beta-Synuclein reduces proteasomal inhibition by alpha-synuclein but not gamma-synuclein

J Biol Chem. 2005 Mar 4;280(9):7562-9. doi: 10.1074/jbc.M412887200. Epub 2004 Dec 9.

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.

Publication types

  • Research Support, U.S. Gov't, Non-P.H.S.
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Brain / metabolism
  • Cell Line
  • Chymotrypsin / chemistry
  • Dose-Response Relationship, Drug
  • Humans
  • Immunoblotting
  • Inhibitory Concentration 50
  • Models, Biological
  • Nerve Tissue Proteins / metabolism
  • Nerve Tissue Proteins / physiology*
  • Proteasome Endopeptidase Complex / chemistry
  • Protein Binding
  • Protein Structure, Tertiary
  • Recombinant Proteins / chemistry
  • Synucleins
  • Time Factors
  • Transfection
  • Ubiquitin / chemistry
  • alpha-Synuclein
  • beta-Synuclein
  • gamma-Synuclein

Substances

  • Nerve Tissue Proteins
  • Recombinant Proteins
  • SNCA protein, human
  • SNCB protein, human
  • Synucleins
  • Ubiquitin
  • alpha-Synuclein
  • beta-Synuclein
  • gamma-Synuclein
  • Chymotrypsin
  • Proteasome Endopeptidase Complex
  • ATP dependent 26S protease