Abstract
Autophagy plays an important role in cellular quality control and is responsible for removing protein aggregates and dysfunctional organelles. Bnip3 is an atypical BH3-only protein that is known to cause mitochondrial dysfunction and cell death. Interestingly, Bnip3 can also protect against cell death by inducing mitochondrial autophagy. The mechanism for this process, however, remains poorly understood. Bnip3 contains a C-terminal transmembrane domain that is essential for homodimerization and proapoptotic function. In this study, we show that homodimerization of Bnip3 is also a requirement for induction of autophagy. Several Bnip3 mutants that do not interfere with its mitochondrial localization but disrupt homodimerization failed to induce autophagy in cells. In addition, we discovered that endogenous Bnip3 is localized to both mitochondria and the endoplasmic reticulum (ER). To investigate the effects of Bnip3 at mitochondria or the ER on autophagy, Bnip3 was targeted specifically to each organelle by substituting the Bnip3 transmembrane domain with that of Acta or cytochrome b(5). We found that Bnip3 enhanced autophagy in cells from both sites. We also discovered that Bnip3 induced removal of both ER (ERphagy) and mitochondria (mitophagy) via autophagy. The clearance of these organelles was mediated in part via binding of Bnip3 to LC3 on the autophagosome. Although ablation of the Bnip3-LC3 interaction by mutating the LC3 binding site did not impair the prodeath activity of Bnip3, it significantly reduced both mitophagy and ERphagy. Our data indicate that Bnip3 regulates the apoptotic balance as an autophagy receptor that induces removal of both mitochondria and ER.