Figure 12. Schema of the proposed relationships between the different forms of BiP.
The monomer (‘A’ form) at the center of the schema binds clients and is shifted to BiP:Substrate complexes by mounting unfolded protein load. With diminishing clients it partitions to inactive oligomers, with whom it exists in equilibrium. This equilibrium is influenced by ER calcium, whose depletion favors the oligomeric form. The ‘A’ form is also in equilibrium with modified BiP (‘B’ form, likely ADP-ribosylated or AMPylated BiP) and this equilibrium too is influenced by unfolded protein burden but on a slower time scale than oligomer formation and disassembly.