Figure 2.

Schematic representation of receptor binding and fusion of influenza virus. Only a few HA molecules can reach the endosomal membrane by their fusion peptides in the course of fusion triggered by low pH in the late endosome. The amplitude of HA-induced deformations of the endosomal membrane should be highly anisotropic. (a) Attachment of the viral particle to the receptor molecule (ganglioside GD1a) in the plasma membrane. (b) Virion enters the cell in the process of endocytosis. The size of the endocytic vesicle closely fits the size of the viral particle. (c) The large endosome fuses with the endocytic vesicle. The virion remains attached to the endosomal membrane due to binding to the receptor molecule. (d) Drop of pH in the late endosome triggers the conformation transition of all hemagglutinins. However, only those HAs that are the closest to the receptor molecule can reach the endosomal membrane by their fusion peptides. More distant HAs fold into the post-fusion conformation and insert their fusion peptides into the own viral membrane.