Pneumoviridae

Pneumoviruses are pleomorphic, capable of producing spherical and filamentous, enveloped virions (virus particles) that vary in size from 150 to 200 nm in diameter. The nucleocapsid consisting of a protein shell and viral nucleic acids has a helical symmetry. Nucleocapsids have a diameter of 13.5 nm and a helical pitch of 6.5 nm.[6]

The genome is composed of negative-sense, single-stranded RNA that is non-segmented. It is about 15 kbp in size, and encodes eleven proteins.[5] A unique feature of the genome is the M2 gene, which encodes proteins M2-1 and M2-2. The pneumovirus M2-1 protein is distinctive, and no homologue has been found in any other virus families. It functions as a processivity factor for the virus RNA-dependent RNA polymerase, and promotes viral RNA synthesis.[7] Viruses in this family are often associated with respiratory infections, and are transmitted through respiratory secretions.[5]

N – Nucleocapsid protein. Essential for viral replication and transcription. Plays a major role in forming a capsid around the viral genome.[8]

P – Phosphoprotein required for replication.[8] Facilitates RNA-dependent RNA polymerase attachment and recruits M2 protein.[9]

M1 – Matrix protein. Facilitates nucleocapsid and envelope interactions.[10]

M2-1 – Matrix protein. Intragenic and intergenic transcription factor required for mRNA transcript elongation.[8] Binds to nascent and provides stability in order to prevent premature termination.[11]

M2-2 – Matrix protein. Involved in regulating transcription and replication. When over expressed, has been shown to inhibit viral replication.[12]

F – Fusion protein. Type I glycoprotein that facilitates fusion between the virus and the host cell membrane.[13]

SH – Small hydrophobic protein. Non essential. Exact function is unknown. Suggested to alter membrane permeability and block apoptosis.[13][14]

G – Type II glycoprotein.[13] Facilitates virus attachment through interactions with glycosaminoglycans.[15]

L – RNA dependent RNA polymerase.[16] Required for replication.[8] Adds a methylated guanosine cap and poly(A) tail to nascent mRNA.[16]

Pneumoviruses replicate in the cytoplasm of the host cell.[1][5] First, the virus binds to HN glycoprotein receptors expressed on the surface of the cell.[5] Then, through the action of the fusion protein, the virus fuses to the host plasma membrane and the nucleocapsid is released.[5] Prior to undergoing replication, mRNA is transcribed and viral proteins are translated. Transcription is dependent on virally encoded RNA-dependent-RNA-polymerase, which binds the genome at the 3' leader region and then sequentially transcribes each gene. Translation of viral proteins is carried out by host cell ribosomes.[17][18] Once sufficient P, N, L, and M2 proteins are available to create a capsid around the newly replicated genome, the virus undergoes replication.[18] After replication, the P, L, and M proteins participate in forming the ribonucleocapsid. Once virion assembly is complete, the virion egresses by budding out of the cell.[18]