Role of extracellular histidines in agonist sensitivity of the rat P2X4 receptor.

Relatively little information is available about the relationship between the molecular structure of each of the seven subtypes of P2X receptors and their function. Here, we investigated the possible function of three histidine residues in the extracellular loop of rat P2X(4) receptors. Mutation of histidine 241 to alanine (H241A) in the rat P2X(4) receptor decreased the EC(50) value of the ATP concentration-response curve from 8.4 to 0.7 microM. In contrast, the histidine mutation H140A or H286A slightly increased the EC(50) value. Maximal current responses were significantly larger in oocytes expressing rat H241A-mutated receptors compared to those expressing wildtype, H140A or H286A receptors. In addition, significantly less receptor protein was detected in H241A-expressing oocytes than in oocytes expressing wildtype, H140A or H286A receptors. Moreover, ATP-activated current in H241A-expressing cells activated faster than in wildtype receptor-expressing cells. The increased maximal current amplitude, the decrease in protein expression and the more rapid activation kinetics suggest that the H241A mutation facilitates opening of the receptor-channel (gating).