Dependence of purinergic P2X receptor activity on ectodomain structure.

Purinergic receptors (P2XRs) activate and desensitize in response to the binding of extracellular nucleotides in a receptor- and ligand-specific manner, but the structural bases of their ligand preferences and channel kinetics have been incompletely characterized. Here we tested the hypothesis that affinity of agonists for binding domain accounts for a ligand-specific desensitization pattern. We generated chimeras using receptors with variable sensitivity to ATP in order: P2X(4)R > P2X(2a)R = P2X(2b)R P2X(7)R. Chimeras having the ectodomain Ile(66)-Tyr(310) sequence of P2X(2)R and Val(61)-Phe(313) sequence of P2X(7)R in the backbone of P2X(4)R were expressed but were non-functioning channels. P2X(2a) + X(4)R and P2X(2b) + X(4)R chimeras having the Val(66)-Tyr(315) ectodomain sequence of P2X(4)R in the backbones of P2X(2a)R and P2X(2b)R were functional and exhibited increased sensitivity to ligands as compared with both parental receptors. These chimeras also desensitized faster than parental receptors and in a ligand-nonspecific manner. However, like parental P2X(2b)R and P2X(2a)R, chimeric P2X(2b) + X(4)R desensitized more rapidly than P2X(2a) + X(4)R, and the rate of desensitization of P2X(2a)+X(4)R increased by substituting its Arg(371)-Pro(376) intracellular C-terminal sequence with the Glu(376)-Gly(381) sequence of P2X(4)R. These results indicate the relevance of interaction between the ectodomain and flanking regions around the transmembrane domains on ligand potency and receptor activation. Furthermore, the ligand potency positively correlates with the rate of receptor desensitization but does not affect the C-terminal-specific pattern of desensitization.