Page 16 - Avian Virology: Current Research and Future Trends

Avian Influenza Virus | 9

shown the ability of NP and CrmI to bind in vitro (Elton et al., linker (L), middle (M), and C-terminal (C) domains, which have
2001). been analysed by X-ray diffraction (Sha and Luo, 1997; Harris et
al., 2001; Arzt et al., 2004). Negative-stain electron microscopy
Segment 6: neuraminidase (NA) has shown that M1 monomers are approximately 6nm long and
Segment 6 encodes a single open reading frame for the mushroom- have a rod-like shape (Ruigrok et al., 2000). M1 monomers
shaped neuraminidase (NA). The NA is the second major undergo oligomerization forming an organized structure that
glycoprotein of IAV and a type II integral membrane protein of underlines the lipid envelope and interacts with the cytoplasmic
~ 470 amino acids long (length depending on subtype) that is tails of surface glycoproteins (HA and NA) and M2 protein from
responsible for the removal of terminal sialic acid residues from the virion’s outer core, and with the vRNPs in the inner core
virus particle, host cell surface and the surrounding environment (Harris et al., 2001; Nayak et al., 2004; Noton et al., 2007; Calder
glycoconjugates (Hirst, 1942; Shaw and Palese, 2013a). There are et al., 2010). Each monomer has positive and negative charges
nine NA subtypes (N1 to N9) found in nature, which are divided on opposite sides (Arzt et al., 2001). The oligomeric structure is
into two major groups based on sequence comparisons: group 1 arranged in such a way that the positive and negative charges are
(N1, N4, N5 and N8) and group 2 (N3, N6, N7 and N9) (Russell directed towards opposite sides of the oligomer, facilitating the
et al., 2006; Shaw and Palese, 2013a). Additionally, two unique interaction with the corresponding structures at each side (Sha
IAVs were identified in bats and their NAs have been provisionally and Luo, 1997; and reviewed in Shaw and Palese, 2013b). M1 has a
designated NL10 and NL11 due the lack of canonical NA activ- zinc-binding motif (cys–cys–his–his type) at residues 148 to 162,
ity (Li et al., 2012; Zhu et al., 2012). The neuraminidase activity which is conserved between influenza A and B viruses (Nasser
is important to remove sialic acids from mucopolysaccharides et al., 1996; Shaw and Palese, 2013b); its function is not clear,
that interfere with virus infection and from newly formed virus but it may be involved in protein–protein interactions (Shaw and
particles from infected cells (Palese et al., 1974), promoting virus Palese, 2013b). There are conflicting reports about the roles of the
spread (Matrosovich et al., 2004; Huang et al., 2008). The essential zinc-binding-motif. Initially, proposed that M1 the zinc-binding-
role of NA in the virus replication cycle is exemplified by the fact motif was involved in RNA binding activity and transcriptase
that commercially available influenza virus antiviral therapy is inhibition (Ye et al., 1989), with the latter being supported by
focused almost exclusively on neuraminidase inhibitors, such as another research group (Nasser et al., 1996). However, a few years
zanamivir and oseltamivir, that act by directly interfering with the later, it was reported that the zinc-binding-motif was not related
enzymatic activity of NA (Moscona, 2005). Interspecies transmis- to neither of those functions (Elster et al., 1994). It has been pro-
sion from the wild bird reservoir into poultry and further poultry posed that M1 has a role on both virion assembly by recruiting the
adaptation is usually accompanied by deletions of various lengths viral components to the assembly site at the plasma membrane,
in the stalk domain of the NA. Although the exact mechanisms and on budding off process (Gómez-Puertas et al., 2000; Latham
that lead to these deletions are not well established, they are and Galarza, 2001). The mechanism by which this is achieved is
usually associated with the respiratory tropism and virulence of yet to be determined; however, a couple of hypotheses have been
IAVs in poultry (Els et al., 1985; Castrucci and Kawaoka, 1993; suggested. One suggests that M1 interacts with the cytoplasmic
Keawcharoen et al., 2005; Munier et al., 2010; Sorrell et al., 2010; domain from the HA, NA and M2 during their transport through
Li et al., 2011, 2014b; Chockalingam et al., 2012; Soltanialvar et al., the exocytic pathway to the assembly site at the apical plasma
2012; Stech et al., 2015). membrane, carrying with it the vRNP–NS2/NEP complexes
Although largely ignored in vaccine development, antibodies (reviewed in Shaw and Palese, 2013b). It has also been suggested
against NA were shown to restrict virus replication and prevent that M1 bound to the vRNPs uses the cell transport machinery
severe disease (Eichelberger and Wan, 2015; Krammer et al., to translocate to the assembly site (reviewed in Shaw and Palese,
2018). Recently, NA has re-emerged as a potential vaccine anti- 2013b). In addition to its role on virus assembly and budding, M1
gen candidate for broadly protective influenza virus vaccines has an important role on vRNPs nuclear transport. Transport of
(Chen et al., 2018; Krammer et al., 2018). vRNPs into the nucleus can be affected by association with M1
(Martin and Helenius, 1991a). After virus internalization and
Segment 7: matrix protein 1 (M1), ion channel M2, release of genetic material into the cytoplasm, M1 dissociates
and M42 from vRNPs for their subsequent transport into the nucleus. It
RNA segment 7 of influenza A virus is about 1027 nucleotides has been shown that amantadine blocks the dissociation prevent-
long and encodes for at least two proteins, the matrix protein M1 ing the nuclear import of vRNPs (Martin and Helenius, 1991a).
(nucleotide positions 26–784) which underlines the envelope, Moreover, this dissociation appears to be pH dependent as shown
and the matrix protein M2 (nucleotide position 26 to 51 and 740 by experiments performed with a recombinant M1 protein, when
to 1007) with ion channel activity. The M1 protein is encoded the pH is altered, the dissociation fails and the nuclear import of
from a continuous open reading frame that gives origin to a co- vRNPs is also affected (Bui et al., 1996). M1 is translocated into
linear mRNA. M1 is the most abundant protein in the virion and is the nucleus of the host cell by its nuclear localization signal (NLS)
composed of 252 amino acid (aa) residues. It confers stability and and interacts with newly formed vRNPs by binding NP protein,
shape to IAVs. Structural analysis has revealed that M1 consists of forming the vRNP–M1 complexes (Martin and Helenius, 1991a;
two globular helical domains linked by a protease-sensitive region Noton et al., 2007). Then, NS2/NEP interacts with vRNP-M1,
at residue 164 (Arzt et al., 2001). It contains N-terminal (N), binding M1 at its NLS (Martin and Helenius, 1991a; Bui et al.,