Endosomal RFFL ubiquitin ligase regulates mitochondrial morphology by targeting mitofusin 2

Mitochondrial homeostasis is ensured through communication between diverse cellular organelles, including mitochondria, the endoplasmic reticulum (ER), lysosomes and endosomes. Although it is known that mitofusins regulate mitochondrial networks and ER contacts, their role in endosomal-mitochondrial interactions remains unclear. Previously, we have reported that vesicles positive for the endosomal ubiquitin ligase RFFL are associated with damaged mitochondria and prime the organelle for PRKN recruitment. Now, we establish that RFFL is a ubiquitin ligase for mitofusin 2 (MFN2). Using electron microscopy and confocal imaging analyses, we demonstrate that RFFL-knockout cells exhibit enlarged mitochondrial morphology. RFFL interacts at an endogenous level with MFN2 and contributes to its ubiquitylation upon mitochondrial damage. Recombinant RFFL interacts and ubiquitylates MFN2 protein in vitro. Furthermore, exogenous RFFL, in a ligase-dependent manner, specifically reduces the exogenous protein levels of both MFN1 and MFN2, but not that of DRP1, and also perturbs lipid homeostasis. Importantly, we show that the hyperfused mitochondria morphology reported with expression of pathogenic disease mutants of MFN2 (T206I and R364W) of Charcot-Marie-Tooth disease type 2A can be rescued by RFFL co-expression. The study unravels novel mechanisms involving endosomal ubiquitin ligases in mitochondrial networks.