Abstract
Autosomal recessive loss-of-function mutations within the PARK2 gene functionally inactivate the E3 ubiquitin ligase parkin, resulting in neurodegeneration of catecholaminergic neurons and a familial form of Parkinson disease. Current evidence suggests both a mitochondrial function for parkin and a neuroprotective role, which may in fact be interrelated. The antiapoptotic effects of parkin have been widely reported, and may involve fundamental changes in the threshold for apoptotic cytochrome c release, but the substrate(s) involved in parkin dependent protection had not been identified. Here, we demonstrate the parkin-dependent ubiquitination of endogenous Bax comparing primary cultured neurons from WT and parkin KO mice and using multiple parkin-overexpressing cell culture systems. The direct ubiquitination of purified Bax was also observed in vitro following incubation with recombinant parkin. We found that parkin prevented basal and apoptotic stress-induced translocation of Bax to the mitochondria. Moreover, an engineered ubiquitination-resistant form of Bax retained its apoptotic function, but Bax KO cells complemented with lysine-mutant Bax did not manifest the antiapoptotic effects of parkin that were observed in cells expressing WT Bax. These data suggest that Bax is the primary substrate responsible for the antiapoptotic effects of parkin, and provide mechanistic insight into at least a subset of the mitochondrial effects of parkin.