Fig. 2. E2 suppresses activating transitions of the ECD upon GABA binding.

a Two different cryo-EM structures were obtained from a single sample of ρ1-EM with GABA and E2, viewed from the membrane plane. Presumed functional state (left, blue: desensitized; right, green: primed) and ensemble contribution as a percent of resolved particles are indicated above each structure. One subunit of each pentamer is colored darker for definition. E2 (yellow) and resolved lipids (gray) are shown as thick and thin sticks, respectively. b Zoom view of a single GABA binding site in the primed state, depicted as in panel a. Density assigned to GABA is shown in transparency. GABA and surrounding residues are shown as sticks and labeled. c Zoom view of a single E2 binding site in the primed state, depicted as in Fig. 1f from the channel pore. The density assigned to E2 is shown in transparency. E2 and surrounding residues are shown as sticks and labeled. d Superimposed structures of ρ1-EM in the apparent desensitized (blue) and primed (green) states, viewed from the membrane plane (left) and extracellular side (right). All but one subunit of each pentamer is rendered transparent for clarity. e GABA concentration-response curves for ρ1-EM in the absence (black) and presence of 30 μM E2 (pink). Error bars represent SEM from 5 individual oocytes. Solid lines represent fits Boltzmann curves with an EC₅₀ of 1.3 μM (GABA alone, 95% confidence interval 1.16–1.41 μM) or 1.8 μM (GABA + E2, 95% confidence interval 1.56–2.21 μM). f Pore-radius profiles of ρ1-EM apo (gray) and E2 (red) structures. Both structures are assigned to a resting-like state. g Pore-radius profiles of ρ1-EM in the presence of GABA alone (black, PDB ID: 8OP9) and GABA + E2 in the primed (green) and desensitized (blue) states.