Hemagglutinin

Hemogglutinin is a membrane fusion protein expressed on the surface of influenza virus. By mediating the fusion of viral and cellular membranes during infection it allows the viral genome to enter the cells.During the fusion reaction, hemoggluttinin inserts a hydrophobic fusion peptide into the host cell membrane and it becomes integral membrane protein in the two lipid bilayers

Tran membrane helix that anchors hemoggluttinin in the viral membrane is omitted from the structure.

Fusion reaction is triggered by low ph, which the virus encounters after the up taking endosomes of the host cell. This change in ph leads to massive structural change including the formation of long alpha helix in the core of protein.

Fusion peptide, which are previously tucked away from protein stock are displayed prominently in the tip of helix, ready to slip into host cell membrane.

Fusion peptide had to removed from the protein to be allowed for crystallization.
On the viral surface of hemogglutinin is a complex with 3 identical sub units.

It is likely the considered action of small number of hemogglutinin trimers is required to trigger a membrane fusion effect.