General morphology and outer envelope
The mature virion (Influenzavirus) most often has a regular spherical shape with a diameter ranging from 80 to 120 nm. However, the virus exhibits marked polymorphism: examination of fresh clinical preparations often reveals long filamentous forms.
The outer layer of the virus is represented by the supercapsid—a lipoprotein envelope of exclusively cellular origin. During the process of exiting the infected cell (via budding), the virus captures a section of the host's cytoplasmic membrane. The presence of this lipid envelope causes the high sensitivity of the influenza virus to ether and other fat solvents. Ion channels formed by the specific viral M2 protein are integrated into this membrane.
Surface glycoproteins: spikes on the envelope
Characteristic "spikes" about 10 nm long are located on the surface of the supercapsid. They are formed by two key virus-specific glycoproteins: hemagglutinin (HA) and neuraminidase (NA). These proteins are encoded by the virus's own genome, synthesized in the host cell, embedded in its membrane, and become part of the virion at the moment of its formation.
The number of hemagglutinin molecules on the surface of the virion is approximately 5 times greater than the number of neuraminidase molecules. Purified forms of these antigens are of paramount practical importance: it is they that are used by biotechnologists to create modern, effective subunit vaccines.
Characteristics of hemagglutinin (HA)
- Structural organization: represents a trimer (a complex of three protein molecules).
- Function: responsible for the primary interaction of the virus with the cell. Hemagglutinin recognizes and binds to specific cellular receptors of susceptible respiratory tract tissues.
Characteristics of neuraminidase (NA)
- Structural organization: represents a tetramer (consists of four protein molecules). It should be noted that in influenza C virus, this enzyme is completely absent.
- Functions:
- Carries out the modification of cellular receptors after the primary binding of hemagglutinin, which is necessary for subsequent entry of the virus inside via endocytosis.
- Reduces the viscosity of respiratory tract mucous secretions, thereby facilitating the movement of viral particles into the lower respiratory tract.
- Participates in the final stage of the reproduction cycle: cleaves sialic acids during the exit of new virions from the cell, preventing their aggregation (mutual sticking).
Internal structures: matrix and nucleocapsid
Immediately beneath the lipid envelope lies a dense layer of matrix protein M1. It underlies the supercapsid from the inside, giving the virion structural rigidity and regular shape. The M1 protein performs an important regulatory function: upon reaching a high level of accumulation in the infected cell, it induces the export of synthesized genome segments from the nucleus to the cytoplasm. This process is carried out with the participation of a special nuclear export protein (NEP).
Beneath the matrix layer lies the core of the virus—the nucleocapsid, which has a helical type of symmetry. Its composition includes the following components:
- Genome: represented by single-stranded fragmented negative-sense RNA. In influenza viruses of types A and B, the genome is divided into 8 independent segments, while in type C virus, it is divided into 7. Genome segmentation has colossal epidemiological significance: it predisposes to frequent genetic recombinations (shift mutations)—the exchange of entire genome segments during simultaneous infection of a cell by different strains.
- Nucleoprotein (NP): a structural protein that forms the capsid itself, tightly binding to the strands of viral RNA.
- Polymerase complex: associated directly with RNA and consists of three large enzymatic proteins—PA, PB1, and PB2. They ensure the processes of transcription and replication of the viral genome inside the host cell.
Briefly: answers to topical questions
What function in the reproduction cycle is performed by the ion channel formed by the M2 protein?
The ion channel formed by the M2 protein ensures the entry of hydrogen ions into the viral particle, which is necessary for its "uncoating" (deproteinization). Acidification of the internal environment of the virion causes the dissociation of ribonucleoproteins from the matrix protein. As a result of this process, the viral RNA is released from the capsid and enters the cell cytoplasm for subsequent transport to the nucleus.
Which specific molecules on the surface of respiratory tract cells does hemagglutinin bind to?
Hemagglutinin binds to specific receptors, which are compounds with sialic acid residues. On the surface of the respiratory epithelium, these molecules are:
- α-2', 6' galactose-containing oligosaccharides—the main receptors for virus attachment to the cells of the upper respiratory tract mucosa.
- α-2', 3' receptors—specific molecules on the epithelium of bronchioles and alveoli, to which the pandemic strain A (pH1N1) has acquired complementarity.
These sialic acids are part of cellular glycoproteins and glycolipids.
What types of antigenic variability are characteristic of the influenza virus?
Influenza A virus is characterized by two main types of antigenic variability of surface antigens—hemagglutinin (HA/H) and neuraminidase (NA/N).
- Antigenic drift (drift)—minor changes in the structure of surface antigens resulting from point mutations in the genes encoding the antigenic determinants of HA and NA. Occurs frequently and constantly, leads to the emergence of new serovariants, and causes periodic epidemics.
- Antigenic shift (shift, reassortment)—significant, radical changes in the structure of surface antigens, up to the complete replacement of HA or NA. Occurs upon the exchange of RNA segments during the simultaneous entry of two different viral subtypes into a single cell. Occurs rarely and is the cause of pandemics.
To avoid confusing the functions of surface proteins, use the first-letter rule: Hemagglutinin (HA) — Hauls virus into the cell (ensures attachment). Neuraminidase (NA) — Nips off for release (prevents aggregation of mature virions upon exit).
Class and exam questions
Why does the influenza virus exhibit such high variability?
The viral genome is not a single molecule, but consists of 7 or 8 separate negative-sense RNA segments. This fragmented structure strongly predisposes to genetic recombination: if two different strains enter the same cell, they can exchange entire segments, generating a fundamentally new viral subtype.
Where does the influenza virus get its outer lipid envelope?
The outer envelope (supercapsid) is not synthesized by the virus from scratch. It has a cellular origin: at the moment of the formed virion's exit from the cell via budding, it captures ("dresses" in) a section of the host cell's lipid membrane.
Which components of the influenza virus are used in the production of modern vaccines?
To create safe subunit vaccines, purified surface glycoproteins are used—hemagglutinin (HA) and neuraminidase (NA). It is they that form a reliable protective immune response.
What else to study on this topic
- Mechanism of genetic recombinations (antigenic shift) in fragmented viruses
- Specific structural differences among influenza viruses of types A, B, and C
- Principles of creating subunit vaccines based on HA and NA
- Mechanism of virion entry into the cell via endocytosis
- Function of ion channels (M2 protein) in the process of viral uncoating inside the cell
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