Describe how SNAREs and viral fusion proteins such as hemagglutinin act to bring specific membranes close together and trigger their fusion.

The influenza virus produces hemagglutinin (HA), a viral fusion protein. It's a fusion protein belonging to the Class 1category. This spike-shaped antigenic glycoprotein is found on the influenza virus's surface. The virus must first connect to the target cell before attacking it. HA hunts for a specific monosaccharide on the surface of the target cell called sialic acid.

Endocytosis occurs when the virus is engulfed by the host cell membrane and taken into an endosome, a new compartment within the host cell.

As it tries to digest the materials inside the endosome, the cell acidifies its inner environment and strives to transform itself into a lysosome. The virus's assault mechanism, however, is aided by the acidic environment. The HA molecule undergoes structural modifications as the pH falls below 6.

A previously buried hydrophobic component of the peptide chain (fusion peptide) that was ordinarily folded against the protein suddenly jumps upward as a result of the partial unfolding that occurs. The fusion peptide has now been released from the hydrophobic pocket. It has a strong affinity for membranes and may be inserted into the endosomal membrane, thus locking the virus within the cell. Following this, the rest of the virus conforms further, bringing the endosomal and viral membranes closer together and triggering fusion. In the cytoplasm of the host cell, the viral RNA is now released.

Thus, the viral fusion protein acts as host to bring the membranes closer.