Huntingtin Polyglutamine Fragments Are a Substrate for Hsp104 in Saccharomyces cerevisiae

The aggregation of huntingtin fragments with expanded polyglutamine repeat regions (HttpolyQ) that cause Huntington's disease depends on the presence of a prion with an amyloid conformation in yeast. As a result of this relationship, HttpolyQ aggregation indirectly depends on Hsp104 due to its essential role in prion propagation. We find that HttQ103 aggregation is directly affected by Hsp104 with and without the presence of [RNQ⁺] and [PSI⁺] prions. When we inactivate Hsp104 in the presence of prion, yeast cells have only one or a few large HttQ103 aggregates rather than numerous smaller aggregates. When we inactivate Hsp104 in the absence of prion, there is no significant aggregation of HttQ103, whereas with active Hsp104, HttQ103 aggregates accumulate slowly due to the severing of spontaneously nucleated aggregates by Hsp104. We do not observe either effect with HttQ103P, which has a polyproline-rich region downstream of the polyglutamine region, because HttQ103P does not spontaneously nucleate and Hsp104 does not efficiently sever the prion-nucleated HttQ103P aggregates. Therefore, the only role of Hsp104 in HttQ103P aggregation is to propagate yeast prion. In conclusion, because Hsp104 efficiently severs the HttQ103 aggregates but not HttQ103P aggregates, it has a marked effect on the aggregation of HttQ103 but not HttQ103P.