Hsp105 reduces the protein aggregation and cytotoxicity by expanded-polyglutamine proteins through the induction of Hsp70

Hsp105α and Hsp105β are major heat shock proteins in mammalian cells and belong to the HSP105/110 family. Hsp105α is expressed constitutively in the cytoplasm of cells, while Hsp105β, an alternatively spliced form of Hsp105α, is expressed specifically in the nucleus of cells during mild heat shock. Here, we show that not only Hsp105β but also Hsp105α accumulated in the nucleus of cells following the expression of enhanced green fluorescent protein with a pathological length polyQ tract (EGFP-polyQ97) and suppressed the intranuclear aggregation of polyQ proteins and apoptosis induced by EGFP-polyQ97. Mutants of Hsp105α and Hsp105β with changes in the nuclear localization signal sequences, which localized exclusively in the cytoplasm with or without the expression of EGFP-polyQ97, did not suppress the intranuclear aggregation of polyQ proteins and apoptosis induced by EGFP-polyQ97. Furthermore, Hsp70 was induced by the co-expression of Hsp105α and EGFP-polyQ97, and the knockdown of Hsp70 reduced the inhibitory effect of Hsp105α and Hsp105β on the intranuclear aggregation of polyQ proteins and apoptosis induced by EGFP-polyQ97. These observations suggested that Hsp105α and Hsp105β suppressed the expanded polyQ tract-induced protein aggregation and apoptosis through the induction of Hsp70.