C-terminal Calmodulin-binding Motif Differentially Controls Human and Rat P2X7 Receptor Current Facilitation

P2X7 receptors (P2X7R) are ATP-gated calcium-permeable cationic channels structurally unique among the P2X family by their much longer intracellular C-terminal tail. P2X7Rs show several unusual biophysical properties, in particular marked facilitation of currents and leftward shift in agonist affinity in response to repeated or prolonged agonist applications. We previously found the facilitation at rat P2X7R resulted from a Ca2+-calmodulin-dependent process and a distinct calcium-independent process. However, P2X7Rs show striking species differences; thus, this study compared the properties of ATP-evoked facilitation of currents in HEK293 cells transiently expressing the human or rat P2X7R as well as rat/human, human/rat chimeric, and mutated P2X7Rs. Facilitation at the human P2X7R was 5-fold slower than at the rat P2X7R. Facilitation did not resulting from an increase of receptor addressing the plasma membrane. We found the human P2X7R shows only calcium-independent facilitation with no evidence for calmodulin-dependent processes, nor does it contain the novel 1-5-16 calmodulin binding domain present in the C terminus of rat P2X7R. Replacement of three critical residues of this binding domain from the rat into the human P2X7R (T541I, C552S, and G559V) reconstituted the Ca2+-calmodulin-dependent facilitation, leaving the calcium-independent facilitation unaltered. The leftward shift in the ATP concentration-response curve with repeated agonist applications appears to be a property of the calcium-independent facilitation process because it was not altered in any of the chimeric or mutated P2X7Rs. The absence of Ca2+-dependent facilitation at the human P2X7R may represent a protective adaptation of the innate immune response in which P2X7R plays significant roles.