Endoplasmic reticulum chaperone that plays a key role in protein folding and quality control in the endoplasmic reticulum lumen (By similarity). Involved in the correct folding of proteins and degradation of misfolded proteins via its interaction with DNAJC10/ERdj5, probably to facilitate the release of DNAJC10/ERdj5 from its substrate (By similarity). Acts as a key repressor of the ERN1/IRE1-mediated unfolded protein response (UPR). In the unstressed endoplasmic reticulum, recruited by DNAJB9/ERdj4 to the luminal region of ERN1/IRE1, leading to disrupt the dimerization of ERN1/IRE1, thereby inactivating ERN1/IRE1. Accumulation of misfolded protein in the endoplasmic reticulum causes release of HSPA5/BiP from ERN1/IRE1, allowing homodimerization and subsequent activation of ERN1/IRE1 (By similarity). May also play a role in apoptosis and cell proliferation (By similarity). ATP + H2O = ADP + H(+) + phosphate The chaperone activity is regulated by ATP-induced allosteric coupling of the nucleotide-binding (NBD) and substrate-binding (SBD) domains (By similarity). In the ADP-bound and nucleotide-free (apo) states, the two domains have little interaction (By similarity). In contrast, in the ATP-bound state the two domains are tightly coupled, which results in drastically accelerated kinetics in both binding and release of polypeptide substrates (By similarity). J domain-containing co-chaperones (DNAJB9/ERdj4 or DNAJC10/ERdj5) stimulate the ATPase activity and are required for efficient substrate recognition by HSPA5/BiP. Homooligomerization inactivates participating HSPA5/BiP protomers and probably act as reservoirs to store HSPA5/BiP molecules when they are not needed by the cell (By similarity). Monomer and homooligomer; homooligomerization via the interdomain linker inactivates the chaperone activity and acts as a storage of HSPA5/BiP molecules (By similarity). Interacts with DNAJC10 (By similarity). Interacts with DNAJB9/ERdj4; leading to recruit HSPA5/BiP to ERN1/IRE1. Interacts with ERN1/IRE1; interaction takes place following interaction with DNAJB9/ERdj4 and leads to inactivate ERN1/IRE1 (By similarity). Identified by mass spectrometry in melanosome fractions from stage I to stage IV (By similarity). Localizes to the cell surface in epithelial cells; high levels of free iron promotes cell surface localization (By similarity). The interdomain linker regulates the chaperone activity by mediating the formation of homooligomers. Homooligomers are formed by engagement of the interdomain linker of one HSPA5/BiP molecule as a typical substrate of an adjacent HSPA5/BiP molecule. HSPA5/BiP oligomerization inactivates participating HSPA5/BiP protomers. HSPA5/BiP oligomers probably act as reservoirs to store HSPA5/BiP molecules when they are not needed by the cell. When the levels of unfolded proteins rise, cells can rapidly break up these oligomers to make active monomers. Belongs to the heat shock protein 70 family.