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 (By similarity). 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. Required for survival of B-cell progenitors and normal antibody production (By similarity). Interacts with HSPA5/BiP; interaction is direct (By similarity). Interacts with ERN1/IRE1 (via the luminal region) (By similarity). Interacts with DERL1 (By similarity). 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. Was initially reported to localize in the nucleus (PubMed:11525638). However, it was later shown to localize in the endoplasmic reticulum lumen.