Voltage clamp fluorometry analysis of the P2X7 receptor suggests a limited conformational interplay between extracellular ATP binding and the intracellular ballast domain

The large intracellular C-terminus of the pro-inflammatory P2x7 ion channel receptor (P2x7R) is associated with diverse P2x7R-specific functions. Cryo-EM structures of the closed and ATP-bound open full-length P2x7R recently identified a membrane-associated anchoring domain, an open-state stabilizing "cap" domain, and a globular "ballast domain" containing GTP/GDP and dinuclear Zn2+-binding sites with unknown functions. To investigate protein dynamics during channel activation, we improved incorporation of the environment-sensitive fluorescent unnatural amino acid L-3-(6-acetylnaphthalen-2-ylamino)-2-aminopropanoic acid (ANAP) into Xenopus laevis oocyte-expressed P2x7Rs and performed voltage clamp fluorometry (VCF). While we confirmed predicted conformational changes within the extracellular and the transmembrane domains, only three out of 41 mutants containing ANAP in the C-terminal domain resulted in ATP-induced fluorescence changes. We conclude that the ballast domain functions rather independently from the extracellular ATP binding domain and might require activation by additional ligands and/or protein interactions. Novel tools to study these are presented.