Aggregated and Monomeric α-Synuclein Bind to the S6′ Proteasomal Protein and Inhibit Proteasomal Function

Abstract

The accumulation of aggregated ␣-synuclein is thought to contribute to the pathophysiology of Parkinson's disease, but the mechanism of toxicity is poorly understood. Recent studies suggest that aggregated proteins cause toxicity by inhibiting the ubiquitin-dependent proteasomal system. In the present study, we explore how ␣-synuclein interacts with the proteasome. The proteasome exists as a 26 S and a 20 S species. The 26 S proteasome is composed of the 19 S cap and the 20 S core. Aggregated ␣-synuclein strongly inhibited the function of the 26 S proteasome. The IC 50 of aggregated ␣-synuclein for ubiquitin-independent 26 S proteasomal activity was 1 nM. Aggregated ␣-synuclein also inhibited 26 S ubiquitin-dependent proteasomal activity at a dose of 500 nM. In contrast, the IC 50 of aggregated ␣-synuclein for 20 S proteasomal activity was > 1 M. This suggests that aggregated ␣-synuclein selectively interacts with the 19 S cap. Monomeric ␣-synuclein also inhibited proteasomal activity but with lower affinity and less potency. Recombinant monomeric ␣-synuclein inhibited the activity of the 20 S proteasomal core with an IC 50 > 10 M, exhibited no inhibition of 26 S ubiquitin-dependent proteasomal activity at doses up to 5 M, and exhibited only partial inhibition (50%) of the 26 S ubiquitinindependent proteasomal activity at doses up to 10 mM. Binding studies demonstrate that both aggregated and monomeric ␣-synuclein selectively bind to the proteasomal protein S6, a subunit of the 19 S cap. These studies suggest that proteasomal inhibition by aggregated ␣-synuclein could be mediated by interaction with S6.

The benefits of local depletion: The centrosome as a scaffold for ubiquitin-proteasome-mediated degradation

“…30 Therefore, aggresomes are largely thought to be cleared by autophagy rather than proteasome-mediated degradation. 31 Since proteasomes are unable to process aggregates, the fact that they are actively targeted to aggresomes presents a paradox.…”

Protein misfolding and dysregulated protein homeostasis in autoinflammatory diseases and beyond

“…Loci multiplication and mutations in the -synuclein gene, SNCA, predispose to autosomal dominant PD [97][98][99]. However, most patients have sporadic forms of PD in which aging and inefficient proteasome degradation results in accumulation of -synuclein and inflammation [100][101][102][103][104]. Misfolded -synuclein accumulates into cytoplasmic protein aggregates called Lewy bodies in presynaptic neurons [105,106].…”

Contribution of β-phenethylamine, a component of chocolate and wine, to dopaminergic neurodegeneration: implications for the pathogenesis of Parkinson’s disease

“…Mitochondrial dysfunction and the resulting oxidative stress have been reported to promote α-synuclein aggregation or Lewy body formation [40][41][42] that culminates in the loss of dopaminergic neurons through impairment of the ubiquitin-proteasome system (UPS) [43] . The aggregated α-synuclein has been shown to inhibit the UPS system by interacting with the proteasomal subunits [44,45] . Moreover, a decrease in cellular energy or ATP generation as a consequence of mitochondrial complex-I inhibition results in α-synuclein aggregation [40][41][42]46,47] .…”