Page 63 - Avian Virology: Current Research and Future Trends

56 | Samal

to take different positions to facilitate F–HN interaction and In paramyxoviruses, the N-terminal region of the L protein
F-triggering (Yuan et al., 2011). has been shown to interact with the P protein (Lamb and Parks,
HN interacts with the F protein through its stalk domain 2013). Biochemical studies have indicated that L protein does not
(Deng et al., 1999). The stalk contains specificity determinants interact directly with N protein on the nucleocapsid. As P protein
for homotypic F and HN interactions and fusion activation also interacts with N protein, it is the dual binding specificity of P
domains (Deng et al., 1995). The crystal structure of the NDV protein that is instrumental in bridging the L protein to the nucle-
HN stalk domain revealed a four-helix bundle (4HB) stalk ocapsid template. The L protein functions as homomultimers and
packed between the two globular head dimers (Yuan et al., 2011). the L–L interaction occurs through an N-terminal self-assembly
The structure showed that both dimeric heads were not in contact domain. The L protein also interacts with host cell proteins and
with each other, and they were backfolded onto the stalk with the other viral proteins (Lamb and Parks, 2013).
lower head of each dimer making a short-range contact with the
upper part of the stalk domain. This structure was referred as the
‘four-heads-down’ conformation (Yuan et al., 2011). Mapping of Overview of NDV replication
the residues identified in mutagenesis studies of NDV HN stalk One of the unique characteristics of NDV is its ability to infect
(Stone-Hulslander and Morrison, 1999; Melanson and Iorio, almost all avian and terrestrial species. NDV replicates in many
2004) onto the HN stalk crystal structure revealed a hydrophobic cell types. Its replication cycle is the fastest among all paramyxo-
F activation region on the surface of 4HB (Yuan et al., 2011). The viruses. NDV replaces the host protein synthesis with its own
NDV HN structure shows an overall twofold symmetry for the protein synthesis within six hours of infection and produces
ectodomains, suggesting a possible stoichiometry of one HN maximum yields of progeny viruses within twelve hours of
tetramer with two F trimers (Yuan et al., 2011). infection. The replication life cycle of NDV is similar to other
The HN cytoplasmic tail contains 26 highly conserved aa and paramyxoviruses.
has been shown to interact with the M protein (García-Sastre
et al., 1989). Mutational analysis showed that only the first two Virus adsorption and entry
aa could be deleted without affecting the viability of NDV. Any NDV attaches to the epithelial cells of respiratory and enteric
further deletion or substitution of aa affected the viability of the tracts by complexing of viral HN protein to sialic acid moieties
virus, indicating that these residues are critical for virus assembly found on cell surface N-glycoproteins and lipids (gangliosides).
(Kim et al., 2009). Whether sialic acid alone functions as the host cell receptor is not
known. It was found that both α2–3- and α2–6-linked sialic acid
The large protein containing glycoproteins and glycoconjugates are used for NDV
The L protein is the largest structural protein of NDV, compris- infection (Sanchez-Felipe et al., 2012). It was further shown that
ing 2204 aa with a molecular weight of 250 kDa. The L protein glycoproteins are more critical than gangliosides for NDV inter-
sequences are highly conserved among NDV strains, indicating actions with the host cell.
that this protein may not tolerate changes without disrupting its NDV employs multiple pathways to enter host cells. NDV
structure and function. The L protein is the least abundant protein predominantly enters host cells by direct fusion of the viral
in infected cells and in virion. Typically, 50 copies of the L protein membrane with the host cell membrane and also by endocytosis.
are present in a virus particle. It is found on the nucleocapsid in The membrane fusion process takes place in a pH-independent
co-localization with the P protein. The L protein possesses all the manner, but the endocytic processes take place at acidic pH.
enzymatic activities necessary for synthesis of viral mRNAs and Therefore, the fusion of NDV with cultured cells is enhanced at
genomic RNA replication, including nt polymerization, mRNA acidic pH (San Roman et al., 1999). NDV uses both pinocytosis
capping, methylation, and polyadenylation of mRNAs. The L and receptor-mediated endocytosis to enter cells. But it utilizes
protein has been shown to contribute to the virulence of NDV different pathways for different cells. It has been reported that
(Rout and Samal, 2008). The L protein of NDV also contains the NDV enters HeLa cells and avian fibroblast cells by cell fusion
six highly conserved domains (I–VI) that have been identified and through receptor-mediated and dynamin-dependent endo-
in the L proteins of other non-segmented negative-sense RNA cytosis (Sanchez-Felipe et al., 2014). NDV infects DF-1 cells via
viruses and are believed to carry out distinct functions (Poch et macropinocytosis and clathrin-mediated endocytosis but not via
al., 1990). Conserved region II is believed to interact with RNA, caveolae-mediated endocytosis (Tan et al., 2016). Recently, it
as it contains highly charged residues (Lamb and Parks, 2013). was reported that NDV enters dendritic cells through macropi-
The polymerase activity maps to conserved region III, and it is nocytosis and clathrin-mediated endocytosis (Tan et al., 2018).
also required for polyadenylation. It contains a highly conserved NDV spreads from infected cells to uninfected cells by release
GDN motif as the active site for nt polymerization (Sleat and into extracellular space and by direct cell–cell fusion (syncytia
Banerjee, 1993). The GXGXG motif present in the conserved formation). Cell-to-cell transmission is resistant to neutralizing
region VI is necessary for cap methylation (Li et al., 2005; Grd- antibodies and can contribute to the pathogenesis of viral infec-
zelishvili et al., 2006). A specific HR motif present in conserved tion. The viral entry releases the helical nucleocapsids into the
region V is required for capping of nascent viral RNA (Ogino and cytoplasm. Although the mechanism of disruption of contact
Banerjee, 2007). The exact functions of other conserved regions between nucleocapsid and M protein is not fully known, it has
are unknown. been demonstrated that the acidic pH of the endocytic pathways