Membrane topology of P2Y and P2X receptor subunits.

(A) P2Y receptors (i) are typical 7-membrane–spanning receptors made of a single polypeptide chain, with the N- and C-termini on the external and cytoplasmic side of the plasma membrane, respectively. P2X receptors (ii) are formed by subunits that span the plasma membrane twice and have both the N- and C-termini on the cytoplasmic side. The P2X₇ subunit differs from the other members of the P2X subfamily (P2X₁-P2X₆) in the extended carboxyterminal tail (iii). (B) It is hypothesized that P2X₇ receptor is generated by the aggregation of an unknown number of subunits (maybe 6) to form an ATP-activated channel. Recruitment of additional subunits causes formation of a nonselective pore (also see Figure 2).