Family Bornaviridae

Susan Payne , in Viruses, 2017

Assembly, Release, and Maturation

Bornavirus nucleocapsids comprise N and RNA, associated with P and Fifty proteins. K is also office of the nucleocapsid. While the details are lacking, the process of nucleocapsid germination is presumed to be similar to other members of the club Mononegavirales. Nucleocapsid germination occurs in the nucleus and the RNP must exist exported from the nucleus for the assembly of complete virions. In improver to transmission by extracellular virions, information technology is likely that nucleocapsids are also the transmissible particle from jail cell to jail cell (Fig. 22.iii). This is most clearly seen in cultured cells where BoDV nucleocapsids associate tightly with chromatin to segregate into girl cells during jail cell sectionalisation.

Figure 22.3. Fluorescent antibody labeling of duck embryo fibroblasts persistently infected an avian bornavirus. A focus of infected cells (green) sits amidst uninfected cells. The uninfected cells are visualized by DAPI staining (blue) of their nuclei.

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The Structure of Viruses

JAMES H. STRAUSS , ELLEN G. STRAUSS , in Viruses and Human Disease (Second Edition), 2008

The Nucleocapsid

The nucleocapsids of enveloped RNA viruses are fairly simple structures that comprise only one major structural poly peptide, frequently referred to as the nucleocapsid protein or core protein. This protein is normally quite basic or has a basic domain. Information technology binds to the viral RNA and encapsidates it to form the nucleocapsid. For most RNA viruses, nucleocapsids can exist recognized as singled-out structures within the infected cell and tin be isolated from virions by handling with detergents that deliquesce the envelope. The nucleocapsids of alphaviruses, and probably flaviviruses and arteriviruses as well, are regular icosahedral structures, and in that location are no other proteins within the nucleocapsid other than the nucleocapsid protein. In contrast, the nucleocapsids of all minus-strand viruses are helical and contain, in addition to the major nucleocapsid protein, ii or more than minor proteins that possess enzymatic activity. As described, the nucleocapsids of minus-strand RNA viruses remain intact within the prison cell during the entire infection cycle and serve as machines that make viral RNA. The coronaviruses also take helical nucleocapsids, but beingness plus-strand RNA viruses they practise not need to carry enzymes in the virion to initiate infection. The helical nucleocapsids of (-) RNA viruses appear disordered within the envelope of all viruses except the rhabdoviruses, in which they are coiled in a regular fashion (come across later).

The nucleocapsids of retroviruses too appear to exist adequately simple structures. They are formed from ane major precursor protein, the Gag polyprotein, that is cleaved during maturation into iv or v components. The precursor nucleocapsid is spherically symmetric only lacks icosahedral symmetry. The mature nucleocapsid produced by cleavage of Gag may or may not be spherical symmetric. The nucleocapsid likewise contains small proteins, produced by cleavage of Gag–Pro–Politician, every bit described in Affiliate 1. These minor proteins include the protease, RT, RNase H, and integrase that are required to cleave the polyprotein precursors, to make a cDNA copy of the viral RNA, and to integrate this cDNA copy into the host chromosome.

The two families of enveloped DNA viruses that nosotros consider here, the poxviruses and the herpesviruses, contain large genomes and complicated virus structures. The nucleocapsids of herpesviruses are regular icosahedrons only those of poxviruses are complicated structures containing a core and associated lateral bodies.

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Virion Structure and Composition

Christopher J. Burrell , ... Frederick A. White potato , in Fenner and White'due south Medical Virology (5th Edition), 2017

Helical Symmetry

The nucleocapsid of many RNA viruses self-assembles very differently, forming a cylindrical structure in which the protein structural units are spatially bundled as a helix, hence the term helical symmetry. The occurrence of identical poly peptide–protein interfaces on the structural units promotes the symmetrical assembly of the helix. In helically symmetrical nucleocapsids, the RNA genome forms a spiral inside the nucleocapsid (Fig. 3.7). Many plant viruses with helical nucleocapsids are rod-shaped, flexible or rigid, and non-enveloped. The helical construction of tobacco mosaic virus was amongst the get-go viral structures adamant by negative staining electron microscopy—its detailed structure was resolved by X-ray crystallography. Notwithstanding, in all viruses of vertebrates with helical symmetry, the nucleocapsid is wound into a secondary ringlet and enclosed inside a lipoprotein envelope, for example rhabdoviruses (see Fig. 27.1); and paramyxoviruses (run across Fig. 26.ii).

