Contribution of the Juxtatransmembrane Intracellular Regions to the Time Course and Permeation of ATP-gated P2X7 Receptor Ion Channels.

P2X7 receptors are ATP-gated ion channels that contribute to inflammation and cell death. They have the novel property of showing marked facilitation to repeated applications of agonist, and the intrinsic channel pore dilates to allow the passage of fluorescent dyes. A 60-s application of ATP to hP2X7 receptors expressed in Xenopus oocytes gave rise to a current that had a biphasic time course with initial and secondary slowly developing components. A second application of ATP evoked a response with a more rapid time to peak. This facilitation was reversed to initial levels following a 10-min agonist-free interval. A chimeric approach showed that replacement of the pre-TM1 amino-terminal region with the corresponding P2X2 receptor section (P2X7-2Nβ) gave responses that quickly reached a steady state and did not show facilitation. Subsequent point mutations of variant residues identified Asn-16 and Ser-23 as important contributors to the time course/facilitation. The P2X7 receptor is unique in having an intracellular carboxyl-terminal cysteine-rich region (Ccys). Deletion of this region removed the secondary slowly developing current, and, when expressed in HEK293 cells, ethidium bromide uptake was only ∼5% that of WT levels, indicating reduced large pore formation. Dye uptake was also reduced for the P2X7-2Nβ chimera. Surprisingly, combination of the chimera and the Ccys deletion (P2X7-2NβdelCcys) restored the current rise time and ethidium uptake to WT levels. These findings suggest that there is a coevolved interaction between the juxtatransmembrane amino and carboxyl termini in the regulation of P2X7 receptor gating.