Abstract
Mitochondrial respiratory chain complexes I, III, and IV can associate into larger structures termed supercomplexes or respirasomes, thereby generating structural interdependences among the individual complexes yet to be understood. In patients, nonsense mutations in complex IV subunit genes cause severe encephalomyopathies randomly associated with pleiotropic complex I defects. Using complexome profiling and biochemical analyses, we have explored the structural rearrangements of the respiratory chain in human cell lines depleted of the catalytic complex IV subunit COX1 or COX2. In the absence of a functional complex IV holoenzyme, several supercomplex I+III2 species coexist, which differ in their content of COX subunits and COX7A2L/HIGD2A assembly factors. The incorporation of an atypical COX1‐HIGD2A submodule attenuates supercomplex I+III2 turnover rate, indicating an unexpected molecular adaptation for supercomplexes stabilization that relies on the presence of COX1 independently of holo‐complex IV formation. Our data set the basis for complex I structural dependence on complex IV, revealing the co‐existence of alternative pathways for the biogenesis of “supercomplex‐associated” versus individual complex IV, which could determine physiological adaptations under different stress and disease scenarios.
Synopsis
The mitochondrial respiratory chain (MRC) drives aerobic energy transduction in eukaryotic cells and consists of five multisubunit enzyme complexes that can associate to form supercomplexes (SCs). Availability of complex IV subunit COX1 and assembly factor HIGD2A promotes the formation of submodules that directly bind and stabilize respiratory SC I+III2 in human cells.
Human cells lacking complex IV display diverse SC I+III2 species.
Alternative SC I+III2 species differ in their COX subunits and assembly factors content.
SC I+III2 stability is enhanced upon binding of an atypical COX1‐HIGD2A submodule.
Functional preservation of SC I+III2 requires COX1 but is independent of holo‐COX formation.
Different assembly lines exist to synthesize free‐ versus SC‐associated complex IV.
Availability of complex IV subunit COX1 and assembly factor HIGD2A promotes the formation of submodules that directly bind and stabilize respiratory supercomplex I+III2 in human cells.
