Fig. 5.

Comparison of P domain monomers and dimers. (A) Cartoon representation of the P domain monomer in the C2 crystal form (yellow). Site 1 residues are shown in magenta for reference. (B) Cartoon representation of the P domain dimer in the C222₁ crystal form. The dimeric subunits are indicated in purple and gray, and black arrows indicate the location of the HBGA binding sites. HBGA binding sites 1 and 2 are indicated in magenta and green, respectively. (C) Cartoon representation of the P domain monomer in the C222₁ crystal form (cyan). Site 1 residues are shown in magenta for reference. (D) Superposition of the C2 monomer (yellow) and one of the dimer subunits (purple) in the C222₁ crystal form. Regions with conformational flexibility, which include site 1, are shown inside a box (side view). The inset shows the top view of this boxed loop region. Site 2 (green) is seen only in the dimer subunit (purple); this region in the C2 monomer is disordered. Arrows indicate the HBGA binding sites. (E) Superposition of the C222₁ monomer (cyan) and a subunit from the dimer (purple) in the same C222₁ crystal form. Regions with conformational flexibility, which include site 1, are shown inside a box (side view). The top view of this boxed region is shown in the inset. As in D, site 2 (green) is seen only in the dimer subunit (purple), and this region in the C222₁ monomer is disordered. Arrows indicate the HBGA binding region. (F) Cation-π interaction between residues H396 and Y444 in the P domain of the 2004 variant with a face-to-face distance of ∼3.8 Å (indicated by a red dashed line). These two highly conserved residues are in close proximity to the dimeric interface, HBGA binding sites 1 (magenta) and 2 (green). Bound H-type HBGA is shown (yellow) for reference. The cation-π interaction is observed only for the dimeric subunits and is not observed for the monomeric state, as the loops containing residues H396 and Y444 in this state are disordered.