Aggregates of yeast mitochondrial cytochrome b observed after electrophoresis

Orientation of complex III in the yeast mitochondrial membrane: labeling with [125I] diazobenzenesulfonate and functional studies with the decyl analogue of coenzyme Q as substrate

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.

In vitro reconstruction of the mitochondrial translation system of yeast

We have isolated the translation system from yeast mitochondria and have reconstructed it in vitro. This submitochondrial system, composed of mitochondrial ribosomes, tRNA, pH 5 fraction and mRNA, is maximally active at 10 mM Mg2+ and 100 mM KCl or NH4Cl. NH4+ is more stimulatory than K+. Added Escherichia coli tRNA gives less than half the activity obtained with added mitochondrial tRNA. Activity is enhanced with protease inhibitors but not with Ca2+, spermine, or spermidine. In contrast to heterologous translation systems, the present system produces products with molecular weights similar to those of products synthesized by yeast mitochondria in vivo and by intact yeast mitochondria in vitro. The results support the idea that the unique coding features of the mitochondrial genome require a unique translation system for accurate translation of mitochondrial mRNAs.