Classification and Major Genera
The family Paramyxoviridae includes two major subfamilies, which differ in their genus composition and surface proteins:
- Subfamily Paramyxovirinae:
- Genus Morbillivirus (measles virus).
- Genus Respirovirus (human parainfluenza viruses 1, 3, Sendai virus).
- Genus Rubulavirus (mumps virus, parainfluenza 2, 4a, 4b).
- Genus Henipavirus (zoonotic Hendra and Nipah viruses).
- Genus Avulavirus (Newcastle disease virus).
- Subfamily Pneumovirinae:
- Genus Pneumovirus (Respiratory Syncytial Virus — RSV).
- Genus Metapneumovirus (human metapneumoviruses).
In addition to classic pathogens, emerging high-consequence paramyxoviruses were discovered in the late 20th and early 21st centuries. For instance, Hendra and Nipah viruses, whose natural reservoirs are fruit bats, cause severe encephalitis in humans and animals. Meanwhile, human metapneumovirus is recognized as a major cause of respiratory infections in infants.
Virion Structure
Paramyxoviruses range in size from 150 to 200 nm.
Internally, they contain a nucleocapsid with helical symmetry enclosing a linear single-stranded negative-sense RNA genome. The genome is associated with three essential proteins: N (nucleoprotein — maintains structure), L (polymerase — required for replication), and P (phosphoprotein).
Externally, the virion is enveloped (supercapsid). The inner surface of the envelope is lined by the matrix protein (M), which links the envelope to the nucleocapsid. Two types of glycoprotein spikes project from the surface:
- Fusion protein (F): mediates viral entry via membrane fusion (requires proteolytic cleavage into F1 and F2 fragments).
- Attachment proteins: vary by viral genus (e.g., HN — hemagglutinin-neuraminidase, H — hemagglutinin in measles virus, G — glycoprotein in RSV).
Replication Cycle
The entire reproductive cycle of paramyxoviruses takes place exclusively in the cytoplasm of the infected cell.
- Attachment: The virus binds to cellular receptors (sialic acid or specific proteins) via attachment proteins (HN, H, or G).
- Penetration: The F protein fuses the viral envelope with the host cell membrane. Notably, this same protein can induce neighboring infected cells to fuse, forming giant multinucleated structures known as syncytia.
- Synthesis: The viral RNA-dependent RNA polymerase (brought into the cell within the virion) transcribes mRNA to synthesize viral proteins, and synthesizes a positive-sense antigenomic template. New negative-sense genomic RNAs are subsequently replicated from this antigenome.
- Assembly and Release: Newly synthesized genomic RNAs associate with N, L, and P proteins to form nucleocapsids, traffic to the plasma membrane (interacting with the M protein), and exit the cell by budding.