Lysine 117 on ataxin-3 modulates toxicity in Drosophila models of Spinocerebellar Ataxia Type 3.

Ataxin-3 (Atxn3) is a deubiquitinase with a polyglutamine (polyQ) repeat tract whose abnormal expansion causes the neurodegenerative disease, Spinocerebellar Ataxia Type 3 (SCA3; also known as Machado-Joseph Disease). The ubiquitin chain cleavage properties of Atxn3 are enhanced when it is ubiquitinated at lysine (K) at position 117. K117-ubiqutinated Atxn3 cleaves poly-ubiquitin more rapidly in vitro compared to its unmodified counterpart and this residue is also important for Atxn3 roles in cell culture and in Drosophila melanogaster . How polyQ expansion causes SCA3 remains unclear. To gather insight into the biology of disease of SCA3, here we posited the question: is K117 important for toxicity caused by Atxn3? We generated transgenic Drosophila lines that express full-length, human, pathogenic Atxn3 with 80 polyQ with an intact or mutated K117. We found that K117 mutation mildly enhances the toxicity and aggregation of pathogenic Atxn3 in Drosophila . An additional transgenic line that expresses Atxn3 without any K residues confirms increased aggregation of pathogenic Atxn3 whose ubiquitination is perturbed. These findings suggest Atxn3 ubiquitination as a regulatory step of SCA3, in part by modulating its aggregation.