Integration of the mitochondrial-processing peptidase into the cytochrome bc(1) complex in plants

The plant mitochondrial cytochrome bc(1) complex, like nonplant mitochondri al complexes, consists of cytochromes b and c(1), the Rieske iron-sulfur pr otein, two Core proteins, and five low-molecular mass subunits. However, in contrast to nonplant sources, the two Core proteins are identical to subun its of the general mitochondrial processing peptidase (MPP). The MPP is a f ascinating enzyme that catalyzes the specific cleavage of the diverse prese quence peptides from hundreds of the nuclear-encoded mitochondrial precurso r proteins that are synthesized in the cytosol and imported into the mitoch ondrion. Integration of the MPP into the be, complex renders the bet comple x in plants bifunctional, being involved both in electron transport and in protein processing. Despite the integration of MPP into the bc(1) complex, electron transfer as well as translocation of the precursor through the imp ort channel are independent of the protein-processing activity. Recognition of the processing site by MPP occurs via the recognition of higher-order s tructural elements in combination with charge and cleavage-site properties. Elucidation of the three-dimensional (3-D) structure of the mammalian cyto chrome bc(1) complex is highly useful for understanding of the mechanism of action of MPP.