Taxonomic Structure of the Family
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Virions are 150 nm or more in diameter, pleomorphic, but usually spherical in shape, although filamentous and other forms are common. Virions consist of a lipid envelope surrounding a nucleocapsid. The envelope is derived directly from the host cell plasma membrane by budding and contains 2 or 3 transmembrane glycoproteins. These are present as homo-oligomers and form spike-like projections, 8-12 nm in length, spaced 7-10 nm apart (depending on the genus). One non-glycosylated membrane or matrix protein is associated with the inner face of the envelope. The viral nucleocapsid consists of a single species of viral RNA and associated proteins. It has helical symmetry and is 13-18 nm in diameter with a 5.5-7 nm pitch (depending on the subfamily); its length can be up to 1,000 nm in some genera. Occasionally, multiploid virions are found, although the vast majority of virions contain a single functional genome. The viral polymerase is packaged in the virion (Fig. 1).
Physicochemical and Physical Properties
Virion Mr is about 500 106, and much greater for multiploid virions. Virion buoyant density in sucrose is 1.18-1.20 g/cm3. Virion S20w is at least 1000S. Virions are very sensitive to heat, lipid solvents, ionic and non-ionic detergents, formaldehyde and oxidizing agents.
Virions contain a single molecule of linear, negative sense, ssRNA that is not infectious alone. The RNA genome size is fairly uniform: 15,384 nts for Sendai virus, (SeV); 15,462 nts for Human parainfluenza virus 3, (HPIV-3); 15,384 nts for Mumps virus, (MuV); 15,246 nts for Simian virus 5, (SV-5); 15,450 nts for Simian virus 41, (SV-41); 15,156 nts for Newcastle disease virus, (NDV); 15,653 nts for Human parainfluenza virus 2, (HPIV-2); 15,894 nts for Measles virus, (MeV); 15,690 nts for Canine distemper virus, (CDV); 15,882 nts for Rinderpest virus, (RPV); 15,702 nts for Cetacean morbillivirus, (CeMV); 15,222 nts for Human respiratory syncytial virus, (HRSV). Some virions may contain positive sense RNA. Thus, partial self-annealing of extracted RNA may occur. The Mr of the genome is 5 106 and this constitutes about 0.5% of the virion by weight. Intracellularly, or in virions, genome-size RNA is found exclusively as nucleocapsids. The genome RNA does not contain a 5-cap, nor a covalently linked protein. The genome 3-end is not polyadenylated.
Members of the subfamily Paramyxovirinae encode 7-9 proteins (Mr 5-250 103) of which 2-4 (or more) are derived from the 2-3 overlapping ORFs in the P locus (Fig. 2). Pneumoviruses encode 9-11 proteins of Mr 4.8-250 103. Virion proteins common to all genera include: three nucleocapsid-associated proteins, i.e., an RNA-binding protein (N or NP), a phosphoprotein (P), and a large putative polymerase protein (L); three membrane associated proteins, i.e., an unglycosylated inner membrane or matrix protein (M), and two glycosylated envelope proteins, comprising a fusion protein (F) and an attachment protein (G, or H, or HN). The F protein is synthesized within an infected cell as a precursor (F0) which is activated following cleavage by cellular protease(s) to produce the virion disulfide-linked F1 and F2 subunits (order: amino F2-S-S-F1 carboxyl). Variable proteins include non-structural proteins (C, NS1, NS2), a cysteine-rich protein that binds zinc (V), a small integral membrane protein (SH), and a transcription processivity factor (M2, formerly called 22 kDa protein) which previously was thought to be a second M-like protein. Virion enzyme activities (variously represented among the genera) include a RNA-dependent RNA transcriptase, an adenylate transferase, mRNA guanylyl and methyl transferases, protein kinase and a neuraminidase.
Virions are composed of 20-25% lipid by weight. The lipids are derived from the host cell plasma membrane.
Virions are composed of 6% carbohydrate by weight; composition is dependent on the host cell. Fusion and attachment proteins are glycosylated by N-linked carbohydrate side chains. In the subfamily Pneumovirinae the attachment protein (G) is heavily glycosylated by O-linked carbohydrate side chains. The SH protein of respiratory syncytial virus contains polylactosaminoglycan.
