Misfolding promotes the ubiquitination of polyglutamine-expanded ataxin-3, the defective gene product in SCA3/MJD.

A major hallmark of the polyglutamine diseases is the formation of neuronal intranuclear inclusions (NIIs) of the disease proteins that are ubiquitinated and often associated with various chaperones and proteasome components. Recently, misfolding has come to be considered one of the primary factors for polyglutamine protein aggregation, although, the nature of misfolding and the relationship between misfolding and ubiquitination of the expanded polyglutamine protein is not yet known. By using ataxin-3, the defective gene product of SCA3/MJD, we demonstrate here that the misfolding propensity and the cellular toxicity of a polyglutamine protein is directly proportional to the length of the glutamine repeats and inversely dependent on the size of the corresponding protein. The size of the polyglutamine bearing protein also inversely influences the binding of 1C2 antibody (an antibody that selectively recognizes polyglutamine expansion) to the polyglutamine protein and determines the minimum length of glutamine expansion to be recognized by 1C2 antibody, which suggests that the critical pathological range of glutamine repeats could also be dependent on the size of the corresponding protein. Ataxin-3 (both full length and truncated) with normal glutamine repeats are not ubiquitinated, however, ataxin-3 with expanded polyglutamine is ubiquitinated and the ubiquitination depends on the misfolding propensity of the polyglutamine expanded ataxin-3.