Cytochrome bc 1 complex from bovine heart has been reconstituted into tubular crystals. The three-dimensional structure of the complex in lipid bilayer has been obtained at an effective resolution of 16 Å by electron cryomicroscopy and helical image reconstruction. The complex is in a dimeric form, in which the monomers are associated closely in extramembrane domains on both sides of the membrane. The large inner domain is distinctively hollow and the small outer domain consists of a flat mass and two bulbous extrusions. These domains are connected by two narrow transmembrane columns. Locations of the subunits and the redox centres in the model are proposed.
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