Genome Organization and Replication
The genome organization is illustrated in Figure 2 for viruses representing the 5 genera of the family. After attachment to cell receptors, virus entry is achieved by fusion of the virus envelope with the cell surface membrane. This can occur at neutral pH. Virus replication occurs in the cell cytoplasm and is thought to be independent of host nuclear functions. The genome is transcribed processively from the 3-end by virion-associated enzymes into 5-10 separate, subgenomic, genome-complementary mRNAs. Transcription is guided by short (10-13 bp) conserved transcription start and termination/polyadenylation signals flanking each transcriptional element. The mRNA are capped and possess a 3-poly(A) tract synthesized by reiterative copying of the polyadenylation site. Intergenic regions are either highly conserved in sequence and length (Respirovirus, Morbillivirus) or are not conserved in sequence and length (Rubulavirus, Pneumovirinae). RNA replication occurs through an intermediate, the antigenome, that is a complete exact positive sense copy of the genome.
Nucleocapsid assembly occurs in the cytoplasm and is tightly linked to RNA synthesis. Nucleocapsids are enveloped at the cell surface at sites containing virus envelope proteins. Members of the subfamily Paramyxovirinae contains 5-7 transcriptional elements that encode 9-11 proteins. Each element encodes a single mRNA with the sole exception of the P/V element. This element is transcribed into an exact-copy mRNA (P or V mRNA, depending on the genus) as well as into an alternative version in which the RNA transcriptase stutters on the template at a motif midway down the element. This results in the insertion of one of more non-templated nucleotides (“RNA editing”) and shifts the reading frame to access an alternative ORF. The exact-copy and edited mRNAs synthesize two alternative proteins, P and V, which have identical amino-terminal domains but have different carboxy-terminal domains due to the frameshift. Other truncated, or chimeric, proteins can be produced by shifting into the third reading frame. The C ORF present in some viruses overlaps the P ORF and can initiate at a non-AUG codon that is accessed by ribosomal choice. Additional truncated P and C proteins can be generated by translation initiation at alternative start codons in the same ORF.
Members of the subfamily Pneumovirinae have 8 (Metapneumovirus) or 10 (Pneumovirus) transcriptional elements each encoding one mRNA. Each mRNA encodes one protein, except for the M2 mRNA which encodes two proteins from separate ORFs. There is overlap between the M2 and L transcriptional elements in some pneumoviruses (Fig. 2), but this does not affect the ORF of either mRNA.
The attachment (HN, or H, or G) and fusion (F) proteins are of primary importance in inducing virus-neutralizing antibodies and immunity against reinfection. Antibodies to N and, variably, to other viral proteins also are induced by infection. Various proteins have been reported to serve as antigens for cytotoxic or helper T cells.
Paramyxoviruses have only been conclusively identified in vertebrates and almost exclusively in mammals and birds. Most viruses have a narrow specific host range in nature, but in cultured cells they display a broad host range. Infection of cultured cells generally is lytic, but temperate or persistent infections in vitro are common. Other features of infection include the formation of inclusion bodies and syncytia. Cell surface molecules reported to serve a receptors for the attachment of respiroviruses and rubulaviruses include sialoglycoproteins and glyco-lipids. The cell surface protein CD46 is a major receptor for measles virus. Nucleocapsids associate with viral membrane proteins at the plasma membrane and are enveloped by budding. Transmission is horizontal, mainly through airborne routes; no vectors are known. Paramyxovirus infection typically begins in the respiratory tract and may remain at that site (e.g., HRSV and HPIV) or may spread to secondary sites (e.g., lymphoid and endothelial tissues for MeV, or the parotid gland, CNS and endothelial tissues for MuV). In general, paramyxovirus infections are limited by, and eliminated by, host immunity. However, virus sometimes can be shed for periods of weeks or months in normal and, especially, in immunocompromised individuals. Latent infection is unknown, and long term persistent infection is known only for subacute sclerosing panencephalitis, a rare complication that involves defective measles virus, and old dog distemper, which can involve persistence of defective or fully infectious virus, weeks or months in normal and, especially, immunocompromised individuals.
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