Superimposition of LukE and LukD with HlgA and HlgB. Left, ribbon representation of LukE colored by sequence conservation with respect to HlgA (red, conserved; blue, nonconserved) superimposed on HlgA (PDB entry 2qk7) in yellow. The missing regions in the structures of HlgA are indicated in the superimpositions on LukE, respectively, with the residues drawn as sticks. Right, structural superimposition of LukD with HlgB (PDB entry 1lkf). LukD is colored according to sequence conservation with HlgB.