Taxonomic Structure of the Family
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Virions are unenveloped, 18-26 nm in diameter, and exhibit icosahedral symmetry (Fig. 1). The particles are composed of 60 copies of the capsid protein. The principal protein appears to be either VP2 or VP3 although 12 of the copies may be VP1.
Physicochemical and Physical Properties
Virion Mr is about 5.5-6.2 106. Virion buoyant density is 1.39-1.42 g/cm3 in CsCl. The S20w is 110-122S. Infectious particles are composed of about 80% protein and about 20% DNA. Infectious particles with buoyant densities about 1.45 g/cm3 may represent conformational or other variants, or precursors to the mature particles. Defective particles with deletions in the genome occur and exhibit lower densities. Mature virions are stable in the presence of lipid solvents, or on exposure to pH 3-9 or, for most species, incubation at 56°C for at least 60 min. Viruses can be inactivated by treatment with formalin, -propriolactone, hydroxylamine, or oxidizing agents.
The genome is a linear, molecule of ssDNA, 4-6 kb in size (Mr 1.5-2.0 106). The G+C content is 41-53%. Some members preferentially encapsidate ssDNA of negative polarity (i.e., complementary to the viral mRNA species; e.g., Mice minute virus, MMV), others may encapsidate ssDNA species of either polarity in equivalent (e.g., Adeno-associated virus, AAV), or different proportions (Bovine parvovirus, BPV). The percentage of particles encapsidating the positive strand can vary from 1 to 50% and may be influenced by the host cell in which the virus is produced (e.g., LUIII virus, LUIIIV). After extraction, and depending on the amounts present, the complementary strands may hybridize in vitro to form dsDNA.
Viruses generally have 2-4 virion protein species (VP1-4). Depending on the species, the Mr of VP1 species is 80-96 103, the VP2 species is 64-85 103, the VP3 species is 60-75 103 and the VP4 species 49-52 103. The viral proteins represent alternative forms of the same gene product. Enzymes are lacking. The principal protein species is VP2 or VP3. Spermidine, spermine, and putrescine have been identified in some virus particles.
Virions lack lipids.
None of the viral proteins is glycosylated.
Genome Organization and Replication
Parvoviruses possess 2 major genes, the REP (or NS) ORF that encodes functions required for transcription and DNA replication, and the CP (or CAP or S) ORF that encodes the capsid proteins. Both genes are present on the same DNA strand in the cases of the vertebrate parvoviruses (Fig. 2) and some densoviruses (e.g., Densovirinae genera Iterovirus and Brevidensovirus, Fig. 3). In the case of Densovirus, the REP function and the CPs are encoded on complementary strands (Fig. 3). Other minor ORFs have been detected in some viruses. For some of these a protein product has been identified (e.g., the ORF for the amino terminus of VP1). The MMV REP ORF produces 2 major non-structural proteins, NS1, NS2.
Mutations within the REP (NS) ORF of MMV block virus replication (Fig. 4) and gene expression (Fig. 2). For some viruses alternative splicing allows different forms of the REP gene products to be produced. The CP ORF of MMV produces up to 3 proteins. MMV VP3 is generated in the intact capsid by proteolytic cleavage of VP2. VP1 and VP2 are identical except for their amino termini. Synthesis of VP1 derives from a spliced mRNA that brings an upstream small ORF with basic amino acids motifs to the 5-end of the VP2-coding sequence. Parvoviruses use an alternative splice donor, while dependoviruses use an alternative splice acceptor for this purpose. VP1, by virtue of its particular position in the capsid structure may facilitate DNA binding. Mutants in REP or CP can be complemented in trans. The palindromic sequences (at both termini) are required in cis for DNA replication to occur.
The processes of adsorption and uncoating are poorly understood. Viral replication takes place in the cell nucleus and appears to require the cell to go through its S phase, indicating a close association between the host and viral replication processes, and probably involving host DNA polymerase(s) (e.g., , or others). Rendering the viral genome into a dsDNA is thought to be required before mRNA transcription occurs. DNA synthesis derives from a self-priming mechanism and the existence of palindromic sequences (Fig. 4). The replicative intermediate is a linear duplex molecule covalently linked at one end by a hairpin primer. The covalent link is broken by the REP protein(s) and the hairpin is transferred to the progeny strand. The resulting 3-terminal gap in the parental strand is repaired using the transferred sequence as a template. In the case of MMV, NS1 (REP equivalent) is covalently bound to the 5-end of the progeny strand. Other replicative intermediates include concatameric structures. Mature ssDNA genome equivalents are removed from the replicative complex in a manner that seems to be dependent on the availability of some species of NS protein and empty capsid assembly.
Rep68 or Rep78, the two large Rep proteins also possess helicase activity, as required for the isomerization to convert structure V to VI. Structures VII and VIII are equivalent to structures II and III. Structure VII can either be encapsidated during strand displacement (resulting in net virion production) or it can enter the template amplification pathway as shown.
Depending on the virus there may be 1 (B19V, Iteravirus and Brevidensovirus), 2 (MMV, Densovirus), or 3 (AAV) promoters for mRNA transcription (Figs. 2 and 3). Some of the mRNAs are spliced allowing alternate forms of the protein products to be produced. The mRNA species are capped and polyadenylated either at a common 3 site near the end of the genome (MMV, AAV), or at an alternative polyadenylation site in the centre of the genome as well as at a site near the end of the genome (B19V, ADNV).
Depending on the species, viruses may benefit from co-infection with other viruses, such as adenoviruses, or herpesviruses, or from the effects of chemical or other treatments of the host. Viral proteins accumulate in the nucleus in the form of empty capsid structures. Progeny infectious virions accumulate in the cell nucleus.
Some, but not all, species in a genus may be antigenically related by epitopes in the NS proteins.
Autonomous parvoviruses require host cell passage through S-phase. Certain parvoviruses replicate efficiently in the presence of helper viruses (e.g., adenoviruses, herpesviruses). These helper functions involve the adenovirus or herpesvirus early gene products and trans-activation of parvovirus replication. The helper functions appear to relate to effects of the helper virus upon the host cell rather than direct involvement of helper virus gene products in parvovirus replication.
Association of parvoviruses with tumor cell lines appears to relate to increased DNA replication and/or the state of differentiation in such cells rather than previous involvement as an etiologic agent of oncogenesis. Co-infection involving certain parvoviruses and selected oncogenic adenoviruses (or other viruses) may reduce the oncogenic effect of those viruses, possibly by promoting cell death.
In certain circumstances parvovirus DNA may integrate into the host genome from which it may be activated by subsequent helper virus infection. The site of integration may be specific in certain hosts (e.g., the q arm of human chromosome 19 for AAV-2).
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