Fig. 2

Acetylcholine CRCs of receptors from two non-dendrobatid frogs. Xenopus tropicalis retain a monophasic ACh CRC best fit with a single EC₅₀ (Table 1 and Additional file 4) even in conditions that induce LS sites in mammalian α4β2 nAChRs (7α:1β) or with the S108C substitution alone [β2(FC)] or in combination with F106L [β2(LC)]. The α:β RNA ratios are 1:3 (A, n = 6–18) and 7:1 (B, n = 13–37). The actual stoichiometry of frog receptors is unknown but the Xenopus α4β2 nAChR behaves as the mammalian 2α:3β stoichiometry (C). This conjecture is indicated as a question mark over the gray arrow. Note that the scale for Xenopus is nanomolar concentration. Nanorana parkeri retain a biphasic CRC best fit with two EC₅₀ values (Table 2 and Additional file 4) in both 1:3 (D, n = 5–9) or 7:1 (E, n = 6–9) α:β RNA ratios, with the S108C substitution alone [β2(FC)] or in combination with F106L [β2(LC)]. The Nanorana α4β2 nAChR behaves as if its stoichiometry is 3α:2β (F) with both ratios of RNA used in this study. Data points represent means ± SD. Red arrows indicate HS-binding sites, and blue arrow indicates LS-binding site. + and − signs indicate the principal and complementary components of the subunit interfaces