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Huntingtons disease is caused by CAG trinucleotide expansions in the gene encoding huntingtin. N- terminal fragments of huntingtin with polyglu-tamine produce aggregates in the endosome-ly-sosomal system, where proteolytic fragments of huntingtin is generated. Heat shock protein 70 (HSP70) prevents the formation of protein aggre-gates, but the effect of HSP70 on the huntingtin in the endosome-lysosomal system is unknown. This study was to determine whether HSP70 alters the distribution of huntingtin in endosome-lysosomal system. HSP70 expressing stable cells (NIH/3T3 or cerebral hybrid cell line A1) were generated, and mutant [(CAG)100] huntingtin was transiently over-expressed. Analysis of subcellular distribution by immnuocytochemistry or proteolysis cleavage by Western blotting was performed. 18 CAG repeat wild type [WT; (CAG)18] huntingtin was used as a control. Cells with huntingtin showed paterns of endosome- lysosomal accumulation, from a dispersed vacuole (DV) type into a coalescent perinuclear vacuole (PV) type over time. In WT huntingtin, HSP70 increased the cells with the PV types that enhanced the proteolytic cleavage of huntingtin. However, HSP70 reduced cells of the DV and PV types expressing mutant ess proteolysis than that of control. In addition, intranuclear inclusions were formed only in mutant cells, which was no t affected by HSP70. These results suggest that HSP70 alters the distribution of huntingtin in the endosome- lysosomal system, and that this contributes to huntingtin proteolysis.