Probably plays a role in facilitating the assembly of multimeric protein complexes inside the ER. Is required for secretory polypeptide translocation. May physically associate with SEC63 protein in the endoplasmic reticulum and this interaction may be regulated by ATP hydrolysis. 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. Interacts with EPS1, PDI1 and YOS9. Present with 337000 molecules/cell in log phase SD medium. Belongs to the heat shock protein 70 family.