Structure and function of the <i>S. pombe</i> III-IV-cyt <i>c</i> supercomplex.

Abstract

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 (CIII₂), via a mobile electron carrier, cytochrome <i>c</i>. In the present study, we isolated the CIII₂CIV supercomplex from the fission yeast <i>Schizosaccharomyces pombe</i> and determined its structure with bound cyt. <i>c</i> 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 Zn²⁺, coordinated in the CIII subunit Cor1, which is encoded by the same gene (<i>qcr</i><i>1</i>) as the mitochondrial-processing peptidase subunit β. Our data show that the isolated CIII₂CIV supercomplex displays proteolytic activity suggesting a dual role of CIII₂ in <i>S. pombe</i>. As in the supercomplex from <i>S. cerevisiae</i>, subunit Cox5 of CIV faces towards one CIII monomer, but in <i>S. pombe,</i> the two complexes are rotated relative to each other by ~45°. This orientation yields equal distances between the cyt. <i>c</i> binding sites at CIV and at each of the two CIII monomers. The structure shows cyt. <i>c</i> 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 CIII₂ respiratory system and indicates a two-dimensional cyt. <i>c</i> diffusion mechanism within the CIII₂-CIV supercomplex.