Heat shock protein 105 kDa is a protein that in humans is encoded by the HSPH1 gene. Prevents the aggregation of denatured proteins in cells under severe stress, on which the ATP levels decrease markedly. Inhibits HSPA8/HSC70 ATPase and chaperone activities. Highly expressed in testis. Present at lower levels in most brain regions, except cerebellum. Overexpressed in cancer cells. Phosphorylation on Ser-509 may be important for regulation of the HSPA8/HSC70 chaperone activity.