Fig. 4.

Monomeric HA structures. (a) Structure of HA pre-fusion state (H5 subtype, PDB code 2IBX[78]). Left: Ribbon diagram of HA pre-fusion trimer. The HA1 subunits are colored in a shade of yellow. The HA2 subunits are colored in a shade of blue. Right: Ribbon diagram of HA protomer. (b) First row: ribbon diagrams of HA2 structure (H1 subtype) as in the pre-fusion protomer [74] and in two monomeric HA structures [74], [77]. Second row: ribbon diagrams of HA2 structure (H3 subtype) [79] as in the pre-fusion protomer, in two monomeric HA0 structures [76] and as in its trimeric post-fusion conformation [80]. Third row: ribbon diagrams of HA2 structure (H5 subtype) as in the pre-fusion protomer [78] and in a monomeric HA structures [75]. The residues are colored by their secondary structure in the pre-fusion protomer. Residues corresponding to helix A are in yellow, to helix B in blue and to the loop located between helices A and B in red. In several monomers, the part of the chain corresponding to the loop between helices A and B is not visible in the crystal. All the structures are aligned on helix A of the prefusion protomer. The corresponding PDB codes are indicated above each structure.