FIG 1.

Cryo-electron tomography used to visualize the high density of HA on the viral surface. (A) An 8.0-nm computational slice through X31 influenza virus imaged by cryo-ET, with an individual HA trimer highlighted (yellow box). (B) Structural features of HA modeled in an expanded view of the highlighted trimer in panel A, with a single HA monomer consisting of the receptor binding domain (HA1) and the fusion domain (HA2) shown in a ribbon diagram (PDB accession number 4FNK). Epitopes for HC19 (PDB accession number 2VIR) and FI6v3 (PDB accession number 3ZTJ) are highlighted using the respective crystal structures. Bar = 25 nm.