent gating. These results show that P2X(2) channel pore is exceptionally flexible, and that the channel activity is dynamically regulated by various factors, including not only ATP but also PIP(ns) and membrane potential.