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 (By similarity). 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. 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. Interacts with EIF2AK3 (By similarity). Interacts with EIF2AK2 (PubMed:8576172). Forms a trimeric complex with DNAJB1 and HSPA8 (PubMed:9920933). Interacts with THAP12 (PubMed:9447982). Widely expressed with high level in the pancreas and testis. Also expressed in cell lines with different levels. Up-regulated during an endoplasmic reticulum stress via ATF6. Activated in response to infection by influenza virus through the dissociation of DNAJB1. Down-regulated by DNAJB1 and THAP12. 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. The disease is caused by variants affecting the gene represented in this entry.