Identification and characterisation of localisation signals in ataxin-3, the affected protein in spinocerebellar ataxia type 3

Spinocerebellar Ataxia Type 3 (SCA3), also known as Machado-Joseph disease (MJD), is an autosomal dominantly inherited neurodegenerative disorder caused by the expansion of a CAG stretch in the MJD1 gene encoding a polyglutamine repeat in the respective ataxin-3 protein. SCA3 therefore belongs to the group of the so called polyglutamine diseases.

Ataxin-3 is mainly localized in the cytoplasm. However, one hallmark of SCA3 is the formation of ataxin-3 containing aggregates in the nucleus of neurons. In addition, we recently demonstrated in vivo using transgenic mouse models that nuclear localization of ataxin-3 is required for the manifestation of symptoms in SCA3 and that cytoplasmically localized ataxin-3 is not able to induce a phenotype even with a very high number of polyglutamine repeats.

Up to now it is not known why and how ataxin-3 leaves the cytoplasm and translocates into the nucleus. The identification of nuclear localization (NLS) or nuclear export signals (NES) within ataxin-3 would make an important contribution to the answer of this question. Recently, using computerized comparison with known sequence profiles, ataxin-3 was proposed to contain both a NLS and a NES. In order to find out whether these proposed signals are functional, we performed nuclear import and export assays in tissue culture. We also tested the consequence of in vitro mutagenesis of crucial amino acids within these signals for the localization of ataxin-3 and the formation of intranuclear inclusion bodies. In addition, we performed own searches for novel potential localization signal in ataxin-3 and characterized these signals likewise as described above. The identification of localization signals in ataxin-3 is an important step to understand the pathogenic mechanisms of SCA3.