Fig. 1.

Detected positions, discriminating between prior human H1N1 circulating strains and the pH1N1 strain, correspond to known antigenic sites. The RBD of the HA protein from the human A/California/04/2009 H1N1 strain (PDB ID code 3lzg) is shown in cartoon representation. Front (A) and side (B) views of the RBD with the identified positions and antigenic sites presented as all-atom spheres. The Sa antigenic site is in pink, Sb site in blue, Ca site in green, and the Cb antigenic site in orange. The identified specificity determinants that overlap with H1N1 antigenic sites are shown using a dark variant of the site color (e.g., a position identified in Ca is colored in dark green, whereas the rest of the site is in pale green). Predictions that do not overlap with a known antigenic site are in dark red. All newly detected positions are located in and around the recognition hot spots of the immune system.