Introduction Spinocerebellar ataxia type 1 (SCA1) is an autosomal dominant, progressive neurodegenerative disease. Primary sites of neurodegeneration in the brain are the Purkinje cells of the cerebellar cortex and specific neuronal populations in the brain stem. Neural dysfunction and cell death at these sites lead to bulbar dysfunction and characteristic ataxia in SCA1 patients. The disease is caused by a CAG repeat expansion in the SCA1 gene, resulting in an expanded polyglutamine tract in ataxin-1, the SCA1 gene product (Orr et al., 1993; Orr and Zoghbi, 2001). Thus, SCA1 is a member of a family of neurodegenerative diseases characterized by polyglutamine expansion that falls into a larger class of neurological disorders caused by triplet nucleotide repeat expansions. In addition to SCA1, this group of polyglutamine expressing disorders includes Huntington’s disease (HD), spinobulbar muscle atrophy (SBMA), dentatorubropallidoluysian atrophy (DRPLA) and the spinocerebellar ataxias (SCA2, SCA3/Machado-Joseph Disease, SCA6, SCA7 and SCA17) (Cummings and Zoghbi, 2000; Nakamura et al., 2001). A pathological hallmark of most of these polyglutamine diseases is the presence of nuclear and cytoplasmic inclusions containing the mutant polyglutamine protein. Ataxin-1 is a nuclear protein whose cellular function is not entirely known. It has been shown to interact with the SMRT transcription co-repressor, suggesting a role in transcriptional regulation (Tsai et al., 2004). Mutant ataxin-1 must enter the nucleus in order for the polyglutamine-induced neurodegenerative disease to develop (Klement et al., 1998), and ataxin-1 has been shown to have an in vitro RNA-binding activity (Yue et al., 2001). In order to assess the potential functions of ataxin-1 in the nucleus, we studied the properties of nuclear ataxin-1 and its co-localization with some known nuclear proteins. We show that ataxin-1 nuclear inclusions were distinct from several known nuclear bodies including speckles (Misteli et al., 1997), Nup98 bodies (Griffis et al., 2002) and hnRNP proteins (Burd and Dreyfuss, 1994; Siomi and Dreyfuss, 1995). However, ataxin-1 did co-localize in nuclear bodies with the mRNA export factor TAP/NXF1 (Gruter et al., 1998; Segref et al., 1997), in a manner that was enhanced by cell heat shock. Importantly, ataxin-1 localization to nuclear bodies was dependent on the presence of RNA. Using a live-cell nucleocytoplasmic transport assay that employs laser confocal microscopy and fluorescence recovery after photobleaching, or FRAP (Howell and Truant, 2002), we demonstrated that wild- type, but not polyglutamine expanded mutant ataxin-1, has the ability to export from the nucleus. By removing the polyglutamine tract in human ataxin-1, leaving only two glutamine residues, we noted that inclusion formation, inclusion movement and TAP/NXF1 recruitment were not affected, indicating that these characteristics are not related to the polyglutamine tract in ataxin-1. The role of active nuclear transport and mRNA in ataxin-1 inclusion formation suggests that lack of export of polyglutamine expanded ataxin-1 may be an important gain of function that leads to SCA1. 233 Spinocerebellar ataxia type 1 (SCA1) is a dominant neurodegenerative disease caused by the expression of mutant ataxin-1 containing an expanded polyglutamine tract. Ataxin-1 is a nuclear protein that localizes to punctate inclusions similar to neuronal nuclear inclusions seen in many polyglutamine expansion disease proteins. We demonstrate that ataxin-1 localization to inclusions and inclusion dynamics within the nucleus are RNA and transcription dependent, but not dependent on the polyglutamine tract. Ataxin-1 nuclear inclusions are distinct from other described nuclear bodies but recruit the mRNA export factor, TAP/NXF1, in a manner that is enhanced by cell heat shock. By FRAP protein dynamic studies in live cells, we found that wild-type, but not mutant, ataxin-1 was capable of nuclear export. These results suggest that the normal role of ataxin-1 may be in RNA processing, perhaps nuclear RNA export. Thus, nuclear retention of mutant ataxin-1 may be an important toxic gain of function in SCA1 disease. Supplementary material available online at http://jcs.biologists.org/cgi/content/full/118/1/233/DC1 Key words: Ataxin-1, Spinocerebellar ataxia type 1, RNA processing, Nuclear transport Summary RNA association and nucleocytoplasmic shuttling by ataxin-1 Stuart Irwin 1 , Mark Vandelft 1 , Deborah Pinchev 1 , Jenny L. Howell 1 , Joanna Graczyk 1 , Harry T. Orr 2 and Ray Truant 1, * 1 McMaster University, HSC 4H45, Department of Biochemistry, Hamilton, Ontario, L8N 3Z5, Canada 2 Department of Laboratory Medicine and Pathology and the Institute of Human Genetics, University of Minnesota, MN 55455-0213, USA *Author for correspondence (e-mail: truantr@mcmaster.ca) Accepted 26 October 2004 Journal of Cell Science 118, 233-242 Published by The Company of Biologists 2005 doi:10.1242/jcs.01611 Research Article Journal฀of฀Cell฀Science