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. Nucleotide release is mediated via its binding to the nucleotide-binding domain (NBD) of HSPA8/HSC70 where as the substrate release is mediated via its binding to the substrate-binding domain (SBD) of HSPA8/HSC70. Has anti-apoptotic activity. Plays a role in the HSF1 nucleocytoplasmic transport. Binds to the ATPase domain of HSP70/HSC70 chaperones. Interacts with BCL2. Interacts with phospholipase C-gamma proteins. Interacts with DNAJB6. Interacts (via BAG domain) with HSF1; this interaction occurs in normal and heat-shocked cells. Interacts with HSPA8 (via NBD), HSPA1A (via NBD) and HSPA1B (via NBD). Interacts (via WW domain 1) with SYNPO2 (via PPPY motif). Colocalizes with HSF1 to the nucleus upon heat stress.