PTEN-induced putative kinase 1 (Pink1) is a recently identified gene linked to a recessive form of familial Parkinson's disease (PD). The kinase contains a mitochondrial localization sequence and is reported to reside, at least in part, in mitochondria. However, neither the manner by which the loss of Pink1 contributes to dopamine neuron loss nor its impact on mitochondrial function and relevance to death is clear. Here, we report that depletion of Pink1 by RNAi increased neuronal toxicity induced by MPP ؉ . Moreover, wild-type Pink1, but not the G309D mutant linked to familial PD or an engineered kinase-dead mutant K219M, protects neurons against MPTP both in vitro and in vivo. Intriguingly, a mutant that contains a deletion of the putative mitochondrial-targeting motif was targeted to the cytoplasm but still provided protection against 1-methyl-4-phenylpyridine (MPP ؉ )/1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced toxicity. In addition, we also show that endogenous Pink1 is localized to cytosolic as well as mitochondrial fractions. Thus, our findings indicate that Pink1 plays a functional role in the survival of neurons and that cytoplasmic targets, in addition to its other actions in the mitochondria, may be important for this protective effect.Parkinson's disease ͉ neurodegeneration ͉ neuroprotection P arkinson's disease (PD) is a movement disorder with progressive loss of dopamine neurons in the substantia nigra pars compacta (SNc). The molecular events responsible for the loss of dopaminergic neurons in PD remain poorly understood. One common feature of PD is the dysfunction of mitochondria, which results in reduced complex I activity in the SNc (1, 2). Experimentally, inhibitors of complex I of the mitochondrial respiratory chain can recapitulate this selective dopaminergic neuronal loss and consequent behavioral deficits (1, 3-5). These observations support the hypothesis that nigral dopamine neurons are highly vulnerable to stress arising from mitochondrial dysfunction.Recently, several genes have been identified that cause PD (6). These genes include ␣-synuclein, parkin, PTEN-induced putative kinase 1 (Pink1), DJ-1, and LRRK2. Although several of the genes have been partially localized to the mitochondria, Pink1 is the only gene with a putative mitochondrial targeting motif. Several studies have shown that the mitochondrial targeting motif at the Nterminal region of Pink1 is sufficient to direct proteins to the mitochondria (7). Pink1 was initially identified as a PTEN-inducible transcript and contains a serine/threonine kinase domain (8). Interestingly, the G309D mutation of the kinase domain leads to a mild decrease in mitochondrial complex I activity, elevation of superoxide radicals, and increased lipid peroxidation (9). Studies with Drosophila lacking Pink1 showed mitochondrial pathology with the similar phenotype as seen in Parkin knockout flies (10, 11). The above observations suggest that mitochondrial dysfunction may be linked to the Pink1 PD phenotype.The mechanisms by which Pi...