Co-chaperone for Hsp70 protein HSPA5/BiP that acts as a key repressor of the ERN1/IRE1-mediated unfolded protein response (UPR) (By similarity). J domain-containing co-chaperones stimulate the ATPase activity of Hsp70 proteins and are required for efficient substrate recognition by Hsp70 proteins (PubMed:11836248). In the unstressed endoplasmic reticulum, interacts with the luminal region of ERN1/IRE1 and selectively recruits HSPA5/BiP: HSPA5/BiP disrupts the dimerization of the active ERN1/IRE1 luminal region, thereby inactivating ERN1/IRE1 (By similarity). Also involved in endoplasmic reticulum-associated degradation (ERAD) of misfolded proteins (PubMed:22267725). Required for survival of B-cell progenitors and normal antibody production (PubMed:25222125). Interacts with HSPA5/BiP; interaction is direct (PubMed:11836248). Interacts with ERN1/IRE1 (via the luminal region) (By similarity). Interacts with DERL1 (PubMed:22267725). The J domain stimulates the ATPase activity of HSPA5/BiP, while the divergent targeting domain is required for efficient substrate recognition by HSPA5/BiP. The divergent targeting domain specifically recognizes and binds to aggregation-prone sequences. Not N-glycosylated. Perinatal death in approximately half of knockout mice (PubMed:24336520). Death is caused by fetal growth restriction, reduced hepatic glycogen stores and hypoglycemia (PubMed:24336520). Surviving adult mice display constitutive endoplasmic reticulum stress in multiple cells and tissues (PubMed:24336520). Elevated endoplasmic reticulum stress in pancreatic beta cells is associated with beta cell loss, hypoinsulinemia and glucose intolerance (PubMed:24336520) Conditional knockout mice lacking Dnajb9 in bone marrow show impaired hematopoiesis: the number of myeloid cells is increased, while the number of erythroid and B lymphoid cells is reduced (PubMed:25222125). B-cell defects cause decreased survival of B-cell precursors, including large and small pre-B, and immature B-cells (PubMed:25222125). Was initially thought to be an integral membrane protein (PubMed:11836248). However, it was later shown that it is a soluble luminal protein localized in the endoplasmic reticulum lumen.