CHARACTERIZATION OF THE BIFUNCTIONAL CYTOCHROME-C REDUCTASE-PROCESSING PEPTIDASE COMPLEX FROM POTATO MITOCHONDRIA

In potato, cytochrome c reductase, a protein complex of the respirator y chain, exhibits processing activity toward mitochondrial precursor p roteins. One of the two cooperating components of the processing pepti dase was shown to be identical with subunit III of the complex. Here w e report that two additional proteins of the complex (subunit I and II ) share 40-50% sequence identity with the processing enhancing protein , the other component of the processing enzyme from fungi and mammals. Thus the composition and structure of the complex integrated processi ng peptidase seems to be different from its fungal and mammalian count erparts. Cytochrome c reductase from potato is extraordinarily stable, and separation of subunit III from the complex leads to aggregation o f the remaining subcomplex and irreversible loss of processing activit y. Expression of the three high molecular weight subunits of the compl ex allowed purification of each individual protein. Neither the indivi dual subunits nor their combinations are active in in vitro processing assays suggesting that they may need the structural support of the co mplex for activity. In contrast to mitochondrial processing peptidases from other organisms, the purified potato enzyme is active in the pre sence of high salt (above 1 M NaCl) and works efficiently without addi tion of metal ions. These data indicate that potato cytochrome c reduc tase is a bifunctional protein complex with unique features. Possibly, there is a more general evolutionary relationship between cytochrome c reductases and mitochondrial processing peptidases than hitherto ass umed.