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

Abstract

The accumulation of aggregated ␣-synuclein is thought to contribute to the pathogenesis of Parkinson's disease. Recent studies indicate that aggregated ␣-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 ubiquitindependent. The IC 50 of aggregated ␣-synuclein for inhibition of the 26 S ubiquitin-independent proteasomal activity is ϳ1 nM. ␣-Synuclein has two close homologues, termed ␤-synuclein and ␥-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 ␣-and ␤-synucleins inhibited the 20 S and 26 S proteasomal activities only weakly, but monomeric ␥-synuclein strongly inhibited ubiquitin-independent proteolysis. The IC 50 of monomeric ␥-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 ␤-synuclein had no direct effect on proteasomal activity, co-incubating monomeric ␤-synuclein with aggregated ␣-synuclein antagonized the inhibition of the 26 S ubiquitin-independent proteasome by aggregated ␣-synuclein when added before the aggregated ␣-synuclein. Co-incubating ␤-synuclein with ␥-synuclein had no effect on the inhibition of the 20 S proteasome by monomeric ␥-synuclein. Immunoprecipitation and pull-down experiments suggested that antagonism by ␤-synuclein resulted from binding to ␣-synuclein rather than binding to S6. Pull-down experiments demonstrated that recombinant monomeric ␤-synuclein does not interact with the proteasomal subunit S6, unlike ␣-synuclein, but ␤-synuclein does bind ␣-synuclein and competes with S6 for binding to ␣-synuclein. Based on these data, we hypothesize that the ␣-and ␥-synucleins regulate proteasomal function and that ␤-synuclein acts as a negative regulator of ␣-synuclein.

α-Synuclein Stimulates Differentiation of Osteosarcoma Cells

“…In contrast to ␣-syn, ␤-syn, a less aggregate-prone protein that is non-amyloidogenic under the physiological conditions, had little effect on proteasome activity. The result is consistent with the previous report by Snyder et al (49).…”

“…Based on two-hybrid screening, ␣-syn was shown to bind to S6Ј, a component of the 19 S subunit in the 26 S proteasome, in cell cultures (41). Subsequently, it was shown that aggregated ␣-syn, but not its monomeric form, efficiently inhibited the activity of the 26 S proteasome under the cell-free conditions (42,43,49). Using ␣-syn-overexpressing MG63 cells, we observed that S6a was co-precipitated with ␣-syn by co-immunoprecipitation experiment, although immunofluorescence/LSCM study revealed that these molecules were partially colocalized in the cytoplasm (Fig.…”

γ-synuclein has a dynamic intracellular localization

“…The reducing effect of g-synuclein on BubR1 protein expression can be prevented by treating g-synuclein-transfected cells with MG-132, a selective 26S proteasome inhibitor, implying that the proteasome machinery may be involved in the g-synuclein-induced reduction of the BubR1 protein. Intriguingly, our recent data showed that synucleins are modulators of the proteasomal activity [Snyder et al, 2005]. Further experiments are necessary to get a deeper understanding how these synuclein functions are coordinated and why their malfunction leads to pathology.…”

“…In some cell types g-synuclein has been found in association with the centrosomes; however, most probably it is not a component of centrioles, but is transiently attached to a pericentriolar material (PCM) [Surgucheva et al, 2003]. g-Synuclein inhibits proteasomal activity as the other member of the synuclein family, asynuclein [Snyder et al, 2005]. Interestingly, b-synuclein may antagonize some of the pathological properties of a-and g-synucleins [da Costa et al, 2003;Snyder et al, 2005].…”

“…g-Synuclein inhibits proteasomal activity as the other member of the synuclein family, asynuclein [Snyder et al, 2005]. Interestingly, b-synuclein may antagonize some of the pathological properties of a-and g-synucleins [da Costa et al, 2003;Snyder et al, 2005].…”