Endoplasmic reticulum chaperone that plays a key role in protein folding and quality control in the endoplasmic reticulum lumen (By similarity). Required for ER dynamics during the first embryonic cell divisions (PubMed:15716356). Specifically, controls ER transition into sheet-like structures at the onset of mitosis, possibly by regulating homotypic membrane fusion (PubMed:15716356). 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. In the ADP-bound and nucleotide-free (apo) states, the two domains have little interaction. 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. J domain-containing co-chaperones stimulate the ATPase activity and are required for efficient substrate recognition. By endoplasmic reticulum stress. RNAi-mediated knockdown prevents the formation of ER sheet-like structures during mitosis in the 1-cell embryo. ER accumulates in foci throughout mitosis and partially fails to disperse at the end of mitosis. Belongs to the heat shock protein 70 family.