Contrasting sites between ROPK and PK: kinase hinge region. (A) Sequence logos of regions surrounding the αC’ helix insert and kinase hinge in ROPK (top) and PK (bottom), with selected contrasting sites highlighted. (B) ROP2 structure with selected contrasting sites shown in “sticks” representation. (C) Inset of the ROP2 structure around the contrasting prolines in the αC’- β4 loop and linker. Two other adjacent residues, I327 and Y357, pack against the prolines on opposite sides. (D) and (E) Two structures of the protein kinase FGFR show the conditional salt bridge between the linker glutamate (E565) and β8 lysine (K641), commonly observed in typical protein kinases, dependent upon kinase activation.