DnaJ homolog subfamily B member 9 (DNAJB9)

Co-chaperone for Hsp70 protein HSPA5/BiP that functions 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:18400946).

In the unstressed endoplasmic reticulum, interacts with the luminal area of ERN1/IRE1 and recruits HSPA5/BiP: HSPA5/BiP disrupts the dimerization of the active ERN1/IRE1 luminal area, thereby inactivating ERN1/IRE1 (By similarity). Also involved in endoplasmic reticulum-associated degradation (ERAD) of misfolded proteins.