Figure three.vii. Helical symmetry. (A) Model of tobacco mosaic virus virion, showing interlocking capsid subunits and internal helical RNA. (B) Negative contrast electron micrograph of TMV particle.

 (A) From David South. Goodsell, RCSB. (B) Reproduced from Williams, R.C. and Fisher, H.Due west. (1974). An electron micrographic atlas of viruses. Charles C. Thomas, Springfield, Il, with permission.

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CYTOMEGALOVIRUSES (HERPERSVIRIDAE) | Murine Cytomegaloviruses

John Staczek , in Encyclopedia of Virology (Second Edition), 1999

Assembly Site, Uptake, Release, Cytopathology

Nucleocapsid self-assembly occurs in the nucleus. Concatemeric viral DNA is cleaved and packaged into preassembled capsids. The association of the nucleocapsids with the nuclear membrane is facilitated past the viral tegument proteins. Viral glycoproteins are targeted to the nuclear membrane and aid in virion associates. The virion buds out into the cisternae of the Golgi apparatus where farther modifications of the glycoproteins may occur.

Infected cells brandish a cytopathology that is characterized by the enlargement (cytomegaly) of the cell. The enlarged cells have characteristic intranuclear inclusion bodies, marginated chromatin, and large cytoplasmic vacuoles.

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Families Paramyxoviridae and Pneumoviridae

Susan Payne , in Viruses, 2017

Assembly, Release, and Maturation

Nucleocapsid assembly occurs in the cytoplasm. The nucleocapsid consists of genomic RNA, N, P, and L proteins. The nucleocapsid travels to the plasma membrane and interacts with M. Virions bud from the PM. Virions containing uncleaved F0 are noninfectious unless F0 is cleaved by extracellular, trypsin-similar proteases. Recollect that F cleavage is a necessary maturation step as it releases the hydrophobic fusion peptide at the North-terminus of Fii, a procedure critical for fusion to an uninfected jail cell.

The cellular location of F cleavage is determined by the amino acid sequence at the cleavage site. Measles and canine distemper viruses have F proteins that are cleaved in the ER/Golgi by furin-like or cathepsin-like proteases. Thus these viruses have a wide tissue tropism in the infected creature. In contrast, human parainfluenza virus F0 is broken by extracellular proteases plant in the respiratory tract.

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Control

B.C. Bonning , in Comprehensive Molecular Insect Science, 2005

6.8.2.i Nucleocapsids

Nucleocapsids are tubular in shape with cap structures at each cease (Effigy 2). The genome is condensed to about 100-fold inside the inner nucleoprotein cadre (Bud and Kelly, 1980). The supercoiled, circular genome of double-stranded Deoxyribonucleic acid is complexed with a highly bones protein, P6.9 in NPV and VP12 in GV (Tweeten et al., 1980; Wilson et al., 1987). Binding of these arginine-rich molecules to the Dna produces a compact, insoluble Dna complex. The viral genomes appear to exist prepackaged within the virogenic stroma (an electron-dense structure produced within the nucleus at the onset of viral Deoxyribonucleic acid synthesis), earlier incorporation into the capsid shells (Fraser, 1986) (Figure iii). The cease structures of nucleocapsids consist of a flat disk at the basal finish and nipple structures at the apical end (Federici, 1986). Nucleocapsids attach to membranes at the apical cease of the capsid. VP39 is the major component of the nucleocapsid in BV and OB of AcMNPV (Pearson et al., 1988; Thiem and Miller, 1989). P80 and P24 are also associated with the capsid and PP78/83 is associated with the basal cease complexed with EC27 and C42 (Braunagel et al., 2001) (Figure 2). VP1054 and VP91 are associated with both BV and OB (Olszewski and Miller, 1997a; Russell and Rohrmann, 1997).

