In vitro import of the Rieske iron-sulfur protein by trypanosome mitochondria

Most of the proteins present in the mitochondrion are imported to that location from the cytosol. While this process has been studied extensively in fungal and mammalian systems, little work has been done in other eukaryotic organisms. We are particularly interested in the Trypanosoma brucei system because this organism developmentally regulates mitochondrial function during its life cycle and because one of the imported proteins lacks a conventional targeting sequence. We report here the development of an in vitro import system using crude trypanosome mitochondria and a nuclear encoded, mitochondrial protein. Import of the Rieske iron-sulfur protein subunit of the cytochrome c reductase complex requires a membrane potential, ATP, and a protein component on the mitochondrial surface. The precursor protein is sequentially processed to the mature form in two steps by peptidases that require divalent metal ions for activity. As in other eukaryotic systems, the first processing event occurs inside the inner membrane and is probably catalyzed by a matrix-processing protease. Surprisingly, the second processing activity is located outside the inner membrane. Both processing steps require ATP but are independent of a membrane potential. We suggest that the trypanosome iron-sulfur protein is imported along a "conservative sorting pathway" but that the assembly mechanism of the reductase complex may be unique to trypanosomes.