Acts as a nucleotide-exchange factor (NEF) for chaperone proteins HSPA1A and HSPA1B, promoting the release of ADP from HSPA1A/B thereby triggering substrate release. 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. Interacts with HSPA8/HSC70. Interacts with HSPA1A (via NBD) and HSPA1B (via NBD). Phosphorylation on Ser-509 may be important for regulation of the HSPA8/HSC70 chaperone activity. Belongs to the heat shock protein 70 family.