Sutton Joanna R, Blount Jessica R, Libohova Kozeta, Tsou Wei-Ling, Joshi Gnanada S, Paulson Henry L, Costa Maria do Carmo, Scaglione K Matthew, Todi Sokol V
Department of Pharmacology, Wayne State University, Detroit MI, USA.
Department of Neurology, University of Michigan, Ann Arbor MI, USA.
Hum Mol Genet. 2017 Apr 15;26(8):1419-1431. doi: 10.1093/hmg/ddx039.
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.
去泛素化酶ataxin-3中的聚谷氨酰胺（polyQ）重复序列扩增会导致3型脊髓小脑共济失调（SCA3）中的神经退行性变，SCA3是由类似突变引起的九种遗传性不治之症之一。ataxin-3与蛋白酶体转运蛋白Rad23通过泛素结合位点2（UbS2）结合，从而抑制其降解。破坏这种相互作用会降低ataxin-3的水平。由于降低聚Q蛋白的水平可以降低其毒性，我们测试了通过基因调控ataxin-3与Rad23的相互作用是否能调节其在果蝇中的毒性。我们发现外源性Rad23会增加致病性ataxin-3的毒性，同时疾病蛋白水平也会升高。相反，降低Rad23水平可减轻该SCA3模型中的毒性。出乎意料的是，在UbS2处具有突变的Rad23结合位点的致病性ataxin-3，尽管其水平明显较低，但被证明比具有完整UbS2的致病对应物更具致病性。进一步的研究表明，UbS2突变后毒性增加是由于破坏了致病性ataxin-3对自身的自我保护作用，而这种作用依赖于共伴侣蛋白DnaJ-1。我们的数据揭示了ataxin-3与Rad23相互作用的致病性和潜在治疗特性之间以前未被认识到的平衡；它们强调这种相互作用对SCA3蛋白的毒性至关重要，并强调在寻求抑制途径时考虑蛋白质背景的重要性。
