Fig. 3.

Loop-C conformation and pore sizes of closed- and open-class structures. (a) Boxed area in Fig. 2 showing densities from the three ~ 6‐Å maps (blue, untreated tubes; grey, closed class; red, open class), and superimposed C^α backbones from the atomic model⁵ (black, PDB ID: 2BG9) and (right) loop C from aligned AChBP¹⁸ (green; PDB ID: 1I9B). (b) Central slab through ACh receptor, showing densities from the closed-class structure, and superimposed C^α backbone from the atomic model (ribbon representation; α, β, γ and δ in red, green, magenta and blue, respectively). The phospholipid headgroup regions (pink contours) are identified by two prominent bands of density running concentrically about the axis of the tube;^22,29 their locations coincide with rings of negatively charged amino acids that affect conductance through the open pore.³⁰ (c and d) Comparison of closed- and open-channel densities from the three ~ 6‐Å maps in a cross section through the extracellular leaflet of the lipid bilayer [level of dashed bar in (b)]; colours as in (a). The broken lines in (d) highlight the pentagonally symmetric arrangement of M2 helices around the pore of the closed channel and the movement outwards of all four helices of β when the channel opens. The mesh interval corresponds to 1 Å; all contours at 1 σ.