Under conditions of ER-stress the dimeric form FICD is favoured (right hand side). Dimeric FICD cannot bind ATP in an AMPylation competent mode but can efficiently catalyse deAMPylation of BiP-AMP (thereby remobilising BiP back into the chaperone cycle). A decrease in unfolded protein load in the ER, possibly associated with a decreased ER ADP/ATP ratio, shifts the FICD monomer-dimer equilibrium towards monomeric FICD. Monomeric FICD can bind MgATP in an AMPylation competent conformation and, as such, AMPylate and inactivate surplus BiP. 