Mitochondrial Ubiquitination as a Signaling Hub: Balancing Mitophagy, Inflammation, and Cell Death

Mitochondria are increasingly recognized as signaling organelles that coordinate cell-fate decisions during stress. Because outer mitochondrial membrane (OMM) proteins are exposed to the cytosol, they are prominent substrates for ubiquitination, a dynamic post-translational modification that encodes information through diverse chain architectures and linkage types. In this review, we examine how ubiquitination of OMM proteins functions as a molecular switch that integrates mitochondrial stress signals and engages three major, often antagonistic, stress-response mechanisms: mitophagy, cell death, and innate immune signaling. We highlight an emerging concept that a stress-responsive "ubiquitin code" is written on OMM substrates, in which pathway selection is coordinated by the identity of ubiquitinated OMM proteins together with the linkage type and branching of attached polyubiquitin chains. We provide an updated overview of the E3 ubiquitin ligases and deubiquitinases (DUBs) that write and erase this code and summarize ubiquitin linkage types reported on key OMM substrates across these pathways. For mitophagy, we cover both PARKIN-dependent and PARKIN-independent mechanisms mediated by other E3 ligases and counteracted by DUBs. For innate immunity, we discuss how ubiquitination of OMM proteins regulates the MDA5/RIG-I-MAVS axis and NF-κB signaling. For cell death, we describe how ubiquitination of anti- and pro-apoptotic BCL-2 family proteins can either lower or increase the threshold for the induction of apoptosis. We also highlight the newfound role of PARKIN to drive apoptosis through a BAX/BAK-independent mechanism. Finally, we discuss therapeutic opportunities to reprogram OMM ubiquitination by targeting E3 ligases or DUBs directly, or by using PROTAC- and DUBTAC-based strategies.