Domain architecture of the polyglutamine protein ataxin-3: a globular domain followed by a flexible tail.

Anomalous expansion of a polyglutamine (polyQ) tract in the protein ataxin-3 causes spinocerebellar ataxia type 3, an autosomal dominant neurodegenerative disease. Very little is known about the structure and the function of ataxin-3, although this information would undoubtedly help to understand why the expanded protein forms insoluble nuclear aggregates and causes neuronal cell death. With the aim of establishing the domain architecture of ataxin-3 and the role of the polyQ tract within the protein context, we have studied the human and murine orthologues using a combination of techniques, which range from limited proteolysis to circular dichroism (CD) and nuclear magnetic resonance (NMR) spectroscopies. The two protein sequences share a highly conserved N-terminus and differ only in the length of the glutamine repeats and in the C-terminus. Our data conclusively indicate that ataxin-3 is composed by a structured N-terminal domain, followed by a flexible tail. Moreover, [(15)N]glutamine selectively labelled samples allowed us to have a direct insight by NMR into the structure of the polyQ region.