Scanning tunneling microscopy imaging ofTorpedo acetylcholine receptor

ABSTRACT Thesynaptic surfaceofthe acetylcholine re- ceptor in membranes from Torpedo calzfornica electric organhasbeenimagedby anigtunneling microscopy. The mol-ecule appears pentameric, with one major and four minor protrusions rising above the surface, and these protrusionsencompass a large central cavity. Theouter diameter ofthemolecule is 69 ± 10 A, while the diameter of the cavity,measured at the widest complete contour line delmiting theopening,is 26 ±7 A.Theimagesanddimensionsobtainedare consistent with the structure determined from hybrid density mapsobtained byx-ray diffraction and electron microscopy.Thus, scanning tunneling microscopy can be used to obtain overall dimensions and low-resolution structuralfeaturesofthe surface of amembrane-embeddedprotein.Giventhedifficulties encounteredin preparingcrystalsfrommembraneproteins suitable for x-ray or electron diffractionanalysis, it is worthwhile to explore other methods forobtaining structural information on these rather intractablesystems. Theintroductionofscanningtunnelingmicroscopy(STM)for imagingofbiological systems (1)