Formation of I2+III2 supercomplex rescues respiratory chain defects

Mitochondrial electron transport chain (ETC) complexes partition between free complexes and quaternary assemblies known as supercomplexes (SCs). However, the physiological requirement for SCs and the mechanisms regulating their formation remain controversial. Here, we show that genetic perturbations in mammalian ETC complex III (CIII) biogenesis stimulate the formation of a specialized extra-large SC (SC-XL) with a structure of I2+III2, resolved at 3.7 Å by cryoelectron microscopy (cryo-EM). SC-XL formation increases mitochondrial cristae density, reduces CIII reactive oxygen species (ROS), and sustains normal respiration despite a 70% reduction in CIII activity, effectively rescuing CIII deficiency. Consequently, inhibiting SC-XL formation in CIII mutants using the Uqcrc1DEL:E258-D260 contact site mutation leads to respiratory decompensation. Lastly, SC-XL formation promotes fatty acid oxidation (FAO) and protects against ischemic heart failure in mice. Our study uncovers an unexpected plasticity in the mammalian ETC, where structural adaptations mitigate intrinsic perturbations, and suggests that manipulating SC-XL formation is a potential therapeutic strategy for mitochondrial dysfunction. [Display omitted] •SC-XL (I2+III2) is a unique mammalian SC that mitigates CIII deficiency•SC-XL formation is a mitohormetic response to coenzyme Q over-reduction•SC-XL supports efficient OXPHOS with reduced CIII reactive oxygen species•SC-XL is associated with protection against ischemic heart failure Respiratory supercomplexes are conserved throughout evolution, but their physiological relevance remains contentious. Here, Liang et al. discover an I2+III2 supercomplex (SC-XL) triggered by complex III deficiency that rescues respiration by enhancing OXPHOS and reducing ROS production. These findings indicate that supercomplexes are indispensable for maintaining mitochondrial homeostasis and integrity during metabolic stress.