The dimerization of the yeast cytochrome bc1 complex is an early event and is independent of Rip1

In Saccharomyces cerevisiae the mature cytochrome bc(1) complex exists as an obligate homo-dimer in which each monomer consists of ten distinct protein subunits inserted into or bound to the inner mitochondrial membrane. Among them, the Rieske iron-sulfur protein (Rip1), besides its catalytic role in electron transfer, may be implicated in the bc(1) complex dimerization. Indeed, Rip1 has the globular domain containing the catalytic center in one monomer while the transmembrane helix interacts with the adjacent monomer. In addition, the lack of Rip1 leads to the accumulation of an immature bc(1) intermediate, only loosely associated with cytochrome c oxidase. In this study we have investigated the biogenesis of the yeast cytochrome bc(1) complex using epitope tagged proteins to purify native assembly intermediates. We showed that the dimerization process is an early event during bc(1) complex biogenesis and that the presence of Rip1, differently from previous proposals, is not essential for this process. We also investigated the multi-step model of bc(1) assembly thereby lending further support to the existence of bona fide subcomplexes during bc(1) maturation in the inner mitochondrial membrane. Finally, a new model of cytochrome bc(1) complex assembly, in which distinct intermediates sequentially interact during bc(1) maturation, has been proposed. (C) 2015 Elsevier B.V. All rights reserved.