Membrane topology of beef-heart ubiquinone-cytochrome c reductase (complex III).

The membrane topology of ubiquinone-cytochrome c reductase (EC 1.10.2.2.) has been investigated with photoreactive lipid analogs (Bisson, R., and Montecucco, C. (1981) Biochem. J. 193, 757-763), both in its isolated form and when part of succinate-cytochrome c reductase (Complex II + III). These probes react specifically with those polypeptide chains exposed to lipids, thereby labeling them radioactively. Highly resolving gel electrophoretic conditions have been used to determine the patterns of labeling. Core protein I, cytochrome b, cytochrome c1, and polypeptides VI, VII, VIII, and IX contribute to the lipid-protein boundary of Complex III. Evidence that the interaction between Complex II and Complex III involves their hydrophobic domains is also presented.