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Hum Mol Genet. 2017 Apr 15;26(8):1419-1431. doi: 10.1093/hmg/ddx039.

Interaction of the polyglutamine protein ataxin-3 with Rad23 regulates toxicity in Drosophila models of Spinocerebellar Ataxia Type 3.

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Department of Pharmacology, Wayne State University, Detroit MI, USA.
Department of Neurology, University of Michigan, Ann Arbor MI, USA.
Department of Biochemistry and the Neuroscience Research Center, Medical College of Wisconsin, Milwaukee WI, USA.
Department of Neurology, Wayne State University, Detroit MI, USA.


Polyglutamine (polyQ) repeat expansion in the deubiquitinase ataxin-3 causes neurodegeneration in Spinocerebellar Ataxia Type 3 (SCA3), one of nine inherited, incurable diseases caused by similar mutations. Ataxin-3's degradation is inhibited by its binding to the proteasome shuttle Rad23 through ubiquitin-binding site 2 (UbS2). Disrupting this interaction decreases levels of ataxin-3. Since reducing levels of polyQ proteins can decrease their toxicity, we tested whether genetically modulating the ataxin-3-Rad23 interaction regulates its toxicity in Drosophila. We found that exogenous Rad23 increases the toxicity of pathogenic ataxin-3, coincident with increased levels of the disease protein. Conversely, reducing Rad23 levels alleviates toxicity in this SCA3 model. Unexpectedly, pathogenic ataxin-3 with a mutated Rad23-binding site at UbS2, despite being present at markedly lower levels, proved to be more pathogenic than a disease-causing counterpart with intact UbS2. Additional studies established that the increased toxicity upon mutating UbS2 stems from disrupting the autoprotective role that pathogenic ataxin-3 has against itself, which depends on the co-chaperone, DnaJ-1. Our data reveal a previously unrecognized balance between pathogenic and potentially therapeutic properties of the ataxin-3-Rad23 interaction; they highlight this interaction as critical for the toxicity of the SCA3 protein, and emphasize the importance of considering protein context when pursuing suppressive avenues.

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