Nonsense mutations in the COX1 subunit impair the stability of respiratory chain complexes rather than their assembly

Abstract

Respiratory chain (RC) complexes are organized into supercomplexes forming 'respirasomes'. The mechanism underlying the interdependence of individual complexes is still unclear. Here, we show in human patient cells that the presence of a truncated COX1 subunit leads to destabilization of complex IV (CIV) and other RC complexes. Surprisingly, the truncated COX1 protein is integrated into subcomplexes, the holocomplex and even into supercomplexes, which however are all unstable. Depletion of the m-AAA protease AFG3L2 increases stability of the truncated COX1 and other mitochondrially encoded proteins, whereas overexpression of wild-type AFG3L2 decreases their stability. Both full-length and truncated COX1 proteins physically interact with AFG3L2. Expression of a dominant negative AFG3L2 variant also promotes stabilization of CIV proteins as well as the assembled complex and rescues the severe phenotype in heteroplasmic cells. Our data indicate that the mechanism underlying pathogenesis in these patients is the rapid clearance of unstable respiratory complexes by quality control pathways, rather than their impaired assembly.

Stress-triggered Activation of the Metalloprotease Oma1 Involves Its C-terminal Region and Is Important for Mitochondrial Stress Protection in Yeast

“…Depletion of Oma1 in coa2⌬ cells leads to stabilization of newly synthesized Cox1 and allows its maturation to proceed (9,14). This situation is analogous to the recently reported case where attenuation of the mammalian m-AAA protease led to partial stabilization of a mutant Cox1 and restoration of cytochrome c oxidase levels (15). In mammalian cells, Oma1 activation was reported to occur in severely depolarized or ATP-depleted mitochondria (10,11).…”

Sealing the Mitochondrial Respirasome

“…Another intriguing CI-CIV interconnection based on a human cybrid cell line containing a truncated CIV Cox1 subunit was recently reported (25). The truncation conferred a marked steadystate CIV instability; although the mutant CIV assembled within the usual CI-CIII-CIV supercomplexes, stability was compromised, leading to a relative increase in the CI-CIII supercomplex.…”

Mitochondrial respiratory chain dysfunction: Implications in neurodegeneration

“…Importantly, this model aids explaining the structural interdependences among OXPHOS complexes, and why certain genetic defects affecting a single complex may lead to combined RC enzyme defects in patients. For instance, mutations in COX1 may lead to pleiotropic complex I defects (D'Aurelio et al, 2006;Hornig-Do et al, 2012) because the insertion of this complex IV subunit into supercomplexes occurs prior or in parallel with the incorporation of subunits from the complex I N catalytic module (NDUFV1 and NDUFS4). Consequently, severe structural abnormalities or the lack of COX1 affect the assembly or stability of NDUFS4 and NDUFV1 within supercomplexes.…”