Page 58 - Avian Virology: Current Research and Future Trends

Newcastle Disease Virus | 51

The P protein is an essential component of viral RNA-dependent has a cysteine-rich carboxy-terminal domain that binds to two
RNA polymerase (Hamaguchi et al., 1983, 1985). The P protein molecules of Zinc (Steward et al., 1993). NDV V protein can be
of NDV is 395 aa long and is heavily phosphorylated at specific found in very small quantity in association with the nucleocapsid
serine and threonine residues. The calculated molecular weight of of the purified virions and thus is a minor structural component
P protein is ≈ 42 kDa, but by SDS-PAGE analysis, the P protein of the virion.
forms multiple bands with molecular weight ranging from 50 to The V protein of NDV is a multifunctional protein that plays
55 kDa. These observations are due to multiple phosphorylated important roles in both virus replication and viral virulence. Sev-
forms of the P protein. The P protein is not known to have any eral recombinant V-deficient NDV mutants have been engineered
intrinsic enzymatic activity, but functions as the bridge between to determine V protein functions (Mebatsion et al., 2001; Huang
N-RNA and the L protein to perform viral transcription and et al., 2003; Park et al., 2003). These studies demonstrated that the
replication (Horikami et al., 1992). The P protein functions as a V protein of NDV is indispensable for virus replication. Deletion
homo-oligomer and in paramyxoviruses it is a tetramer (Longhi of the entire V and W proteins or only the C-terminal portion of
et al., 2017). Although the P protein of paramyxoviruses lack V protein severely impaired virus replication in vitro and in vivo
sequence homology, they contain some common modules that and virus pathogenicity, indicating that the C-terminus of the V
carry out specific functions. Bioinformatics analyses showed that protein is responsible for these incompetencies.
paramyxovirus P protein consists of an intrinsically disordered The V protein plays a crucial role in NDV pathogenesis by
N-terminal domains (PNT) and a C-terminal domain (PCT) antagonizing the antiviral effects of IFN. It interacts specifically to
(Karlin et al., 2003). The intrinsically disordered protein regions avian cell proteins to abrogate IFN response in a species-specific
(IDPRs) play key roles in the formation of N–P–L complex manner. It does not interfere with the IFN response in non-avian
(Habchi and Longhi, 2015). The IDPRs lack a stable structure cells (Park et al., 2003). The V-deficient NDV mutants are highly
in the absence of a protein/ligand under physiological condi- attenuated in chickens (Mebatsion et al., 2001; Huang et al., 2003;
tions. The first 40–50 aa of paramyxovirus PNT contain IDPRs, Park et al., 2003). The V protein of NDV can block IFNα signal-
0
which are responsible for binding to N (Karlin et al., 2003; ling in avian cells by targeting signal transducer and activator of
Yabukarski et al., 2014). Computational approaches showed transcription 1 (STAT1) degradation (Huang et al., 2003). It was
that paramyxoviral P proteins share a short (11–16 residues) further shown that NDV V protein targets specifically phospho-
sequence motif within PNT which is proposed to be the region STAT1 degradation to block IFNα signalling (Qiu et al., 2016c).
0
responsible for binding to N (Karlin and Belshaw, 2012). The It also inhibits IFN-β induction through interaction with mela-
PNT might also bind to L and/or host factors. It is not known noma differentiation-associated gene 5 (MDA5), leading to the
whether non-overlapping regions of PNT or different monomers inhibition of IFN regulatory factor 3 (IRF3) activation (Park
0
of P bind to N and L. The PCT also contains IDPRs between et al., 2003). A good correlation has been found between viral
ordered regions. Consequently, 70–80% of the residues in P virulence properties and IFN antagonistic activities of the cor-
protein are disordered, indicating that these regions are heavily responding V protein (Alamares et al., 2010). In addition, it plays
used for chaperon activities (Longhi et al., 2017). The ordered an important role in preventing apoptosis in a species-specific
domains of PCT are the P multimerization domain (PMD) and manner (Park et al., 2003; Hengel et al., 2005). The V-deficient
the C-terminal X domain (XD). PMD is involved in oligomeriza- NDV mutant viruses induce apoptotic cell death more rapidly in
tion and XD binds to the C-terminal domain of N (i.e. N TAIL ) chicken cells than by wild type NDV. These studies suggest that
(Houben et al., 2007; Longhi et al., 2017). PMD and XD are the host range of NDV is limited by the ability of its V protein to
separated by IDPR. The C-terminal 45 aa (residues 247–291) efficiently prevent innate host defences, such as IFN response and
of the NDV P protein have been shown to be involved in P-P apoptosis (Park et al., 2003).
and P–N interactions (Jahanshiri et al., 2005). Bioinformatics The V protein of paramyxoviruses has been shown to be a neg-
analysis showed that 26 aa residues of the NDV P protein were ative regulator of viral RNA synthesis (Lamb and Parks, 2013).
possibly phosphorylated by protein kinase C (PKC) (Qiu et al., The V protein shares the N-terminal domain of P protein that is
2016b). The role of phosphorylation in the functions of NDV known to interact with N to form P-N . Therefore, the N-terminal
0
0
0
0
P protein is not clearly known. Phosphorylation of S48, T111 of V protein can also interact with N to form V-N that is not
and T271 was shown to be involved in NDV transcription and suitable for RNA encapsidation (Horikami et al., 1996). The V
replication (Qiu et al., 2016b). protein of some paramyxoviruses have been shown to inhibit
genome replication by binding to the L protein (Sweetman et al.,
The V protein 2001; Nishio et al., 2008). The V protein of paramyxovirus has
The V mRNA is generated by the insertion of a non-templated G also been shown to inhibit viral RNA synthesis by directly bind-
residue at a conserved editing site on the P mRNA during tran- ing to RNA (Lin et al., 1997). However, the exact role of NDV V
scription (Steward et al., 1993). The V protein of NDV is 239 aa protein in RNA synthesis remains to be established.
long and has a molecular weight of 36 kDa. The P and V pro- Insertion of two G residues at the P gene editing site produces
teins share 135 aa at their amino terminal ends but vary at their W mRNA. However, insertion of two G residues into the mRNA
carboxy-terminal ends in length and in aa composition (Fig. 2.5). is relatively rare. The W protein of NDV is 221 aa long. The role
The NDV V protein, similar to other paramyxovirus V proteins, of W protein in NDV replication cycle has not been established.