Bnip3 interacts with LC3 to induce selective removal of endoplasmic reticulum and mitochondria via autophagy

Bcl-2 family proteins are known to regulate mitochondrial integrity and apoptosis. More recently, they have been found to play a role in regulating autophagy. Autophagy is a process involved in removing excess or damaged organelles. Bnip3 is a pro-apoptotic BH3-only protein which is known to cause mitochondrial dysfunction and cell death. Bnip3 is also a potent inducer of mitochondrial autophagy. In this study I have investigated the mechanism by which Bnip3 promotes removal of mitochondria via autophagy. Bnip3 contains a C-terminal transmembrane (TM) domain that is essential for homodimerization and pro- apoptotic function. Here, I show that Bnip3 homodimerization is also a requirement for induction of autophagy. Mutations in Bnip3 that disrupt homodimerization, but do not interfere with mitochondrial localization, failed to induce autophagy. In addition, I found that endogenous Bnip3 was localized to both the mitochondria and the endoplasmic reticulum (ER) in HeLa cells. To investigate the effects of Bnip3 at ER on autophagy, Bnip3 was targeted specifically to mitochondria or ER by substituting the Bnip3 TM domain with that of Acta or cytochromeb5, respectively. Interestingly, Bnip3 induced significant autophagy in cells from both sites. Moreover, Bnip3 induced removal of mitochondria and ER by autophagy via binding to LC3 on the autophagosome. Ablation of the Bnip3-LC3 interaction had no effect on the induction of general autophagy but significantly reduced autophagy of mitochondria and ER. Thus, our data suggest that the Bnip3 homodimer functions as an autophagy receptor to ensure removal of mitochondria and ER