Influenza neuraminidase is on the surface of influenza viruses that enables the virus to be released from the host cell. Neuraminidases are enzymes that cleave sialic acid groups from glycoproteins and are required for influenza virus replication. In addition to the mutations that arise due to antigenic drift, the NA of influenza A viruses (IAVs) can exist in different forms.Based on HA and NA antigenicity using serologic tests with hyperimmune sera, there have been a total of 16 HA (H1-16) and 9 NA (N1-9) subtypes identified in birds.Nine subtypes of influenza A NA are divided into two phylogenic groups. The first group consists of the neuraminidases of N1, N4, N5 and N8 subtypes, and the second one consists of N2, N3, N6 N7 and N9 subtypes.These are expressed in numerous combinations of viruses isolated from aquatic avian species, and an additional two combinations, H17N10 and H18N11, have been identified in bats
The neuraminidase (NA) assembles as a tetramer of four identical polypeptides.The four monomers fold into four distinct structural domains:
• cytoplasmic tail
• transmembrane region
• catalytic head
NA tetramer exists in local clusters on the virion surface or as isolated spikes surrounded by HA. Reduced stalk length may impact the ability of NA to contact sialic acids on mucins or cellular receptors as neighboring HA may sterically hinder its approach. Depending on the length of the stalk region, the NA may protrude slightly more or less above the viral envelope than the HA, which may influence the overall enzymatic activity of the virus.
Picture 1: NA structure
Picture 2: Stalk length of NA
Neuraminidase (NA) has several functions in virus replication and infection:
• Virus Entry
• Receptor Binding
• Virus Internalization
• Catalytic Activity
• NA Substrate Specificity
Rather than just a sialidase that facilitates virus release from infected cells, the NA is a complicated multifunctional protein with an important role at many stages of the infectious process. While the NA is the main target for current antiviral therapies, recent approaches to new influenza therapy include targeting the HA with monoclonal antibodies. However, given the NA also has the capacity to bind receptors, there needs to be caution in this approach, as it is possible that compensating mutations in the NA may allow escape from inhibition of the HA.
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