title: Siah-1 facilitates ubiquitination and degradation of synphilin-1.
abstract: Parkinson's disease is a common neurodegenerative disorder characterized by loss of dopaminergic neurons and appearance of Lewy bodies, cytoplasmic inclusions that are highly enriched with ubiquitin. Synphilin-1, alpha-synuclein, and Parkin represent the major components of Lewy bodies and are involved in the pathogenesis of Parkinson's disease. Synphilin-1 is an alpha-synuclein-binding protein that is ubiquitinated by Parkin. Recently, a mutation in the synphilin-1 gene has been reported in patients with sporadic Parkinson's disease. Although synphilin-1 localizes close to synaptic vesicles, its function remains unknown. To investigate the proteins that interact with synphilin-1, the present study performed a yeast two-hybrid screening and identified a novel interacting protein, Siah-1 ubiquitin ligase. Synphilin-1 and Siah-1 proteins were endogenously expressed in the central nervous system and were found to coimmunoprecipitate each other in rat brain homogenate. Confocal microscopic analysis revealed colocalization of both proteins in cells. Siah-1 was found to interact with the N terminus of synphilin-1 through its substrate-binding domain and to specifically ubiquitinate synphilin-1 via its RING finger domain. Siah-1 facilitated synphilin-1 degradation via the ubiquitin-proteasome pathway more efficiently than Parkin. Siah-1 was found to not facilitate ubiquitination and degradation of wild type or mutant alpha-synuclein. Synphilin-1 inhibited high K+-induced dopamine release from PC12 cells. Siah-1 was found to abrogate the inhibitory effects of synphilin-1 on dopamine release. Such findings suggest that Siah-1 might play a role in regulation of synphilin-1 function.
mesh_terms: D000818:Animals; D001665:Binding Sites; D001921:Brain / Q000378:metabolism; D002352:Carrier Proteins / Q000235:genetics / Q000378:metabolism*; D002460:Cell Line; D003546:Cysteine Endopeptidases / Q000378:metabolism; D003593:Cytoplasm / Q000378:metabolism; D004298:Dopamine / Q000378:metabolism; D018492:Dopamine Antagonists / Q000378:metabolism; D006801:Humans; D009097:Multienzyme Complexes / Q000378:metabolism; D009419:Nerve Tissue Proteins / Q000235:genetics / Q000378:metabolism*; D009687:Nuclear Proteins / Q000235:genetics / Q000378:metabolism / Q000502:physiology*; D046988:Proteasome Endopeptidase Complex; D011485:Protein Binding; D051381:Rats; D014018:Tissue Distribution; D014162:Transfection; D025801:Ubiquitin / Q000378:metabolism*; D044767:Ubiquitin-Protein Ligases; D000098563:Seven in Absentia Proteins
chemical_list: D002352:Carrier Proteins; D018492:Dopamine Antagonists; D009097:Multienzyme Complexes; D009419:Nerve Tissue Proteins; D009687:Nuclear Proteins; C118958:SNCAIP protein, human; C498463:Sncaip protein, rat; D025801:Ubiquitin; D044767:Ubiquitin-Protein Ligases; D000098563:Seven in Absentia Proteins; D003546:Cysteine Endopeptidases; D046988:Proteasome Endopeptidase Complex; D004298:Dopamine