Involved in the unfolded protein response (UPR) during endoplasmic reticulum (ER) stress. Acts as a negative regulator of the EIF2AK4/GCN2 kinase activity by preventing the phosphorylation of eIF-2-alpha at 'Ser-52' and hence attenuating general protein synthesis under ER stress, hypothermic and amino acid starving stress conditions (PubMed:25329545). 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 EIF2AK4/GCN2; this interaction occurs under endoplasmic reticulum (ER) stress, hypothermic and amino acid starving stress conditions and inhibits EIF2AK4/GCN2 kinase activity (PubMed:25329545). Interacts with EIF2AK3 (PubMed:12446838). Interacts with EIF2AK2. Forms a trimeric complex with DNAJB1 and HSPA8. Interacts with THAP12 (By similarity). Widely expressed, with high level in the liver. Up-regulated during an endoplasmic reticulum stress. 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.