Mitochondrial cytochrome bc1 complex performs two functions: It is a respiratory multienzyme complex and it recognizes a mitochondrial targeting presequence. Refined crystal structures of the 11-subunit bc1 complex from bovine heart reveal full views of this bifunctional enzyme. The 'Rieske' iron-sulfur protein subunit shows significant conformational changes in different crystal forms, suggesting a new electron transport mechanism of the enzyme. The mitochondrial targeting presequence of the 'Rieske' protein (subunit 9) is lodged between the two 'core' subunits at the matrix side of the complex. These 'core' subunits are related to the matrix processing peptidase, and the structure unveils how mitochondrial targeting presequences are recognized.