Parkinson's disease (PD) is a neurodegenerative disease characterized by Lewy body formation and death of dopaminergic neurons. Mutations in ␣-synuclein and parkin cause familial forms of PD. Synphilin-1 was shown to interact with ␣-synuclein and to promote the formation of cytosolic inclusions. We now report that synphilin-1 interacts with the E3 ubiquitin-ligases SIAH-1 and SIAH-2. SIAH proteins ubiquitylate synphilin-1 both in vitro and in vivo, promoting its degradation by the ubiquitin-proteasome system. Inability of the proteasome to degrade synphilin-1͞SIAH complex leads to a robust formation of ubiquitylated cytosolic inclusions. Ubiquitylation is required for inclusion formation, because a catalytically inactive mutant of SIAH-1, which still binds to synphilin-1, fails to promote inclusions. Like synphilin-1, ␣-synuclein associates with SIAH in intact cells, but the interaction with SIAH-2 was much stronger that with SIAH-1. In vitro experiments show that SIAH-2 monoubiquitylates ␣-synuclein. Further evidence that SIAH proteins may play a role in inclusion formation comes from the demonstration of SIAH immunoreactivity in Lewy bodies of PD patients.␣-synuclein-interacting protein ͉ ubiquitin ͉ inclusion bodies

␣-Synucleinopathies are a group of neurological disorders characterized by the presence of intracellular inclusion bodies containing ␣-synuclein. We previously demonstrated that synphilin-1 interacts with ␣-synuclein, implying a role in Parkinson's disease. We now report the identification and characterization of synphilin-1A, an isoform of synphilin-1, which has enhanced aggregatory properties and causes neurotoxicity. The two transcripts encoding synphilin-1A and synphilin-1 originate from the SNCAIP gene but differ in both their exon organization and initial reading frames used for translation. Synphilin-1A binds to ␣-synuclein and induces the formation of intracellular aggregates in human embryonic kidney 293 cells, primary neuronal cultures, and human dopaminergic cells. Overexpression of synphilin-1A in neurons results in striking cellular toxicity that is attenuated by the formation of synphilin-1A inclusions, which recruit ␣-synuclein. Synphilin-1A is present in Lewy bodies of patients with Parkinson's disease and Diffuse Lewy Body disease, and is observed in detergent-insoluble fractions of brain protein samples obtained from Diffuse Lewy Body disease patients. These findings suggest that synphilin-1A may contribute to neuronal degeneration in ␣-synucleinopathies and also provide important insights into the role of inclusion bodies in neurodegenerative disorders.␣-synuclein ͉ cell death ͉ Lewy body ͉ Parkinson's disease