Effigy 2. Structure of budded and apoplexy-derived virus. (a) Transmission electron micrograph (TEM) of budded virus (BV) of Lymantria dispar MNPV showing glycoprotein spikes, or peplomers at the anterior end. Scale bar = 50 nm. (Reproduced with permission from Adams, J.R., Goodwin, R.H., Wilcox, T.A., 1977. Electron microscopic investigations on invasion and replication of insect baculoviruses in vivo and in vitro. Biol. Cellulaire 28, 261–268.) (b) TEM of occlusion-derived virus (ODV) of Agrotis ipsilon MNPV (AgipMNPV) in a hemocyte of Agrotis ipsilon. Scale bar 100 nm. (c) Schematic representation of BV and ODV indicating virus-encoded structural components common to both or unique to each structure (after Funk et al., 1997). References for structural proteins: P6.9 (Wilson et al., 1987); VP39 (Thiem and Miller, 1989); P80 (Lu and Carstens, 1992); PP78/83 (Vialard and Richardson, 1993); ODV-E25 (Russell and Rohrmann, 1993); ODV-E66 (Hong et al., 1994); ODV-E56 (Braunagel et al., 1996a; Theilmann et al., 1996); ODV-E18, -E35, and -E27 (Braunagel et al., 1996b); GP41 (Whitford and Faulkner, 1992); p74 (Kuzio et al., 1989); GP64 (group I NPVs: Whitford et al., 1989) or F protein (group II NPVs and GVs: Pearson et al., 2001).

Figure 3. (a) Detail of the virogenic stroma (VS) and polyhedra (PH) within the nucleus of an AgipMNPV-infected hemocyte of Agrotis ipsilon. Scale bar = ane μm. (b) Rod-shaped nucleocapsids (NC) produced within the virogenic stroma (VS). Scale bar = 500 nm.

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PARAINFLUENZA VIRUSES (PARAMYXOVIRIDAE) | Animal

Kailash C. Gupta , in Encyclopedia of Virology (2d Edition), 1999

Genetic Manipulation

Purified nucleocapsids associated with P and 50 proteins are capable of initiating virus infection. Recently, success has been achieved in constructing nucleocapsids from synthetic RNA and viral proteins in vitro. These nucleocapsids independent a reporter gene flanked past 5′ and 3′ termini of the regulatory virus genome sequences. These RNA minigenomes were rescued on transfection of virus-infected cells. Similarly, cDNA minigenomes were rescued on cotransfection with NP, P and L genes. Now infectious virus cDNA clones have been produced. Mutant virus can be derived from these clones. These experimental approaches make it possible to alter specifically the virus gene or regulatory sequences to ascertain their function more precisely and to create virus strains that could exist potentially useful for vaccine development.

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BACULOVIRUSES (BACULOVIRIDAE) | NUCLEOPOLYHEDROVIRUS

George F. Rohrmann , in Encyclopedia of Virology (Second Edition), 1999

Structural proteins of the nucleocapsid

The nucleocapsids of BV and ODV announced to be very similar in composition. The nucleocapsid is composed of at to the lowest degree 1 pocket-size putative Deoxyribonucleic acid bounden poly peptide, a protein plainly specific to the basal end structure, and a number of capsid proteins, and the viral genome (Fig. ane).

Baculoviruses accept big genomes that must exist highly condensed to be efficiently packaged inside a nucleocapsid. Histones practise not appear to be associated with Dna packaging inside nucleocapsids. In AcMNPV, a small gene has been identified that encodes an arginine/serine–threonine rich protein of 54 amino acids (termed p6.nine) that is idea to exist a DNA-binding protein. Homologues of the AcMNPV p6.9 gene take been isolated from other NPVs. These proteins consist of more than xl% arginine and approximately 30% serine or threonine residues. It has been suggested that the basic arginine residues of the Deoxyribonucleic acid-binding protein neutralize the acidic residues of the viral Dna to enable condensation of the viral genome. Upon entry into an insect cell, serine and threonine residues on the Deoxyribonucleic acid binding protein may get phosphorylated by a protein kinase . This would outcome in the unpackaging of the viral DNA. This hypothesis is supported by the observation that protein kinase activity is associated with purified capsids of granulosis viruses and with both BV and ODV of AcMNPV.

