Mechanistic Insights in the Biogenesis and Function of the Respiratory Chain

Sammanfattning: Mitochondria fulfill a plethora of functions, including harboring metabolic pathways and converting energy stored in metabolites into ATP, the common energy source of the cell. This last function is performed by the oxidative phosphorylation system, consisting of the respiratory chain and the ATP synthase. Electrons are channeled through the complexes of the respiratory chain, while protons are translocated across the inner mitochondrial membrane. This process establishes an electrochemical gradient, which is used by the ATP synthase to generate ATP. The subunits of two of the respiratory chain complexes, the bc1 complex and the cytochrome c oxidase, are encoded by two genetic origins, the nuclear and the mitochondrial genome. Therefore, the assembly of these complexes needs to be coordinated and highly regulated.Several proteins are involved in the biogenesis of the bc1 complex. Amongst these proteins, the Cbp3-Cbp6 complex was shown to regulate translation and assembly of the bc1 complex subunit cytochrome b. In this work, we established a homology model of yeast Cbp3. Using a site-specific crosslink approach, we identified binding sites of Cbp3 to its obligate binding partner Cbp6 and its client, cytochrome b, enabling a deeper insight in the molecular mechanisms of bc1 complex biogenesis. The bc1 complex and the cytochrome c oxidase form macromolecular structures, called supercomplexes. The detailed assembly mechanisms and functions of these structures remain to be solved. Two proteins, Rcf1 and Rcf2, were identified associating with supercomplexes in the yeast Saccharomyces cerevisiae. Our studies demonstrate that, while Rcf1 has a minor effect on supercomplex assembly, its main function is to modulate cytochrome c oxidase activity. We show that cytochrome c oxidase is present in three structurally different populations. Rcf1 is needed to maintain the dominant population in a functionally active state. In absence of Rcf1, the abundance of a population with an altered active site is increased. We propose that Rcf1 is needed, especially under a high work load of the respiratory chain, to maintain the function of cytochrome c oxidase.This thesis aims to unravel molecular mechanisms of proteins involved in biogenesis and functionality of respiratory chain complexes to enable a deeper understanding. Dysfunctional respiratory chain complexes lead to severe disease, emphasizing the importance of this work.