Structural basis and regulation of the reductive stress response

Although oxidative phosphorylation is best known for producing ATP, it also yields reactive oxygen species (ROS) as invariant byproducts. Depletion of ROS below their physiological levels, a phenomenon known as reductive stress, impedes cellular signaling and has been linked to cancer, diabetes, and cardiomyopathy. Cells alleviate reductive stress by ubiquitylating and degrading the mitochondrial gatekeeper FNIP1, yet it is unknown how the responsible E3 ligase CUL2FEM1B can bind its target based on redox state and how this is adjusted to changing cellular environments. Here, we show that CUL2FEM1B relies on zinc as a molecular glue to selectively recruit reduced FNIP1 during reductive stress. FNIP1 ubiquitylation is gated by pseudosubstrate inhibitors of the BEX family, which prevent premature FNIP1 degradation to protect cells from unwarranted ROS accumulation. FEM1B gain-of-function mutation and BEX deletion elicit similar developmental syndromes, showing that the zinc-dependent reductive stress response must be tightly regulated to maintain cellular and organismal homeostasis. [Display omitted] •CUL2FEM1B recognizes reduced FNIP1 through two interface zinc ions•Zn2+ is essential for reductive stress signaling•FNIP1 access to CUL2FEM1B is gated by BEX protein pseudosubstrate inhibitors•Mutation of FEM1B and BEX deletion cause similar developmental syndromes Different from protein interactions mediated by amino acid side chains, the E3 ligase CUL2FEM1B selectively recruits the reduced FNIP1 that emerges upon reductive stress through zinc ions at the interface of E3 ligase and substrate. This interaction is gated by pseudosubstrate inhibitors of the BEX family.