Mild phenotypes and proper supercomplex assembly in human cells carrying the homoplasmic m.15557G > A mutation in cytochromebgene

Abstract

Respiratory complex III (CIII) is the first enzymatic bottleneck of the mitochondrial respiratory chain both in its native dimeric form and in supercomplexes. The mammalian CIII comprises 11 subunits among which cytochrome b is central in the catalytic core, where oxidation of ubiquinol occurs at the Qo site. The Qo-or PEWY-motif of cytochrome b is the most conserved through species. Importantly, the highly conserved glutamate at position 271 (Glu271) has never been studied in higher eukaryotes so far and its role in the Q-cycle remains debated. Here, we showed that the homoplasmic m.15557G > A/MT-CYB, which causes the p.Glu271Lys amino acid substitution predicted to dramatically affect CIII, induces a mild mitochondrial dysfunction in human transmitochondrial cybrids. Indeed, we found that the severity of such mutation is mitigated by the proper assembly of CIII into supercomplexes, which may favor an optimal substrate channeling and buffer superoxide production in vitro.

Fine-tuning of the respiratory complexes stability and supercomplexes assembly in cells defective of complex III

“…This allows a residual CI+CIII combined redox activity, and an associated low but detectable ATP synthesis driven by CI substrates only (Carossa et al [27] and this work). This result supports our previous hypothesis that the assembly of CIII 2 into the CI + CIII 2 SC could mitigate the detrimental effect of MT-CYB mutations [29,37]. This observation nicely corroborates the assumption that the evolutionarily conserved CI+CIII 2 core complexes might play an important role in respiratory electron transfer and respiratory complexes stability [4,5,23,38].…”

Mitochondrial dysfunction: A hidden trigger of autism?

“… 40 When mutational analysis was conducted, homoplasmic mutations were observed in the cytb gene affecting CIII activity. 41 A reduction in the mt-DNA content of the cytb gene was also observed along with mitochondrial hyperproliferation and respiratory chain block in CIII in 25 children with autism. 42 Additionally, quantity of CI, CIII and CV proteins were higher in the cecum biopsies of 10 autistic children as compared to controls.…”

Organization of the Respiratory Supercomplexes in Cells with Defective Complex III: Structural Features and Metabolic Consequences

“…These data suggest that regardless of the mutation, there is a tendency to maintain respiratory complex stability and SC assembly to mitigate CIII dysfunction. This latter piece of evidence is supported by recent papers showing that missense mutations in MTCYB that induce defective CIII enzymatic activity when detected in the isolated complex, are mitigated when CIII activity is measured under conditions in which the respiratory complex is organized into SCs [129,132]. Although further work is clearly needed, recent structural details obtained from cryo-EM analysis of active SC particles from sheep mitochondria highlighted the specific involvement of cytochrome b in the crosstalk between CI and CIII 2 , confirming its role in structural/functional interactions between the two complexes [74].…”

“…MTCYB that induce defective CIII enzymatic activity when detected in the is plex, are mitigated when CIII activity is measured under conditions in which tory complex is organized into SCs [129,132]. Although further work is clea recent structural details obtained from cryo-EM analysis of active SC particles mitochondria highlighted the specific involvement of cytochrome b in the c tween CI and CIII2, confirming its role in structural/functional interactions two complexes [74].…”