Fig. 2. Modeling of H1N1 glycosylations over time.

(A) Time line illustrating the year of acquisition of glycosylations in the globular head of the HA protein. Numbers in red indicate the amino acid position of the glycosylation site that are conserved amongst human H1N1 isolates since their emergence in 1918, and numbers in black indicate the amino acid position of the glycosylation sites that appeared in the specific years shown at the bottom. Arrows denote the persistence of glycosylation sites through time, and circles represent their disappearance. Discontinuous lines indicate the time period from 157 to 177 when H1N1 viruses did not circulate in humans. (B) Representation of an HA monomer with the antigenic sites highlighted in red and the glycosylation sites in yellow. The stem region of HA is denoted in silver. Amino acid positions refer to the H1 nomenclature (sites 71, 142, 144, 172 and 177 correspond to H3 numbering 58, 128, 130, 158 and 163, respectively). Numbers in red indicate conserved glycosylation sites. (C) Structural modeling of the trimeric HA with glycosylations as they appeared over time from 1918 to the emergence of the 2009 pH1N1 virus. The glycan structures for the sites shown in yellow have been modeled onto the Cal/09 HA (PDB 3LZG) and are also depicted in yellow. All models were made with MacPyMol.