(A) Cartoon representation of the ATPase domain and peptide-binding domain of BiP. (B) Structures of the ATPase domain (PDB: 3QFU) and peptide binding domain (PDB 3H0X) of yeast BiP. ADP is represented as pink sticks and the cysteine is represented in red. (C) ATPase cycle of BiP. When the ATPase domain binds to ATP, the substrate-binding domain has low substrate binding affinity. When the ATPase domain is bound to ADP, the substrate- binding domain has high substrate binding affinity. The ATPase cycle allows BiP to act as a foldase and a holdase.