Reactive oxygen species control protein degradation at the mitochondrial import gate

Abstract

While reactive oxygen species (ROS) have long been known to drive aging and neurodegeneration, their persistent depletion below basal levels also disrupts organismal function. Cells counteract loss of basal ROS via the reductive stress response, but the identity and biochemical activity of ROS sensed by this pathway remain unknown. Here, we show that the central enzyme of the reductive stress response, the E3 ligase Cullin 2-FEM1 homolog B (CUL2 FEM1B ), specifically acts at mitochondrial TOM complexes, where it senses ROS produced by complex III of the electron transport chain (ETC). ROS depletion during times of low ETC activity triggers the localized degradation of CUL2 FEM1B substrates, which sustains mitochondrial import and ensures the biogenesis of the rate-limiting ETC complex IV. As complex III yields most ROS when the ETC outpaces metabolic demands or oxygen availability, basal ROS are sentinels of mitochondrial activity that help cells adjust their ETC to changing environments, as required for cell differentiation and survival. • TOM20 recruits the E3 ligase CUL2 FEM1B to the mitochondrial outer membrane • CUL2 FEM1B targets key substrates selectively at mitochondrial TOM complexes • CUL2 FEM1B senses complex III-derived ROS • Regulated protein degradation by CUL2 FEM1B controls mitochondrial import This work identifies reactive oxygen species released by the electron transport chain as sentinel molecules that regulate localized protein degradation at mitochondrial TOM complexes, a mechanism that controls mitochondrial protein import and adjusts the abundance of the electron transport chain to cellular needs.