Opening the acetylcholine receptor. Cross sections from electron microscopic reconstructions of the acetylcholine receptor in (top) its closed form and (bottom) its open form. (The open form corresponds to the structure shown in Figure 13.27). The areas labeled M1, M2, M3, and M4 correspond to the four membrane-spanning α helices of one subunit. The cross section of the open channel was generated by treating the receptor with acetylcholine and freezing the sample within 20 ms. Notice that the hole in the center of the channel is substantially larger in the open structure. The enlargement of the hole is due to the rotation of the M2 helices by approximately 15 degrees along their long axes. [Courtesy of Nigel Unwin.]