Page 59 - Avian Virology: Current Research and Future Trends

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The matrix protein thought that the M protein transits to the nucleus and nucleolus
The M protein is the most abundant protein in the virion and at early stages of infection to inhibit host cell transcription and
functions as a bridge between the viral envelope and the nucle- protein synthesis (Duan et al., 2014). The nuclear localization of
ocapsid. The M protein of NDV is 364 aa long and has a molecular M protein also ensures that viral replication and transcription in
weight of 40 kDa (Seal et al., 2000). X-ray crystallography and the cytoplasm proceed smoothly, since M protein has been shown
cryoelectron tomography showed that the M protein of NDV to bind RNA directly and inhibit viral transcription (Iwasaki et al.,
forms a 4- to 5-nm-thick layer lining the inner surface of the viral 2009). The nuclear localization of M protein is required for NDV
membrane. However, in most particles, M was observed to be replication and propagation.
dissociated from the membrane and disassembled, probably to
allow conformational change of F protein (Battisti et al., 2012). The fusion protein
The monomeric subunit of NDV M protein is composed of two The F protein mediates entry of the virus into host cell by fusion
beta-sheets containing folded domains that are connected by an of the viral envelope with the plasma membrane. Later in the
unstructured, flexible linker region. The structural plasticity of the infection, the F protein expressed on the cell surface mediates
linker region allows the M protein to interact with multiple part- fusion with neighbouring cells. The cell–cell fusion allows the
ners (Battisti et al., 2012). The biologically relevant assembly unit virus to spread in the presence of neutralizing antibodies. Like the
of NDV M protein is a dimer that has an inherent property to oli- F protein of other paramyxoviruses, the NDV F protein does not
gomerize into tetramers at neutral pH. The 25 carboxyl-terminal require the acid pH of endosomes to activate its fusion function,
residues of M protein are involved in the monomer-to-monomer which allows it to mediate fusion at neutral pH with neighbouring
contact. The dimers form a grid-like array on the inner surface of cells. The NDV F protein is the major contributor to induction
the viral membrane. Each repeating unit of M protein is nearly of neutralizing antibodies and protective immunity (Kim et al.,
square in shape, which forms an array. The cytoplasmic tails of 2013). The F protein is also the major individual contributor to
the HN or F glycoproteins are anchored in the spaces between NDV virulence and pathogenesis (Paldurai et al., 2014b).
the M protein dimers in the array. The size of the HN globular The NDV F protein is a typical type I integral membrane
head would not allow placement of another HN protein in the protein. Type I proteins cross the membrane only once with a
neighbouring space. Because the globular head of F is smaller cytoplasmic tail at the C terminus. The N terminus contains a
than that of HN, the placement of F would not be a problem. The cleavable signal sequence that targets the polypeptide to the ER.
space could not be occupied by a foreign transmembrane protein The F protein of NDV contains a 470-aa extracellular domain,
because of steric hindrance (Battisti et al., 2012). a transmembrane (TM) domain of 21 aa near the C terminus,
The M protein plays a central role in virus assembly and bud- and a 31-aa cytoplasmic tail (CT). The functional F protein is a
ding. Expression of NDV M protein alone is sufficient to induce homotrimer. Each F monomer is first synthesized as a 553-aa bio-
the assembly and release of virus-like particles (VLPs) from logically inactive precursor, F . In the ER, F is glycosylated and
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transfected cells, indicating that M protein can associate with cell forms homotrimers. F trimers are transported through the Golgi
membrane, induce membrane curvature and promote scission complex, where a ubiquitous endoprotease furin cleaves F at a
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(Pantua et al., 2006). The M protein interacts with the cytoplas- multibasic cleavage site to convert it into biologically active F1
mic tail of HN (Pantua et al., 2006) and with the RNP by binding and F2 subunits (Fig. 2.6A), which are held together by disulfide
to the C-terminal tail region of N protein (Schmitt et al., 2010). bonds (Morrison, 2003). Proteolytic processing is required for
It has been suggested that 15-residue-long DLD-like sequence in fusion activity because it exposes a 25-aa hydrophobic region
the C-terminal of NDV N protein is involved in M–N interaction called the fusion peptide (FP) at the amino terminus of the F1
(Ray et al., 2016). The M–N interaction not only recruits RNP to subunit that is needed for membrane insertion. However, the
the sites of particle assembly, but also thought to trigger particle cleavage of F is not required for transport of F trimers to the
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release (Schmitt et al., 2002). In addition, the M protein contains cell surface. The molecular weights of F , F1, and F2 are 66 kDa,
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positively charged areas on the surface that allows interaction 55 kDa and 12.5 kDa, respectively.
with negatively charged cell membrane. A second topological form of NDV F protein, that is 10% to
Although the entire replication cycle of NDV occurs in the 50% of the total F protein, has also been found in which approxi-
cytoplasm, the M protein has been observed to localize in the mately 200 aa of the amino terminus as well as the CT domain
nucleus early in infection and becomes associated with nucleoli of the protein were translocated across membranes, such that
and remains in the nucleus throughout infection (Peeples, 1988). the CT is exposed at the cell surface. Most of the F1 residues are
Studies have shown that the NDV M protein enters the nucleus in the cytoplasm (McGinnes et al., 2003). It has been suggested
through a bipartite nuclear localization signal (Coleman and that this second form of the F protein may be involved in cell–cell
Peeples, 1993) and shuttles to the cytoplasm using three nuclear fusion (Pantua et al., 2005). However, the second topological
export signal sequences (Duan et al., 2013). It was found that form of F protein has not been found for other paramyxovirus F
NDV M protein accumulated in the nucleolus by binding to a proteins and it is not known why NDV requires the second form
nucleolar phosphoprotein B23 early in infection by interacting for fusion activity.
with amino acid 30–60, a putative nucleolar localization signal The C-terminal F1 subunit contains two hydrophobic domains,
(NoLS), but resulted in the redistribution of B23 from the nucle- the FP at the N terminus and the TM domain, which is located at
oli to the nucleoplasm later in infection (Duan et al., 2014). It is the C terminus and anchors the protein in the membrane of the