C Unlike the monomeric or the dimer relay FICD mutants, dimeric wild-type FICD binds ATP in a configuration that would prevent BiP substrate AMPylation. The position of the α-phosphate in the FICD:ATP structure would preclude in-line nucleophilic attack The left panel represents the superposition of the structures in the upper panel of (B), with ATP interacting residues shown as sticks and annotated. Only Glu234 deviates significantly in sidechain position. Note, however, that the FICD:ATP His363 sidechain is also flipped, forming a hydrogen bond to a ribose interacting water Mg2+ and ATP are coloured to match the corresponding ribbons. Active site waters are omitted for clarity. Distances are indicated by dashed black lines. The inset is a blow-up displaying distances i-iv between the γ-phosphates and Glu234 residues. Note, distances i and ii are derived from the γ-phosphate and Glu234 of different superimposed structures. Distances between Val316(Cγ1) and the corresponding Pα are shown in the right-hand side panel. 