In add-on to a Deoxyribonucleic acid-bounden protein, a protein of approximately 39   kDa (vp39) has been identified that appears to be a major component of the nucleocapsid of NPVs. This protein is present in both BV and ODV virions at a relatively loftier concentration. Immunoelectron microscopy confirmed that the vp39 poly peptide is a component of the nucleocapsid and showed that it was randomly distributed over the entire surface of the nucleocapsid . In improver to vp39, vp80 (p87 in OpMNPV), p24 (OpMNPV) and vp91 (OpMNPV) have been plant associated with nucleocapsids (Fig. ane). A phosphoprotein called ORF 1629 or pp78/83 has been shown to be associated with capsid basal cease structures.

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Icosahedral Enveloped dsRNA Bacterial Viruses

R. Tuma , in Encyclopedia of Virology (Third Edition), 2008

Membrane Acquisition and Host Cell Lysis

NC is enveloped with a lipid bilayer, which is derived from the host cell plasma membrane. Information technology contains only the viral membrane proteins. The virion-associated lytic enzyme P5 is also incorporated at this phase. Structural poly peptide P9 and the nonstructural protein P12 are essential and sufficient for membrane envelopment in ϕ6. These two proteins alone can produce lipid vesicles inside the host prison cell suggesting that envelopment takes place within the cytoplasm. The envelope is afterwards decorated with P3 receptor binding fasten which is anchored by the integral membrane protein P6. Virions are released by host cell lysis which is assisted by phage-encoded lytic proteins P5 and P10.

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Associates and Budding of Negative-Strand RNA Viruses

Douglas Due south. Lyles , in Advances in Virus Research, 2013

six Selection of Nucleocapsids

Intracellular nucleocapsids of negative-strand RNA viruses serve several of import functions in the virus replication wheel. Nucleocapsids containing positive-strand RNA antigenomes serve as templates for genome RNA replication. Nucleocapsids containing negative-strand RNA serve every bit templates for viral mRNA synthesis and for replication into antigenomes. They are also the principal nucleocapsids involved in virus assembly. Nearly of the nucleocapsids generated during the replication cycles of negative-strand RNA viruses remain intracellular, and just a pocket-size fraction is released in virions. Ane of the key questions about associates of these viruses is how nucleocapsids are selected for assembly. Part of the respond to this question involves compositional differences among intracellular nucleocapsids, particularly in the sequence of their RNAs. For the viruses with nonsegmented genomes, this involves discrimination between nucleocapsids containing genomes versus antigenomes. For the viruses with segmented genomes, there is the additional complication of the differences in RNA sequence amidst the different genome segments. There is wide variation amongst different viruses in the specificity of assembly of these various nucleocapsid species into virions, even among viruses in the aforementioned family unit. In keeping with a theme of this review, this variation probable reflects interactions with a continuum of affinities and specificities.

Among the viruses with nonsegmented genomes, VSV displays perhaps the highest level of specificity for envelopment of nucleocapsids with negative-strand versus positive-strand RNA. This preference is due to an RNA sequence most the 5′ end of the genome, which promotes incorporation of nucleocapsids into infectious particles (Whelan & Wertz, 1999). This sequence is not present in antigenomes. For other nonsegmented viruses, the predominance of negative-strand RNA in virus particles primarily reflects the relative abundance of negative-strand genomic RNA in intracellular nucleocapsids. The predominance of genomes over antigenomes is due to the greater strength of the antigenomic promoter compared to the genomic promoter in viral RNA replication. This principle was illustrated very well by engineering a recombinant rabies virus containing the genomic promoter in both genomes and antigenomes (Finke & Conzelmann, 1997). This virus generated approximately equal amounts of intracellular genomes and antigenomes. The nucleocapsids released in virus particles had a similar ratio of genomes to antigenomes. Thus, in that location is trivial, if any, discrimination betwixt nucleocapsids containing genomes versus antigenomes in the case of rabies virus, fifty-fifty though it is in the same family unit as VSV. Other viruses appear to be intermediate between these two extremes. For instance, Sendai virus particles have a weak preference for nucleocapsids containing negative-strand versus positive-strand RNA compared to intracellular nucleocapsids (Kolakofsky & Bruschi, 1975; Mottet & Roux, 1989). The RNA sequences of the 5′ ends of antigenomes are similar to those of genomes, due to the fractional final complementarity of these RNAs. Thus, in that location may be similar RNA sequences present in both genomes and antigenomes that promote nucleocapsid assembly into virus particles analogous to that in genomic RNA of VSV. If so, these RNA sequences are not likely to be essential for incorporation of nucleocapsids into particles of some viruses. When nucleocapsid proteins of negative-strand RNA viruses are expressed in transfected cells in the absence of viral RNA, they volition usually assemble into nucleocapsid-like structures containing cellular RNA. For several paramyxoviruses and filoviruses, these nucleocapsids are incorporated into virus-like particles (VLPs) when coexpressed with viral envelope glycoproteins and matrix proteins (Casabona, Levingston Macleod, Loureiro, Gomez, & Lopez, 2009; Ciancanelli & Basler, 2006; Coronel, Murti, Takimoto, & Portner, 1999; Johnson, Bell, & Harty, 2006; Johnson, McCarthy, Godlewski, & Harty, 2006; Li et al., 2009; Licata, Johnson, Han, & Harty, 2004; Pantua et al., 2006; Patch et al., 2008; Schmitt, Leser, Waning, & Lamb, 2002; Shtanko et al., 2010).

