Protein that can both mediate the addition of adenosine 5'-monophosphate (AMP) to specific residues of target proteins (AMPylation), and the removal of the same modification from target proteins (de-AMPylation), depending on the context (PubMed:29089387). The side chain of Glu-247 determines which of the two opposing activities (AMPylase or de-AMPylase) will take place (By similarity). Acts as a key regulator of the unfolded protein response (UPR) by mediating AMPylation or de-AMPylation of Hsc70-3/BiP (PubMed:25395623, PubMed:29089387). In unstressed cells, acts as an adenylyltransferase by mediating AMPylation of Hsc70-3/BiP at 'Thr-518', thereby inactivating it (PubMed:29089387). In response to endoplasmic reticulum stress, acts as a phosphodiesterase by mediating removal of ATP (de-AMPylation) from Hsc70-3/BiP at 'Thr-518', leading to restore HSPA5/BiP activity (PubMed:29089387). ATP + L-tyrosyl-[protein] = diphosphate + O-(5'-adenylyl)-L-tyrosyl-[protein] ATP + L-threonyl-[protein] = 3-O-(5'-adenylyl)-L-threonyl-[protein] + diphosphate 3-O-(5'-adenylyl)-L-threonyl-[protein] + H2O = AMP + H(+) + L-threonyl-[protein] The side chain of Glu-247 determines which of the two opposing activities (AMPylase or de-AMPylase) will take place. In response to endoplasmic reticulum stress, mediates de-AMPylase activity (By similarity). Adenylyltransferase activity is inhibited by the inhibitory helix present at the N-terminus: Glu-247 binds ATP and competes with ATP-binding at Arg-386, thereby preventing adenylyltransferase activity (By similarity). In unstressed cells, disengagement of Glu-247 promotes adenylyltransferase activity (By similarity). Activation dissociates ATP-binding from Glu-247, allowing ordered binding of the entire ATP moiety with the alpha-phosphate in an orientation that is productive for accepting an incoming target hydroxyl side chain (By similarity). Homodimer; homodimerization may regulate adenylyltransferase and phosphodiesterase activities. Up-regulated in response to endoplasmic reticulum stress. The fido domain mediates the adenylyltransferase activity. No visible phenotype. Flies are viable. Belongs to the fic family. Was initially thought to mediate AMPylation of Hsc70-3/BiP at 'Thr-366' (PubMed:25395623). However, it was later shown that it catalyzes AMPylation of Hsc70-3/BiP at 'Thr-518'.