Acts as a nucleotide-exchange factor (NEF) for chaperone proteins HSPA1A and HSPA1B, promoting the release of ADP from HSPA1A/B thereby triggering client/substrate protein release (PubMed:24318877). 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 (By similarity). Interacts with HSPA8/HSC70 (By similarity). Interacts with HSPA1A (via NBD) and HSPA1B (via NBD) (PubMed:24318877). 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. Belongs to the heat shock protein 70 family.