Respiratory Chain Complexes and Supercomplexes Organization in Cells with Defective Complex III

Cytochrome b is the only subunit of complex III (CIII) encoded by the mitochondrial DNA. Constituting the central core of the enzyme, the protein is essential for both assembly and catalytic activity of the complex. CIII can associate with complex I (CI) and complex IV to form supercomplexes (SCs). MTCYB mutations can affect CIII only or both CIII and CI, as a consequence of the importance of CIII on the stability of CI. Here, we have investigated the effects of two pathogenic mutations affecting MTCYB: the p.278Y>C missense mutation, causing the substitution of conserved Tyr278 close to the QO site, and the ΔI300-P305 microdeletion, producing the loss of six aminoacids in the sixth transmembrane helix, but leaving the remaining of the MTCYB in frame. We have demonstrated that both MTCYB mutations severely impaired the activity of CIII: the missense mutation produced an oxidative damage of CIII due to increased superoxide production, whereas in cells bearing the ΔI300-P305 microdeletion, CIII was not detected, with consequent derangement also of CI. The detailed analysis of SCs organization revealed in both cases a strong perturbation of the CIII2+IV SC, together with an attempt to preserve the respirasome. These results favor the hypothesis that SCs not only preserve the structure and stability of respiratory complexes, but are essential for attenuating the mitochondrial dysfunction due to pathogenic mutations affecting the respiratory enzymes. Furthermore, the cells bearing ΔI300-P305 deletion showed a marked increase in complex II (CII) redox activity, associated with significant hydrogen peroxide production. It has been suggested that the enhanced CII activity is a compensatory mechanism due to the lacking of CI. Our results instead suggest that it might be a more general phenomenon for cell adaptation to respiratory chain dysfunction, being detected also in CIII-deficient cells where the hydrogen peroxide production is increased.