Ataxin 7

Ataxin-7 (ATXN7) is a protein of the SCA7 gene, located on chromosome 3. It is a subunit of the SAGA chromatin remodeling complex, which regulates gene expression; it contains 892 amino acids with an expandable poly(Q) region close to the N-terminus. The expandable poly(Q) motif region in the protein contributes crucially to spinocerebellar ataxia (SCA) pathogenesis by the induction of intranuclear inclusion bodies. ATXN7 is associated with both olivopontocerebellar atrophy type 3 (OPCA3) and spinocerebellar ataxia type 7 (SCA7).

Several CAG repeats within the coding region of the SCA genes will lead to pathological protein misfolding. The allele linked to SCA7 carries 37—306 CAG repeats near the N-terminus, whereas the normal allele has only 4—35 repeats. The CAG repeats in the ATXN7 gene have been linked to cerebellar and brainstem degeneration as well as retinal conerod dystrophy. The polyglutamine (polyQ) expansion at the N-terminus causes protein aggregation, impairing the gene expression of photoreceptor cell survival, leading to the symptoms of ataxia and vision loss. Research suggest that silencing of ATXN7 in the retina by RNAi can be a possible therapeutic strategy for patients with SCA7 retinal degeneration.

The N-terminus of ATXN7 is attached to a structural scaffold protein in the SAGA complex, SUPT20H. This interaction positions ATXN7 so that it can connect the deubiquitination (DUB) module to the complex, which is needed to remove ubiquitin modifications from histones, an essential step in transcription. Without the interaction between an arginine (Arg531) on ATXN7's N-terminus and a serine (Ser182) on the SUPT20H protein, the DUB module would not be anchored to the SAGA complex correctly, leading to defects in histone deubiquitination and gene regulation. Because of the length of the interaction being 3.3Å, it is characterized as a hydrogen bond keeping the two proteins attached.