Novel insights into the assembly and function of human nuclear-encoded cytochrome c oxidase subunits 4, 5a, 6a, 7a and 7b
Abstract
Mammalian CcO (cytochrome c oxidase) is a complex composed of 13 different structural subunits encoded by both the mitochondrial and nuclear genomes. Knockdown of Cox4, Cox5a and Cox6a resulted in reduced CcO activity, diminished affinity of the residual enzyme for oxygen, decreased holoCcO and CcO dimer levels, increased accumulation of CcO subcomplexes and an altered pattern of respiratory supercomplexes.
An analysis of the patterns of CcO subcomplexes found in both knockdowns and cells overexpressing epitope-tagged Cox6a, Cox7a and Cox7b identified a novel CcO assembly intermediate, identified the entry points of three late-assembled subunits and demonstrated the interdependence of Cox4 and Cox5a assembly. The ectopic expression of the heart/muscle-specific isoform of the Cox6 subunit (COX6A2) resulted in restoration of both CcO holoenzyme and activity in COX6A1-knockdown cells contrary to the unaltered levels of COX6A2 mRNA in these cells, suggesting the existence of a fixed expression programme.