Involved in the unfolded protein response (UPR) during ER stress. Co-chaperone of HSPA8/HSC70, it stimulates its ATPase activity. May inhibit both the autophosphorylation of EIF2AK2/PKR and the ability of EIF2AK2 to catalyze phosphorylation of the EIF2A. May inhibit EIF2AK3/PERK activity (By similarity). Interacts with EIF2AK2 and EIF2AK3. Forms a trimeric complex with DNAJB1 and HSPA8. Interacts with THAP12 (By similarity). The J domain mediates interaction with HSPA8. Binding to misfolded proteins is mediated by a hydrophobic patch forming a large groove within the first two TPR repeats. Contaminating sequence. Potential poly-A sequence.