Figure 2

The organization of the P2X receptor in Aplysia californica. (A,B) Structural features of the P2X monomer models with regions recognized for mammalian homologs in crystallography studies^16,17. TM1 and TM2 are transmembrane regions (see Fig. 1C). (B) 3D modeling of the trimeric P2X organization with suggested functional regions^16,17,18,77. ‘Left flipper’ (chain A) and ‘dorsal fin’ (chain B) together with a head (chain A) of these chains form the ATP binding site (see also Fig. 1C). (C) Comparisons between Aplysia and Lymnaea receptors based on the predicted difference of salt bridges (yellow—‘dorsal fin’). (D) The alignment for P2X receptors in gastropod molluscs (Aplysia californica, Lymnaea stagnalis and Elysia chlorotica) with domain for the ATP binding site (brown—I-VI domains). Of note, Aplysia (middle in the alignment) has significantly less polar acidic and more polar basic amino acids in the ‘dorsal fin’ region [11pb:2pa] compared to other species (Lymnaea [7pb:6pa] and Elysia [5pb:8pa]), suggesting different kinetic and pharmacological properties of P2X receptors. In summary, there are 13 polar charged amino acids in the ‘dorsal fin’. See also Supplementary Figs. S1 and S2.