β-Synuclein Reduces Proteasomal Inhibition by α-Synuclein but Not γ-Synuclein*

The accumulation of aggregated α-synuclein is thought to contribute to the pathogenesis of Parkinson's disease. Recent studies indicate that aggregated α-synuclein binds to S6′, a component of the 19 S subunit in the 26 S proteasome and inhibits 26 S proteasomal degradation, both ubiquitin-independent and ubiquitin-dependent. The IC50 of aggregated α-synuclein for inhibition of the 26 S ubiquitin-independent proteasomal activity is ∼1nm. α-Synuclein has two close homologues, termed β-synuclein and γ-synuclein. In the present study we compared the effects of the three synuclein homologues on proteasomal activity. The proteasome exists as a 26 S and a 20 S species, with the 26 S proteasome containing the 20 S core and 19 S cap. Monomeric α- and β-synucleins inhibited the 20 S and 26 S proteasomal activities only weakly, but monomeric γ-synuclein strongly inhibited ubiquitin-independent proteolysis. The IC50 of monomeric γ-synuclein for the 20 S proteolysis was 400 nm. In monomeric form, none of the three synuclein proteins inhibited 26 S ubiquitin-dependent proteasomal activity. Although β-synuclein had no direct effect on proteasomal activity, co-incubating monomeric β-synuclein with aggregated α-synuclein antagonized the inhibition of the 26 S ubiquitin-independent proteasome by aggregated α-synuclein when added before the aggregated α-synuclein. Co-incubating β-synuclein with γ-synuclein had no effect on the inhibition of the 20 S proteasome by monomeric γ-synuclein. Immunoprecipitation and pull-down experiments suggested that antagonism by β-synuclein resulted from binding to α-synuclein rather than binding to S6′. Pull-down experiments demonstrated that recombinant monomeric β-synuclein does not interact with the proteasomal subunit S6′, unlike α-synuclein, but β-synuclein does bind α-synuclein and competes with S6′ for binding to α-synuclein. Based on these data, we hypothesize that the α- and γ-synucleins regulate proteasomal function and that β-synuclein acts as a negative regulator of α-synuclein.

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