Emerging evidence has demonstrated a growing genetic component in Parkinson disease (PD). For instance, loss-of-function mutations in PINK1 or PARKIN can cause autosomal recessive PD. Recently, PINK1 and PARKIN have been implicated in the same signaling pathway to regulate mitochondrial clearance through recruitment of PARKIN by stabilization of PINK1 on the outer membrane of depolarized mitochondria. The precise mechanisms that govern this process remain enigmatic. In this study, we identify Bcl2-associated athanogene 2 (BAG2) as a factor that promotes mitophagy. BAG2 inhibits PINK1 degradation by blocking the ubiquitination pathway. Stabilization of PINK1 by BAG2 triggers PARKIN-mediated mitophagy and protects neurons against 1-methyl-4-phenylpyridinium-induced oxidative stress in an in vitro cell model of PD. Collectively, our findings support the notion that BAG2 is an upstream regulator of the PINK1/PARKIN signaling pathway. Parkinson disease (PD)2 is the second most common neurodegenerative disorder, characterized by selective loss of the pigmented dopaminergic neurons of the substantia nigra pars compacta (1). Although most PD cases are sporadic in nature (2), mutations in several genes have been linked to familial forms of PD (reviewed in Ref.3). Indeed, these familial genes serve as important vehicles to study the potential mechanisms of pathogenesis in PD. In this regard, increasing evidence suggests that mitochondrial dysfunction may play a critical role in both the inherited and sporadic forms of PD, although the precise role of mitochondrial dysfunction in PD is unclear (4). Recently, two familial recessive PD genes, PTEN-induced putative kinase 1 (PINK1), a mitochondrially localized serine/threonine kinase gene, and PARKIN, an E3 ubiquitin ligase gene, have been identified as acting along similar pathways in regulating mitochondrial quality control in mammalian systems (5-8). These findings are supported by genetic studies in Drosophila models of PD that show that PARKIN and PINK1 function in a common pathway, with PARKIN being a downstream player of PINK1 (9 -11). A third recessive PD gene, DJ-1, associated with the regulation of oxidative stress, also regulates PINK1/PARKIN-mediated control of mitochondrial health (12).PINK1 is normally shuttled to the inner mitochondrial membrane where it is processed by multiple proteases (13). The endogenous role of PINK1 at this site is unknown. However, PINK1-deficient cells display altered calcium homeostasis at the mitochondria as well as altered mitochondrial function (14). Processed PINK1 at the inner mitochondrial membrane has also been proposed to be shuttled to the cytosol, where it is degraded by the proteasome (15). However, under conditions of mitochondrial stress such as treatment with the mitochondrial uncoupler carbonyl cyanide p-chlorophenylhydrazone (CCCP) or with the complex I inhibitor 1-methyl-4-phenylpyridinium (MPP ϩ ), PINK1 can also regulate mitophagy in a PARKIN-dependent fashion (12, 16 -19). The framework by which this mitochondrial ...