Parkinson disease (PD) is the most common neurodegenerative movement disorder, yet the pathogenic mechanisms underlying the disease remain unclear. Although the majority of PD cases are idiopathic in nature, a small percentage of PD is associated with the inheritance of genetic mutations and is referred to as familial PD (1
). Approximately 50% of all recessively transmitted early-onset familial PD cases are caused by homozygous mutations in parkin
). Homozygous mutations in PTEN-induced putative kinase 1 (PINK1)
are the second most common cause of early-onset recessive PD (5
). In addition, heterozygous mutations in parkin
have been implicated as significant risk factors in the development of late-onset PD (2
). Despite compelling genetic evidence linking parkin
mutations to PD, the molecular mechanisms by which the pathogenic mutations trigger neurodegeneration are poorly understood.
gene encodes a 465 amino acid cytosolic E3 ubiquitin-protein ligase that is expressed in many tissues and cell types (3
). Parkin has been reported to regulate K48-linked polyubiquitination and proteasomal degradation of several putative substrate proteins (4
), although it remains controversial regarding the validity of these proteins as physiological parkin substrates (8
). We and others have recently shown that parkin can catalyze monoubiquitination and K63-linked polyubiquitination and function in a proteasome-independent manner for regulating endocytosis (11
), NF-κB signaling (12
) and aggresome formation (13
). The importance of parkin E3 ligase activity to neuronal survival is highlighted by the finding that most PD-linked pathogenic parkin
mutations cause severe impairment of parkin E3 ligase function (2
). At present, very little is known about how parkin E3 ligase function is regulated in cells.
gene encodes a 581 amino acid putative mitochondrial serine/threonine kinase that is expressed in many tissues and cell types, including dopaminergic neurons (5
). We and others have shown that recombinant PINK1 protein exhibits kinase activity in vitro
) and that the PINK1 kinase activity is required for its cytoprotective function (20
). Consistent with the presence of a mitochondrial targeting sequence at its N-terminus, PINK1 is localized to the mitochondria, where it is mainly found in the mitochondrial inner membrane and intermembrane space (14
), although a fraction of PINK1 may exist in the mitochondrial outer membrane with the kinase domain facing the cytosol (22
). There is also a cytosolic pool of PINK1 (16
), as expected because PINK1 is encoded by the nuclear genome. Together, current data suggest that PINK1 could exert cytoprotective action through phosphorylation of mitochondrial as well as cytosolic substrate proteins.
The fact that homozygous mutations in parkin
cause autosomal recessive PD indicates that the normal function of these proteins is required for cell survival, particularly the survival of dopaminergic neurons. Recent genetic studies in Drosophila
reveal that loss of parkin or PINK1 function leads to a similar phenotype that includes reduced lifespan, motor deficits, mitochondrial abnormalities and degeneration of muscle and dopaminergic neurons (25
). The Drosophila
PINK1 mutant phenotype can be rescued by overexpression of parkin, suggesting that PINK1 acts upstream of parkin in a common pathway (27
). Recent studies in mammalian cells further support a functional link between PINK1 and parkin (31
) and suggest that PINK1 is capable of interacting with parkin (32
) and phosphorylating parkin (35
). However, the cellular role of PINK1–parkin interaction is unclear and whether PINK1-mediated parkin phosphorylation regulates the E3 ligase function of parkin remains to be determined.
In this study, we investigated the interaction of parkin with and the phosphorylation of parkin by wild-type and PD-linked mutant PINK1 proteins and their functional consequences. Our results reveal that PINK1-mediated phosphorylation activates parkin E3 ligase function for catalyzing K63-linked polyubiquitination and enhances parkin-mediated ubiquitin signaling through the NF-κB pathway. Moreover, our findings provide evidence supporting a link between the deregulation of the PINK1/parkin/NF-κB neuroprotective pathway and PD pathogenesis.