Elucidation of ataxin-3 and ataxin-7 function by integrative bioinformatics

The spinocerebellar ataxias (SCAs) are a class of hereditary neurodegenerative diseases, which are caused by the pathological expansion of unstable CAG triplet repeats found in a number of apparently unrelated genes. The proteins encoded by the SCA genes typically translate this expanded (CAG)n repeat into an expanded poly(Q) stretch. Several pathological features are common to all SCAs, irrespective of the gene harbouring the expansion. The specific contributions of the mutated genes are currently hard to assess, as the physiological role of most of the so-called ataxins is not known. By combining the results of profile-based sequence analysis with genome-wide functional data available for model organisms, we have derived detailed predictions of the physiological function of two SCA gene products. Ataxin-3, the protein mutated in Machado Joseph Disease (SCA3), belongs to a novel group of cysteine-proteases and is predicted to be active against ubiquitin chains or related substrates. The catalytic site of this enzyme class is similar to that found in UBP and UCH type ubiquitin proteases. For ataxin-7, the gene product of the SCA7 gene, we have identified an orthology relationship to the yeast open reading frame Ygl066c. Recently published evidence from genome-wide studies suggests that Ygl066c is a component of the SAGA histone acetyltransferase complex. By analogy, a similar role for the mammalian ataxin-7 can be expected. The functional predictions reported here are sufficiently precise to allow a direct experimental verification. Moreover, both findings have implications for the general pathogenesis of spinocerebellar ataxias by providing a direct connection of these diseases with ubiquitin metabolism and histone acetylation.