Failure to insert the iron-sulfur cluster into the Rieske iron-sulfur protein impairs both center N and center P of the cytochrome bc1 complex.

Mutation of a serine that forms a hydrogen bond to the iron-sulfur cluster of the Rieske iron-sulfur protein to a cysteine results in a respiratory-deficient yeast strain due to formation of iron-sulfur protein lacking the iron-sulfur cluster. The Rieske apoprotein lacking the iron-sulfur cluster is inserted into both monomers of the dimeric cytochrome bc(1) complex and processed to mature size, but the protein lacking iron-sulfur cluster is more susceptible to proteolysis. In addition, the protein environment of center P in one half of the dimer is affected by failure to insert the iron-sulfur cluster as indicated by the fact that only one molecule of myxothiazol can be bound to the cytochrome bc(1) dimer. Although the bc(1) complex lacking the Rieske iron-sulfur cluster cannot oxidize ubiquinol through center P, rates of reduction of cytochrome b by menaquinol through center N are normal. However, less cytochrome b is reduced through center N, and only one molecule of antimycin can be bound at center N in the bc(1) dimer lacking iron-sulfur cluster. These results indicate that failure to insert the [2Fe-2S] cluster impairs assembly of the Rieske protein into the bc(1) complex and that this interferes with proper assembly of both center P and center N in one half of the dimeric enzyme.