Loss of COX4i1 leads to combined respiratory chain deficiency and impaired mitochondrial proteosynthesis

Oxidative phosphorylation (OXPHOS) system localized in the inner mitochondrial membrane secures production of the majority of ATP in mammalian organisms. Individual OXPHOS complexes were shown to form supramolecular assemblies termed supercomplexes. It has been repeatedly shown that complexes are not linked only by their function but also by interdependence of individual complex biogenesis or maintenance. For instance, cytochrome c oxidase (cIV, COX) or cytochrome bc1 complex (cIII) deficiencies affect the level of fully assembled NADH dehydrogenase (cI) in monomer as well as within supercomplexes. It was hypothesized that cI is affected at the level of enzyme assembly as well as at the level of cI stability and maintenance. However, the true nature of interdependency between cI and cIV is not fully understood yet. We used HEK293 cellular model with complete knockout of COX4 subunit, which serves as an ideal system to study interdependency of cI and cIV, as early phases of cIV assembly process are disrupted. Total absence of cIV was accompanied by profound deficiency of cI, documented by selective decrease in cI subunits amount and significantly reduced amount of assembled cI. Supercomplexes assembled from cI, cIII and cIV were missing in COX4dKO due to loss of cIV and decrease in cI amount. Pulse-chase metabolic labelling of mtDNA-encoded proteins uncovered decrease of cIV and cI subunits translation. Moreover, partial impairment of mitochondrial proteosynthesis correlated with decreased level of mitochondrial ribosomal proteins. In addition, complexome profiling approach uncovered accumulation of cI assembly intermediates indicating that cI biogenesis was affected rather than stability. We propose that impairment of mitochondrial proteosynthesis caused by cIV deficiency represents one of the mechanisms which may couple biogenesis of cI and cIV.