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Newcastle Disease Virus | 57

facilitates NDV entry into host cell through disintegration of M glycoproteins are concentrated (Battisti et al., 2012). The M pro-
protein oligomeric structure for the nucleocapsid release (Shtyk- tein also interacts with viral RNPs via the N protein, thus allowing
ova et al., 2019). the RNPs to associate with a region at the plasma membrane
where the surface glycoproteins are present, which becomes the
Viral RNA synthesis budding site. It is thought that the M–N interaction is responsible
Fusion of viral and plasma membranes releases the RNP into the for incorporation of RNPs into virus particles and multiple copies
cytoplasm. Viral transcription and replication are mediated by of RNPs can be packaged into a single virus particle (Goff et al.,
the viral RdRp, which is associated with the RNP. The negative- 2012). Co-immunoprecipitation experiments showed that the F
polarity of the genome imposes transcription as the first step in protein of NDV interacts with the N protein and not with the M
the virus gene expression. Thus, the viral mRNAs are first synthe- protein, suggesting that F–N interaction may also be involved in
sized from the parental RNPs while the positive-strand replicative localization of RNPs at plasma membrane assembly sites (Pantua
intermediates (RI) are produced at a later stage. Although both et al., 2006). It is believed that the HN protein is incorporated
the RNAs are positive-polarity, they differ widely. While the viral into the envelope by interaction with the M protein; whereas, the
mRNAs are 5′-capped and 3′-polyadenylated like cellular mRNAs, F protein is incorporated into the envelope by interaction with
the RIs are assembled into RNPs like the genomic RNPs. the HN protein.
The level of viral transcription and replication is controlled by NDV, like other enveloped RNA viruses, assembles in
cis-acting elements (leader, trailer, GS and GE) present within plasma membrane domains with properties of membrane lipid
the viral RNA. Paramyxoviruses have evolved mechanisms to rafts (Laliberte et al., 2006). During NDV infection, F and HN
regulate their RNA synthesis to maximize use of available tem- interact with lipid rafts to facilitate the incorporation of F–HN
plates. In the initial stages of infection, the ratio of antigenome complexes into virions (Laliberte et al., 2007). The mechanisms
to genome RNA increases significantly, reflecting high use of of NDV budding and particle release remains poorly understood.
leader promoter. But at the later stage of infection, the ratio As previously mentioned, the M protein of NDV is necessary and
reverses due to very active production of genomic RNAs from sufficient for budding (Pantua et al., 2006).
the trailer promoter. Thus, there are at least two mechanisms
by which paramyxovirus transcription and replication are regu-
lated. First, the switch from mRNA to antigenome synthesis. Genetics
There is evidence that the level of N protein, which is required Viral genetic diversity is determined by mutation rate. Mutation
–6
–4
for encapsidation, is responsible for this transition (Baker and rate of RNA viruses range between 10 to 10 nt substitutions
Moyer, 1988; Horikami et al., 1992). The second transition per site, per generation (Jenkins et al., 2002; Hanada et al., 2004).
from positive-sense to negative-sense RNA synthesis at late The mutation rate of an RNA virus depends on viral factors such
stage of infection ensures sufficient genomic RNAs are available as polymerase fidelity, 3′-exonuclease activity and the mode
for packaging into virus particles. Available evidence indicates of replication. In addition, the host cell also contributes to the
that this switch could be due to differences in the strengths of mutation rate. NDV infects many domestic and wild bird species.
the promoters. Therefore, NDV replicates in widely different cellular environ-
ments, but the impact of this host range on its mutation rate is
Virus assembly and budding unknown. NDV is a highly genetically stable virus. NDV strains
Interactions among surface glycoproteins (F and HN), the M show very little sequence variations over long periods of times,
protein and the RNPs are critical for virus assembly. All these both in laboratory and in the field. For example, partial and com-
components are synthesized at distinct sites in the cytoplasm and plete genomic sequences of recent virulent isolates from China,
are transported to the plasma membrane for assembly and bud- Egypt, and India were found to be nearly identical to those of his-
ding utilizing various cellular pathways. The F and HN proteins of torical viruses isolated in the 1940s (Dimitrov et al., 2016). The
NDV were shown to interact following their synthesis in the ER high genetic stability of NDV indicates that the virus has already
(Stone-Hulslander and Morrison, 1997) and thus are transported reached a high fitness value and further accumulations of muta-
to the cell surface as a metastable protein complex. Although tions has no additional growth advantage for this virus in nature.
most of the F and HN proteins are transported to the cell surface The substitution rate for NDV estimated in different studies has
as metastable complexes, a small portion of each protein also traf- been inconsistent. Chong et al. (2010) estimated the rate of muta-
–3
fics alone. It was shown that a dileucine motif in the CT of NDV F tions for NDV to be 0.98 × 10 –3– 1.56 × 10 substitutions per site,
protein mediate their targeting to the basolateral site of polarized per generation. Miller et al. (2009) estimated a substitution rate of
–3
epithelial cells (Samal et al., 2013). The RNPs are formed in the 1.32 × 10 (strict) and 1.7 × 10 (relaxed) for virulent viruses and
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–4
–4
cytoplasm by interactions of the P protein with N-RNA template a substitution rate of 2.28 × 10 (strict) and 2.92 × 10 (relaxed)
and L protein. for low virulence viruses. Dimitrov et al. (2016) observed rates of
The M protein plays a coordinating role in virus assembly and changes for the full fusion coding regions of the virulent viruses
budding. The M protein interacts with the cytoplasmic surface of of genotype II and IX were 7.05 × 10 (relaxed) and 2.05 × 10
–5
–5
the plasma membrane via hydrophobic interactions with the lipid (relaxed), respectively. However, the substitution rate of NDV is
bilayer and with the cytoplasmic tail of HN protein. The NDV significantly lower than estimates of other RNA viruses such as
M protein binds to the plasma membrane where the viral surface HIV-1, influenza virus A and foot-and-mouth disease virus which