Complex II assembly drives metabolic adaptation to OXPHOS dysfunction

During acute oxidative phosphorylation (OXPHOS) dysfunction, the reverse activity of succinate dehydrogenase (Complex II) maintains the redox state of the Coenzyme Q pool by utilizing either fumarate or oxygen as terminal electron acceptors. The tendency for one over another has been suggested to be tissue specific. We identified the action of SDHAF2 protein, a Complex II assembly factor that enhances the flavination of catalytic subunit SDHA, as critical for metabolic adaptation during Complex III dysfunction in HEK293T cells. Loss of SDHAF2 during Complex III inhibition led to a net reductive TCA cycle from loss of succinate oxidation, loss of SDHA active site derived reactive oxygen species (ROS)-signaling, insufficient adaptation to glycolysis, and a severe growth impairment. Cell lines adapted to glycolysis did not accumulate SDHAF2 upon Coenzyme Q-pool stress, exhibited a net reductive TCA cycle and mild growth phenotypes with or without SDHAF2 being present. Thus, our study reveals how Complex II assembly controls a balance between dynamics of TCA cycle directionality, protection from Q-pool stress, and an ability to utilize ROS-meditated signaling to overcome acute OXPHOS dysfunction in cells reliant on mitochondrial respiration.