FICD acts bifunctionally to AMPylate and de-AMPylate the endoplasmic reticulum chaperone BiP.

Protein folding homeostasis in the endoplasmic reticulum (ER) is defended by an unfolded protein response that matches ER chaperone capacity to the burden of unfolded proteins. As levels of unfolded proteins decline, a metazoan-specific FIC-domain-containing ER-localized enzyme (FICD) rapidly inactivates the major ER chaperone BiP by AMPylating T518. Here we show that the single catalytic domain of FICD can also release the attached AMP, restoring functionality to BiP. Consistent with a role for endogenous FICD in de-AMPylating BiP, FICD^-/- hamster cells are hypersensitive to introduction of a constitutively AMPylating, de-AMPylation-defective mutant FICD. These opposing activities hinge on a regulatory residue, E234, whose default state renders FICD a constitutive de-AMPylase in vitro. The location of E234 on a conserved regulatory helix and the mutually antagonistic activities of FICD in vivo, suggest a mechanism whereby fluctuating unfolded protein load actively switches FICD from a de-AMPylase to an AMPylase.

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