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 (By similarity). 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 (PubMed:14644449, PubMed:15292236). Interacts with HSPA8/HSC70 (PubMed:15292236, PubMed:9675148). Interacts with HSPA1A (via NBD) and HSPA1B (via NBD) (By similarity). Strictly cytoplasmic in neurons. Highly expressed in testis. Present at lower levels in most brain regions, except cerebellum. Within the brain, expression is restricted to neurons (at protein level). Overexpressed in cancer cells. By heat shock. Hsp105-alpha also induced by other stresses. Phosphorylation on Ser-509 may be important for regulation of the HSPA8/HSC70 chaperone activity. Belongs to the heat shock protein 70 family.