The first transmembrane domain of the P2X receptor subunit participates in the agonist-induced gating of the channel

Based on pharmacological properties, the P2X receptor family can be subdivided into those homo-oligomers that are sensitive to the ATP analog alpha beta -methylene ATP(alpha beta meATP) (P2X(1) and P2X(3)) and those that are not (P2X(2), P2X(4), P2X(5), P2X(6), and P2X(7)). We exploited this dichotomy through the construction of chimeric receptors and site-directed mutagenesis in order to identify domains responsible for these differences in the abilities of extracellular agonists to gate P2X receptors. Replacement of the extracellular domain of the alpha beta meATP-sensitive rat P2X(1) subunit with that of the alpha beta meATP-insensitive rat P2X2 subunit resulted in a receptor that was still alpha beta meATP-sensitive, suggesting a non-extracellular domain was responsible for the differential gating of P2X receptors by various agonists. Replacement of the first transmembrane domain of the rat P2X2 subunit with one from an alpha beta meATP-sensitive subunit (either rat P2X(1) or P2X3 subunit) converted the resulting chimera to alpha beta meATP sensitivity. This conversion did not occur when the first transmembrane domain came from a non-alpha beta meATP-sensitive subunit. Site-directed mutagenesis indicated that the C-terminal portion of the first transmembrane domain was important in determining the agonist selectivity of channel gating for these chimeras. These results suggest that the first transmembrane domain plays an important role in the agonist operation of the P2X receptor.