Page 301 - Avian Virology: Current Research and Future Trends

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(Sheppard et al., 1998a; Ojkic et al., 2002; Both, 2004; Wold and FAdV-1 and between E4 ORFs 4 and 7 of EDSV (Larsson et
Ison, 2013). al., 1986; Hess et al., 1997; Harrach et al., 2012). VA RNAs of
Transcription from MLP is mediated by the viral IVa2 and 22K HAdV lack nucleotide sequence homology to those of FAdV-1
proteins by direct binding to the first intron between leader exons and EDSV, but the predicted secondary structures are similar.
1 and 2. These proteins also bind the ITR of the left end of the VA RNAs of EDSV and FAdV-1, on the other hand, have 74%
genome for packaging into the virion. homology (Hess et al., 1997). Both HAdV and FAdV-1 VA
Late in infection, the viral mRNAs encoding L proteins are RNAs stimulate mRNA translation in vitro, though FAdV-1 VA
preferentially translated while translation of the cellular mRNAs RNA seems to be less efficient (Larsson et al., 1986).
is inhibited. First, L mRNAs are preferentially exported from
the nucleus to the cytoplasm as a result of physical interactions Assembly
between the viral 100K and the bipartite or tripartite leader After genome replication and expression of structural proteins,
sequences. Second, inhibition of translation of cellular mRNAs virus assembly takes place in the nucleus of the infected cell.
takes place due to dephosphorylation of eIF4E, a subunit of the Besides its role on viral mRNA transport and translation, the viral
multimeric initiation factor eIF4F that binds the 5′m7GPPPN 100K protein assists the nuclear localization and folding of the
cap structure, by the viral 100K. The tripartite, or likely the bipar- hexon protein and acts as scaffold to facilitate assembly of the
tite, leader sequences provide an alternative cap-independent hexon trimers (Hong et al., 2005). The penton capsomers con-
translation through ribosome shunting. Ribosome shunting is sisting of a pentameric penton base and trimeric fibre assemble in
also mediated by the viral 100K through physical interaction with the cytoplasm and translocate to the nucleus for virion assembly.
the tripartite, or likely the bipartite, leader sequences and the Encapsidation of the viral DNA is directed by cis-acting
eIF4G scaffold subunit of the eIF4F complex. packaging sequences at the left end of the viral genome. These
Upon infection, interferon (IFN) signalling stimulates the packaging signals are AT-reach sequences at nucleotides (nts)
expression of the cellular protein kinase PKR. PKR activation 200–400 in HAdV-5, 250–300 in FAdV-1, and likely nts 330–400
takes place by autophosphorylation upon binding to dsRNAs in FAdV-9 (Barra and Langlois, 2008; Corredor and Nagy, 2010a;
– which are bioproducts generated during virus infection. Phos- Berk, 2013). Studies based on chromatin immunoprecipita-
phorylated PKR phosphorylates eIF-2α resulting in inhibition of tion assays in infected cells and mutational studies suggest that
the initiation complex for translation. viral proteins IVa2, 22K and 52/55K bind packaging signals
Virus-associated (VA) RNAs, transcribed by the host RNA to promote encapsidation of the viral genome into the virion
polymerase III, are GC rich with a stable secondary structure (Ewing et al., 2007). Protein IIIa interacts with 52/55K protein
and play important roles in the translation of viral mRNAs in to aid the packaging processes (Ma and Hearing, 2011). Before
some adenoviruses. VA RNA synthesis begins early in infection encapsidation, the viral DNA associates with pVII, the precursor
and significantly increases as the viral DNA replicates (Berk, of VII protein, as well as with core proteins V and the precursor
2013). The strong secondary structure of VA RNAs binds of μ (Berk, 2013). pVII interacts with IVa2 and 52/55K during
PKR rendering its inactivation and thus alleviation of eIF-2α packaging (Zhang and Arcos, 2005).
-mediated inhibition of global protein synthesis in response to The final stage of assembly consists of maturation of non-
infection. During adenovirus replication, the dsRNA molecules infectious to infectious virions by cleavage of precursors of
are also suggested to induce the RNA interference mecha- proteins VI, VII, VIII, μ and pTP by the virally encoded 23K
nism to inhibit translation of viral mRNAs. VA RNAs have protease (Weber, 2007; Berk, 2013).
been shown to interfere with the processing of micro RNAs
(miRNAs) through two mechanisms: (1) competitive binding Virus release
to exportin 5, which is required for pre-miRNAs export from Four processes are known to facilitate virus release and spread
the nucleus to the cytoplasm and (2) competitive binding to to nearby cells. The first process involves the cleavage of the cel-
Dicer, which is required for the incorporation of miRNAs into lular cytokeratin K18 at amino acid 74 by the viral 23K protease.
RISC complexes. Cytokeratin K18 polymerizes and form filaments that help main-
Some viruses such as HAdV-2 and HAdV-5 encode two VA tain the integrity of cells. Cleavage of cytokeratin K18 by 23K
species, VAI and VAII, while others, including FAdV-1 (CELO protease renders cells susceptible to lysis as a consequence of its
virus) and EDSV, encode one species (Larsson et al., 1986; inability to polymerase and form filaments.
Hess et al., 1997). Mastadenovirus and EDSV VA RNAs are The second process for virus release in mastadenoviruses
transcribed rightward, whilst FAdV-1 VA RNA is transcribed involves the virally encoded E3 11.6 K protein, known as adeno-
leftward (Hess et al., 1997). To date, no VA RNA genes have virus death protein (ADP). Expression of ADP seems to be cell
been identified in other avian adenoviruses. VA RNAs encoded type-dependent and its role in accelerating cell lysis is not well
by HAdV, FAdV-1 and EDSV vary in length and location. understood. Cells infected with ADP-overexpressing HAdV-5
HAdV VA RNAs are around 160 nucleotides in length and die quicker than those infected with the wild-type virus. Cells
map near the left end of the genome, between 52 K and pTP infected with ADP-deleted mutant virus, on the other hand, sur-
genes. FAdV-1 and EDSV VA RNAs, on the other hand, are vive the infection longer than those infected with the wild-type
100 and 91 nucleotides in length, respectively, and map at the virus (Murali et al., 2014). ADP is expressed at low levels under
right end of the viral genome between ORF9 and ORF16 of the control of the viral E3 promoter, whilst its expression under