Mitochondria (or mitoplasts) and submitochondrial particles from yeast were treated with [125I] diazobenzenesulfonate to label selectively proteins exposed on the outer or inner surface of the inner mitochondrial membrane. Polyacrylamide gel analysis of the immunoprecipitates formed with antibodies against Complex III or cytochrome b revealed that the two core proteins and cytochrome b were labeled in both mitochondria and submitochondrial particles, suggesting that these proteins span the membrane. Cytochrome c1 and the iron sulfur protein were labeled in mitochondria but not in submitochondrial particles, suggesting that these proteins are exposed on the cytosolic side of the inner membrane. The steady-state reduction of cytochromes b and c1 was determined with succinate and the decyl analogue of coenzyme Q as substrates. Addition of the coenzyme Q analogue to mitochondria caused reduction of 15-30% of ;the total dithionite-reducible b and 100% of the cytochrome c1: Addition of the coenzyme Q analogue to submitochondrial particles led to the reduction of 70% of the total dithionite-reducible cytochrome b but insignificant amounts of cytochrome c1. A model to explain the topography of Complex III in the inner membrane is proposed based on these results.
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