Abstract
P2X receptors are cation selective channels that are activated by extracellular nucleotides. These channels are likely formed by three identical or related subunits, each having two transmembrane segments (TM1 and TM2). To identify regions that undergo rearrangement during gating and to probe their secondary structure, we performed tryptophan scanning mutagenesis on the two putative TMs of the rat P2X4 receptor channel. Mutant channels were expressed in Xenopus oocytes, concentration-response relationships constructed for ATP, and the EC50 estimated by fitting the Hill equation to the data. Of the 22 mutations in TM1 and 24 in TM2, all but one in TM1 and seven in TM2 result in functional channels. Interestingly, the majority of the functional mutants display an increased sensitivity to ATP, and in general these perturbations are more pronounced for TM2 when compared with TM1. For TM1 and for the outer half of TM2, the perturbations are consistent with these regions adopting alpha-helical secondary structures. In addition, the greatest perturbations in the gating equilibrium occur for mutations near the outer ends of both TM1 and TM2. Surface biotinylation experiments reveal that all the nonfunctional mutants traffic to the surface membrane at levels comparable to the WT channel, suggesting that these mutations likely disrupt ion conduction or gating. Taken together, these results suggest that the outer parts of TM1 and TM2 are helical and that they move during activation. The observation that the majority of nonconducting mutations are clustered toward the inner end of TM2 suggests a critical functional role for this region.