Updated project metadata. The Hsp70 chaperone BiP is covalently modified with adenosine monophosphate (referred to as AMPylation) in order to adapt its activity to the fluctuating folding load within the endoplasmic reticulum. This modification is catalyzed by the only human representative of the family of filamentation induced by cyclic adenosine monophosphate (Fic) enzymes HYPE/FICD. The structural basis for BiP binding and AMPylation has remained elusive due to the low affinity of enzyme substrate complexes.