General Morphology and Outer Envelope
A mature virion (Influenzavirus) typically has a regular spherical shape with a diameter ranging from 80 to 120 nm. However, the virus exhibits marked polymorphism: examination of fresh clinical specimens often reveals long filamentous forms.
The outer layer of the virus is represented by the envelope (supercapsid), a lipoprotein membrane of exclusively cellular origin. During budding from the infected cell, the virus acquires a patch of the host cytoplasmic membrane. This lipid envelope renders influenza viruses highly sensitive to ether and other lipid solvents. Integrated into this membrane are ion channels formed by the specific viral M2 protein.
Surface Glycoproteins: Envelope Spikes
Characteristic spikes about 10 nm in length project from the surface of the envelope. They are formed by two key virus-specific glycoproteins: hemagglutinin (HA) and neuraminidase (NA). These proteins are encoded by the viral genome, synthesized in the host cell, integrated into its membrane, and incorporated into the virion during assembly.
The number of hemagglutinin molecules on the virion surface is approximately 5 times greater than the number of neuraminidase molecules. Purified forms of these antigens are of paramount practical importance: they are utilized by biotechnologists to create modern, effective subunit vaccines.
Characteristics of Hemagglutinin (HA)
- Structural organization: A trimer (a complex of three protein molecules).
- Function: Mediates primary viral attachment to the cell. Hemagglutinin recognizes and binds to specific cellular receptors on susceptible tissues of the respiratory tract.
Characteristics of Neuraminidase (NA)
- Structural organization: A tetramer (composed of four protein molecules). Notably, this enzyme is completely absent in influenza C virus.
- Functions:
- Modifies cellular receptors following primary hemagglutinin binding, which is required for subsequent viral entry via endocytosis.
- Reduces the viscosity of respiratory tract mucus, thereby facilitating the movement of viral particles into the lower respiratory tract.
- Participates in the final stage of the replication cycle: cleaves sialic acid residues during the release of new virions from the cell, preventing their aggregation (clumping together).
Internal Structures: Matrix and Nucleocapsid
Directly beneath the lipid envelope lies a dense layer of matrix protein M1. It lines the inner surface of the envelope, providing the virion with structural rigidity and regular shape. The M1 protein performs an important regulatory function: upon reaching high accumulation levels in an infected cell, it induces the export of synthesized genome segments from the nucleus to the cytoplasm. This process is mediated by a specialized nuclear export protein (NEP).
Beneath the matrix layer lies the viral core — the nucleocapsid, which possesses helical symmetry. Its components include:
- Genome: Single-stranded fragmented negative-sense RNA. In influenza viruses types A and B, the genome is divided into 8 independent segments, whereas in type C it is divided into 7. Genome segmentation has immense epidemiological significance: it predisposes the virus to frequent genetic recombination (shift mutations) — the exchange of entire genomic segments during simultaneous coinfection of a cell by different strains.
- Nucleoprotein (NP): A structural protein that forms the capsid itself, binding tightly to the viral RNA strands.
- Polymerase complex: Directly bound to the RNA and composed of three large enzymatic proteins — PA, PB1, and PB2. They drive transcription and replication of the viral genome within the host cell.