The Gp78 ubiquitin E3 ligase drives mitophagy by regulating omegasome formation
Autophagy is initiated at endoplasmic reticulum (ER)-associated omegasomes, yet how the ER contributes to mitochondrial cargo degradation during early autophagosome formation remains unclear. Gp78 is an ER-resident RING E3 ubiquitin ligase involved in ER-associated degradation (ERAD) that also promotes Parkin-independent mitophagy. However, the spatial relationship between Gp78 ubiquitin ligase activity, omegasome formation, and mitochondrial degradation has yet to be defined. Combining targeted mitochondria labeling and super-resolution STED microscopy, we show that the Gp78 ubiquitin ligase is localized to and regulates DFCP1-positive omegasome formation, where it drives mitochondrial protein breakdown. Using fluorescent mitophagy reporters and acute ivermectin-induced mitophagy, generation of mitophagy intermediates lacking mitochondrial outer membrane (TOMM20) was found to be dependent on the Gp78 RING finger as well as the BAG6-UBL4A-USP13 ubiquitin-ligase module, and independent of PINK1 and Parkin. Object-based analysis of 3D STED images further showed that both DFCP1- and LC3-positive structures associate primarily with intact mitochondrial fragments which show progressive degradation of mitochondrial content. Gp78 localizes to and promotes, via BAG6, the formation of DFCP1-positive omegasomes and mitochondrial protein degradation in these early autophagic structures. ER-derived omegasome biogenesis is therefore coupled to Gp78-dependent ubiquitin ligase activity and local degradation of mitochondrial cargo during mitophagy.