Extended Data Fig. 5|. Gate loop motion may be restrained in the inactive state by an ionic interaction with the finger loop.

In certain inactive-state crystal structures of arrestin-1 (for example, PDB entry 1CF1, chain D) and arrestin-2 (for example, PDB entry 1G4M, chain A), a lysine in the gate loop (K298 in arrestin-1, K292 or K294 in arrestin-2) forms an ionic interaction with a carboxylic acid in the finger loop (D71 in arrestin-1, E66 in arrestin-2). Simulations initiated from these structures with the C tail removed exhibited less frequent transitions of the gate loop to fully active conformations than simulations initiated from crystal structures in which this ionic interaction between the gate loop and the figure loop was not formed (for example, PDB entry 1CF1, chain A). Thus a particular finger loop conformation might mildly increase the stability of the inactive-state gate loop conformation. In simulations, we observed additional sets of ionic interactions between gate loop lysines and either D67 on the finger loop or D135 on the middle loop (according to arrestin-2 numbering), which also appeared to prevent motion of the gate loop towards the active state. Certain finger loop conformations might also favour the inactive state through interactions with the C tail of arrestin⁶⁴.