title: beta-Synuclein reduces proteasomal inhibition by alpha-synuclein but not gamma-synuclein.
abstract: The accumulation of aggregated alpha-synuclein is thought to contribute to the pathogenesis of Parkinson's disease. Recent studies indicate that aggregated alpha-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 IC(50) of aggregated alpha-synuclein for inhibition of the 26 S ubiquitin-independent proteasomal activity is approximately 1 nm. alpha-Synuclein has two close homologues, termed beta-synuclein and gamma-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 alpha- and beta-synucleins inhibited the 20 S and 26 S proteasomal activities only weakly, but monomeric gamma-synuclein strongly inhibited ubiquitin-independent proteolysis. The IC(50) of monomeric gamma-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 beta-synuclein had no direct effect on proteasomal activity, co-incubating monomeric beta-synuclein with aggregated alpha-synuclein antagonized the inhibition of the 26 S ubiquitin-independent proteasome by aggregated alpha-synuclein when added before the aggregated alpha-synuclein. Co-incubating beta-synuclein with gamma-synuclein had no effect on the inhibition of the 20 S proteasome by monomeric gamma-synuclein. Immunoprecipitation and pull-down experiments suggested that antagonism by beta-synuclein resulted from binding to alpha-synuclein rather than binding to S6'. Pull-down experiments demonstrated that recombinant monomeric beta-synuclein does not interact with the proteasomal subunit S6', unlike alpha-synuclein, but beta-synuclein does bind alpha-synuclein and competes with S6' for binding to alpha-synuclein. Based on these data, we hypothesize that the alpha- and gamma-synucleins regulate proteasomal function and that beta-synuclein acts as a negative regulator of alpha-synuclein.
authors.affiliation: Department of Pharmacology, Loyola University Medical Center, Maywood, Illinois 60153, USA.
authors.forename: Heather
authors.initials: H
authors.lastname: Snyder
authors.forename: Kwame
authors.initials: K
authors.lastname: Mensah
authors.forename: Cindy
authors.initials: C
authors.lastname: Hsu
authors.forename: Makoto
authors.initials: M
authors.lastname: Hashimoto
authors.forename: Irina G
authors.initials: IG
authors.lastname: Surgucheva
authors.forename: Barry
authors.initials: B
authors.lastname: Festoff
authors.forename: Andrei
authors.initials: A
authors.lastname: Surguchov
authors.forename: Eliazer
authors.initials: E
authors.lastname: Masliah
authors.forename: Andreas
authors.initials: A
authors.lastname: Matouschek
authors.forename: Benjamin
authors.initials: B
authors.lastname: Wolozin
mesh_terms: D001921:Brain / Q000378:metabolism; D002460:Cell Line; D002918:Chymotrypsin / Q000737:chemistry; D004305:Dose-Response Relationship, Drug; D006801:Humans; D015151:Immunoblotting; D020128:Inhibitory Concentration 50; D008954:Models, Biological; D009419:Nerve Tissue Proteins / Q000378:metabolism / Q000502:physiology*; D046988:Proteasome Endopeptidase Complex / Q000737:chemistry; D011485:Protein Binding; D017434:Protein Structure, Tertiary; D011994:Recombinant Proteins / Q000737:chemistry; D051843:Synucleins; D013997:Time Factors; D014162:Transfection; D025801:Ubiquitin / Q000737:chemistry; D051844:alpha-Synuclein; D051846:beta-Synuclein; D051845:gamma-Synuclein
chemical_list: D009419:Nerve Tissue Proteins; D011994:Recombinant Proteins; C497604:SNCA protein, human; C497607:SNCB protein, human; D051843:Synucleins; D025801:Ubiquitin; D051844:alpha-Synuclein; D051846:beta-Synuclein; D051845:gamma-Synuclein; D002918:Chymotrypsin; D046988:Proteasome Endopeptidase Complex; C061553:ATP dependent 26S protease