Organization of assembly factors Cbp3p and Cbp4p and their effect on bc(1) complex assembly in Saccharomyces cerevisiae.

The bc(1) complex (complex III) of Saccharomyces cerevisae is composed of ten subunits that are assembled in the inner mitochondrial membrane. Cbp3p and Cbp4p are two mitochondrial proteins which are postulated to act as chaperones in bc(1) complex formation. Here, we show by blue native PAGE that cbp3Delta and cbp4Delta mutants are disturbed in complex III assembly and accumulate intermediate-sized forms of the complex. Moreover, deletion of CBP3 interferes with the formation of complex III/IV supracomplexes. Our studies show that Cbp3p and Cbp4p interact and are present in high-molecular-weight complexes, some of which might represent intermediates of complex III assembly. Overexpression of Cbp4p cannot substitute for the function of Cbp3p, but high-level expression of Cbp3p can partially compensate for the lack of Cbp4p. The finding that mitochondria of cbp3Delta and cbp4Delta mutants exhibit a wild-type lipid composition favors the idea that Cbp3p and Cbp4p are specific assembly factors for complex III rather than components of the mitochondrial lipid metabolism.