Host Adaptation of Influenza, Part II

Other Influenza Proteins


Neuraminidasen (NA)


Neuraminidase is the other protein major glycoprotein on the surface of the virion. Unlike hemagglutinin, it's not involved in binding to the surface of the cell. Instead, this protein will actually cleave sialic acid from other carboyhydrate linkages. This feature is especially important in virus release; as new virons are made, hemagglutinin binds to sialic acid on the infected cells. The virus wouldn't be able to escape and spread without neuraminidase cutting sialic acid and freeing the virus.


The same function helps with virus entry. Think of hemagglutinin sticking too strongly; it wouldn't be able to change its conformation and fuse with the plasma membrane. It’s true, but I haven't come across too many papers on this subject.


In terms of host range, I'm not aware of any drastic changes that need to be made in NA in order for an avian strain to infect humans or vice versa. But keep in mind that the neuraminidase activity of any virus will evolve to be directly proportional to HA's ability to bind sialic acid (i.e. a weaker binding hemagglutinin has a weak neuraminidase)

Matrix (MA) and Nonstructural (NS) Proteins

Ok, these proteins are important. The matrix proteins do two things: M2 provides an ion channel. When the virus invades the cell, the low pH of the endosome is essentially transported into the virus (H+) resulting in release of ribonucleotide particles (the genome is released). M1 is definitely matrix and forms the outer capsid-like structure of the virus.

At least one of the nonstructural proteins slows down the host immune response by interfering with the interferon pathway (It stops RIG-I recognition of nucleic acid) The other frameshift, which results in NS1, does something else, like assisting in viral replication, but it's pretty vague for me.

So, while these proteins play major roles in virus biology, I'm not aware of any specific changes that need to occur in order for influenza to adapt from human to avian and vice versa. However, these changes do occur, they are just not studied extensively.

Next: Polymerase proteins and nucleoprotein.