Co-chaperone for HSP70 and HSC70 chaperone proteins. Acts as a nucleotide-exchange factor (NEF) promoting the release of ADP from the HSP70 and HSC70 proteins thereby triggering client/substrate protein release. Binds to the ATPase domain of HSP/HSC70 chaperones. May interact with NWD1. Interacts with HSPA1A (via NBD), HSPA1B (via NBD) and HSPA8. May interact with DNJC9; the interaction seems to be histone-dependent (By similarity).