The mitochondrial targeting presequence of the Rieske iron-sulfur protein is processed in a single step after insertion into the cytochrome bc1 complex in mammals and retained as a subunit in the complex.

The amino acid sequence of subunit 9 of the bovine heart cytochrome bc1 complex is identical to the 78-amino acid presequence that is removed post-translationally from the Rieske iron-sulfur protein as it is imported and targeted to the mitochondrial cytochrome bc1 complex. Iron-sulfur protein precursor, generated by in vitro transcription and translation, is processed to mature size in a single step when incubated with rat liver mitochondria, and generates a peptide that comigrates on SDS-polyacrylamide gel electrophoresis with subunit 9. These results suggest that the Rieske protein is processed in a single proteolytic step after it is inserted into the cytochrome bc1 complex in mammals, and that the processed presequence remains as a subunit of the complex. This is apparently the first instance in which a cleaved targeting presequence has been shown to be retained in the cell, possibly exhibiting a second function in addition to its function in protein trafficking.