Knockdown of human COX17 affects assembly and supramolecular organization of cytochrome c oxidase

Assembly of cytochrome c oxidase, the terminal enzyme of the mitochondrial respiratory chain, requires a concerted activity of a number of chaperones and factors for the insertion of subunits, accessory proteins, cofactors and prosthetic groups. It is now well accepted that the multienzyme complexes of the respiratory chain are organized in vivo as supramolecular functional structures, so-called supercomplexes. Here, we investigate the role of COX17 in the biogenesis of the respiratory chain in HeLa cells. In accordance with its predicted function as a copper chaperone and its role in formation of the binuclear copper centre of cytochrome c oxidase, COX17 siRNA knockdown affects activity and assembly of cytochrome c oxidase. While the abundance of cytochrome c oxidase dimers seems to be unaffected, blue native gel electrophoresis reveals the disappearance of COX-containing supercomplexes as an early response. We observe the accumulation of a novel approximately 150 kDa complex that contains Cox1, but not Cox2. This observation may indicate that the absence of Cox17 interferes with copper delivery to Cox2, but not to Cox1. We suggest that supercomplex formation is not simply due to assembly of completely assembled complexes. An interdependent assembly scenario for the formation of supercomplexes that rather requires the coordinated synthesis and association of individual complexes, is proposed.