Fig. 5
From: A potential cost of evolving epibatidine resistance in poison frogs
Summary figure. Results from Tarvin et al. [13] are shown in the graphs on the left, from Epipedobates anthonyi (top, n = 5–9) and human (bottom, n = 6–7) nAChRs (RNA ratio 1α:7β for Epipedobates and 1α:3β for human). Epi-S and Epi-R refer to the epibatidine-sensitive and epibatidine-resistant characteristics of the receptor. These results led us to hypothesize that the mutation S108C in the human β2 subunit [α4β2(FC)] resulted in an altered stoichiometry: instead of a monophasic curve (characteristic of α4β2 nAChRs composed of 2 α4 and 3 β2 subunits), the ACh concentration–response curve for this mutant was biphasic and shifted to the right (characteristic of α4β2 nAChRs composed of 3 α4 and 2 β2 subunits. Furthermore, this alteration in the ACh sensitivity was not observed in Epipedobates receptors, which showed the same monophasic curve at all RNA ratios tested (Additional file 2). In the central panels, the predicted stoichiometry is shown as a diagram of the receptor. We now report that there is a reduction of maximal ACh-induced currents in Epipedobates receptors with C108-containing β2 subunits, likely due to reduced availability of C108-containing β2 subunits (Fig. 3). The relative number of receptors in the plasma membrane is shown on the right diagrams. No differences in ACh sensitivity were observed after biasing α4 and β2 RNA injection ratios, indicating that Epipedobates α4β2 nAChRs functionally assemble in a single stoichiometry. However, the reduced cell-surface expression of α4β2(FC) nAChRs (also observed for Epipedobates α4β2 nAChRs containing cysteine in position 108 of the β2 subunit) alters the concentration–response profile of human α4β2(FC) receptors from the monophasic (HS-like) of the wild-type nAChR to biphasic (LS-like) CRC, indicating an alternative stoichiometry. The presence of an additional mutation [β2(LC)] confers an HS-like stoichiometry but does not correct the β2-reduced availability. The studies on the human receptor numbers in the plasma membrane were obtained using concatenated receptors (Fig. 4)