Model of the bc1-complex from electron microscopy


The UQ:cyt c reductase complex was isolated from Neurospora crassa mitochondria as a protein detergent complex, and dissociated by mild salt treatment. Three parts were obtained and characterized. Firstly, a complex containing subunits III (cyt b), IV (cyt c1), VI, VII, VIII and IX (called cytochrome bc1 complex in the figure legends); secondly, a complex containing subunits I and II (the "core" subunits); and thirdly, the single subunit V (FeS). Membrane crystals were prepared from the cytochrome bc1 subunit complex, and, by combining tilted electron microscopic views of the crystals, a low resolution 3-D structure was calculated. This structure was compared to that of the whole cytochrome reductase (previously determined). Protein density absent from the structure of the subunit complex was then attributed to the missing subunits according to their size and shape and their association with the phospholipid bilayer.

3-D Model of the complete complex, and the complex (mainly cyt b and cyt c1 subunits) without the "core" subunits and the FeS subunit.
View parallel to the plane of the membrane
View perpendicular to the plane of the membrane (from the inter-membrane space).
Three-dimensional models of one (dimeric) cytochrome bc1 complex (left) and the dimeric cytochrome reductase (right).

The unit cell of the "stripped-down" complex.
Two views of the reconstruction for the cytochrome bc1 complex membrane crystal. Each dimer contains 2 well defined lobes of density that correspond in size to the smaller peripheral part of cytochrome reductase and which are shown in black. The remaining, less well-defined region through which the dimers are in contact, is shown in white. In the upper view, the carbon film would be beloww the model as seen here.

The pictures and figure legends are taken from:

  1. Weiss, H. (1987) Structure of mitochondrial ubiquinol-cytochrome c reductase (Complex III). Curr. Topics in Bioenergetics, 15, 67-90
  2. Karlsson, B., Hovmoller, S., Weiss, H. and Leonard, K. (1983) Structural studies of cytochrome reductase. Subunit topography determined by electron microscopy of membrane crystals of a subcomplex. J. Mol. Biol. 165, 287-302