Structural alignment of the receiver domain dimer of PhoP with that of activated PhoB from E. coli (PDB code 1ZES). The PhoP structure is colored in green, and PhoB is colored in orange. The structural superposition was carried out on one of the subunits, shown on the left, with 111 matched Cα atoms and an rmsd of 1.5 Å. The other subunit, shown on the right, has a shift between the two proteins. Panel (a) has the two-fold symmetric axis of the PhoP dimer perpendicular to the paper similar to the view in Figure 2, and panel (b) is a 90° rotation from the view in (a). Despite that both involve α4-β5-α5, the interactions at the dimer interface are different in PhoP and PhoB. The activated PhoB dimer has a more compact interface, with helices from subunits packing more closely.