Structure and function of the III-IV-cyt supercomplex.

The respiratory chain in aerobic organisms is composed of a number of membrane-bound protein complexes that link electron transfer to proton translocation across the membrane. In mitochondria, the final electron acceptor, complex IV (CIV), receives electrons from dimeric complex III (CIII), via a mobile electron carrier, cytochrome . In the present study, we isolated the CIIICIV supercomplex from the fission yeast and determined its structure with bound cyt. using single-particle electron cryomicroscopy. A respiratory supercomplex factor 2 was found to be bound at CIV distally positioned in the supercomplex. In addition to the redox-active metal sites, we found a metal ion, presumably Zn, coordinated in the CIII subunit Cor1, which is encoded by the same gene () as the mitochondrial-processing peptidase subunit β. Our data show that the isolated CIIICIV supercomplex displays proteolytic activity suggesting a dual role of CIII in . As in the supercomplex from , subunit Cox5 of CIV faces towards one CIII monomer, but in the two complexes are rotated relative to each other by ~45°. This orientation yields equal distances between the cyt. binding sites at CIV and at each of the two CIII monomers. The structure shows cyt. bound at four positions, but only along one of the two symmetrical branches. Overall, this combined structural and functional study reveals the integration of peptidase activity with the CIII respiratory system and indicates a two-dimensional cyt. diffusion mechanism within the CIII-CIV supercomplex.