1. (A) Crystal structure of the hemagglutinin (HA) trimer.
(A) Crystal structure of the hemagglutinin (HA) trimer. Shown in this figure is a group 1 HA (H1 subtype) (Protein Data Bank [PDB] accession number 3LGZ), and only one monomer is colored for clarity. The receptor-binding domain (yellow spheres) and the antigenic sites (for H1N1, Sa [blue], Sb [cyan], Cb [magenta[, Ca2 [white[, and Ca1 [orange]) are all located within the globular head of the protein. The stalk region, consisting of long alpha-helix loops (in red) and the fusion peptide, is more conserved across the different subtypes and plays an important role in the fusion process during virus uncoating. Epitopes/regions in the stalk region that induce broadly cross-reactive antibodies to group 1 (circled in blue) and both group 1 and 2 (circled in red) have been identified (as indicated). (B) A group 2 HA monomer (H3 subtype) (modified from data reported under PDB accession number 3DSY), with the group-reactive epitope highlighted in magenta. References are indicated in parentheses.
Sook-San Wong, and Richard J. Webby Clin. Microbiol. Rev. 2013; doi: /CMR