The question of whether the sequences of the RNAs in nucleocapsids affect their incorporation into virions has long been debated in the case of viruses with segmented genomes. This is particularly truthful of influenza viruses, which must package viii unlike genome segments (seven in the instance of influenza C viruses) in social club to generate an infectious virion. The two basic models are random packaging of genome segments versus selective incorporation based on their RNA sequences. Again the bachelor information can exist understood equally indicating a combination of mechanisms with varying degrees of specificity. The about convincing bear witness for the selective model comes from experiments showing that segment-specific RNA sequences in both the coding region equally well as the iii′- and five′-noncoding sequences promote incorporation of genome segments into virions (Fujii, Goto, Watanabe, Yoshida, & Kawaoka, 2003; Fujii et al., 2005; Gog et al., 2007; Liang, Hong, & Parslow, 2005; Marsh, Hatami, & Palese, 2007; Marsh, Rabadan, Levine, & Palese, 2008; Muramoto et al., 2006). Furthermore, some segments are required for incorporation of other segments. For instance, the genome segment encoding PB2 contains sequences that are important for incorporation of about of the other genome segments (Marsh et al., 2008; Muramoto et al., 2006). The basic thought to explicate these results is that nucleocapsids containing the eight genome segments collaborate with each other either directly or indirectly through their RNA sequences. Support for such interactions comes from electron microscopy of serial sections and cryo-electron tomography analysis of flu virions, which demonstrate a parallel organisation of viral RNPs of different lengths with a central RNP surrounded by seven additional RNPs (Noda et al., 2006, 2011). However, such interactions must be of relatively low affinity, since they have not been demonstrated to occur in infected cells or purified virion nucleocapsids. Virions containing such a regular arrangement of nucleocapsids announced to exist common in preparations of some strains of influenza virus, but relatively rare in others (Harris et al., 2006; Nayak, Balogun, Yamada, Zhou, & Barman, 2009), suggesting that the extent to which these higher order interactions occur varies amid strains of influenza virus.

The best evidence for the random incorporation of genome segments in influenza viruses comes from the genetic engineering of viruses with more than than eight genome segments (Bancroft & Parslow, 2002; Enami, Sharma, Benham, & Palese, 1991). Virion RNA segments encoding reporter genes are efficiently incorporated into influenza virus particles with a frequency that leads to gauge that the average virion contains ix–11 segments (Bancroft & Parslow, 2002). Furthermore, different reporters containing sequences of two dissimilar genome segments appear to compete with each other for incorporation rather than being packaged additively, as would be expected if their segment-specific sequences promote incorporation. Because all of the available data, the likely explanation is that both specific and random incorporation mechanisms occur in virtually flu viruses, but to varying extents, depending on variables such as virus strain and mayhap host jail cell type. This would be analogous to the envelopment of antigenomic RNA likewise as genomic RNA to unlike extents by different viruses with nonsegmented genomes described before. The existence of both types of mechanism would not downplay the importance of either type. For example, the predominance of sequence-specific versus random incorporation mechanisms would probable be an important determinant of the frequency by which virus reassortants could be generated in cells coinfected with 2 dissimilar viruses, and thus the frequency with which novel pandemic strains could be generated in nature.

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