Abstract
The synaptic surface of the acetylcholine receptor in membranes from Torpedo californica electric organ has been imaged by scanning tunneling microscopy. The molecule appears pentameric, with one major and four minor protrusions rising above the surface, and these protrusions encompass a large central cavity. The outer diameter of the molecule is 69 +/- 10 A, while the diameter of the cavity, measured at the widest complete contour line delimiting the opening, is 26 +/- 7 A. The images and dimensions obtained are consistent with the structure determined from hybrid density maps obtained by x-ray diffraction and electron microscopy. Thus, scanning tunneling microscopy can be used to obtain overall dimensions and low-resolution structural features of the surface of a membrane-embedded protein.