Image analysis of the heavy form of the acetylcholine receptor from Torpedo marmorata

The structure of the heavy (H) form of the acetylcholine receptor, which comprises two covalently linked 250,000 M r oligomers, has been investigated by numerical analysis of electron microscope images. Na-cholate solubilized Torpedo marmorata H-form receptor was reintegrated into artificial lipid vesicles and negatively stained with uranyl acetate prior to imaging in a conventional transmission microscope. The reconstituted preparations exhibited the standard polypeptide composition of the purified receptor ( α 2 βγδ) and the same transmembrane arrangement as in the native subsynaptic membrane. Covalent disulfide linkage between the two oligomers took place exclusively through the δ chains. In agreement with previous work (Cartaud et al., 1980) the H-form appeared as “doublets” of two coplanar 9 nm rosettes at a center-to-center distance of 9.2 ± 1.1 nm. The relative angular orientation of the two rosettes in a doublet was examined by correlation analysis in the real space. It exhibited a marked variability, few of the doublets featuring any kind of symmetry, suggesting that the two oligomers of a doublet are connected via an extended and flexible chain or loop. The area of contact between the two rosettes of a doublet therefore does not necessarily represent a reliable clue as to the location of the δ chain within the structure. Averaged images obtained after reorientation and summation of up to 132 rosettes revealed the three major peaks and the two grooves already observed in previous studies. Two additional smaller peaks were identified. Tentative assignment of structural details to individual subunits was deduced from an examination of α-bungarotoxin-labeled doublets. The α subunits, which carry part or all of the acetylcholine binding sites, are probably located in nonadjacent positions in the vicinity of the newly found peaks. This assignment is consistent with the image analysis of receptor-toxin complexes recently reported by Zingsheim et al. (1982b).