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The role of Ubiquinone Synthesis Complex in bc1 complex assembly of Saccharomyces cerevisiae




Hashemi, Hayaa

Type of Degree



Biological Sciences


Mitochondria are dynamic organelles bounded by double-membranes found in eukaryotic cells carrying various critical cellular processes. Energy generation is arguably the most important of its functions. The Electron Transport Chain (ETC) is a series of protein complexes found in the inner mitochondrial membrane that possesses redox activities, by virtue of which they transfer electrons to the final electron acceptor, oxygen. This creates a proton motive force that in turn drives ATP production. The bc1 complex is the central component of the ETC chain and it functions in transporting electrons from Ubiquinone, a mobile electron carrier to Cytochrome c. It is made up of ten protein subunits, out of which only Cytochrome b (Cob) is encoded in the mitochondrial genome. Cob needs heme b as a co-factor and its translation is under a feed-back control that is activated in the absence of other bc1 subunits. In addition to Cob, Cytochrome c1 (cyt1) and Reiske Iron-Sulphur protein (Rip1) are the other two catalytic redox active subunits. The other seven subunits support the structure of bc1. The mechanism in which the bc1 complex subunits assemble is not fully defined. It is known to proceed in a step-wise fashion where intermediate sub-complexes of various bc1 subunits progressively combine to form a fully functional enzyme. The goal of this study was to identify and define mechanisms of bc1 assembly steps. The effect of deletion of Ubiquinone synthesis complex on the assembly of the bc1 complex was investigated. The Ubiquinone synthesis complex is a ten-protein subunit (Coq1-Coq10) complex that is needed for synthesizing ubiquinone, an anti-oxidant molecule that participates in ETC by donating electrons to bc1. Protein subunits of this complex are interdependent for stability. A coq9 deletion strain was found to be defective in respiratory growth and bc1 activity. The respiratory activities of this mutant could not be rescued by ubiquinone supplementation or COQ8 over-expression that generally stabilizes several Coq subunits indicating that the entire ubiquinone synthesis complex is necessary for stability of the bc1 complex. Transient physical interactions between bc1 complex and Coq complex are suggested by the co-migration of Qcr7 and Coq9 on sucrose density gradients. Basing on the data, a revised model for bc1 assembly is proposed where the ubiquinone synthesis proteins interact with the early bc1 assembly intermediate containing Cob and Qcr7. This interaction is a chaperoning function that stabilizes Qcr7 whose availability allows Cob translation. In the absence of a stabilizing Coq complex, Qcr7 is degraded allowing a feedback inhibition of Cob translation and stalling of bc1 assembly. Therefore, this study has identified a novel role of Coq complex proteins in the bc1 complex assembly that had not been described previously. Identification of this step can not only be helpful in understanding bc1 disorder pathologies but it may also be useful in devising strategies for therapy by providing an alternative target for bc1 disorders- the ubiquinone biosynthesis proteins.