Fusion of the Influenza Virion to the Host Cell Endosome After the virus binds to the target cell surface and is endocytosed, the low pH of the endosome causes the viral HA (hemagglutinin) to undergo a structural change which frees the fusion peptide of its HA2 subunit allowing it to interact with the endosome membrane. The transmembrane domain of the HA2 (inserted into the viral membrane) and the fusion peptide (inserted into the endosomal membrane) are in juxtaposition in the acidic pH structure of HA. The concerted structural change of several hemagglutinin molecules then opens a pore through which the viral RNP will be able to pass into the host cell cytosol. GENE ONTOLOGYGO:0019064 Pubmed15609499 Pubmed2500008 Pubmed3663665 Reactome Database ID Release 43168288 Reactome, http://www.reactome.org ReactomeREACT_6351 PathwayStep3679 PathwayStep3681 PathwayStep3680 Fusion of the influenza virion HA2 protein transmembrane domain to the host cell endosome membrane Concerted hemagglutinin pore formation Conformation change in hemagglutinin freeing the fusion peptide of HA2