Page 14 - Avian Virology: Current Research and Future Trends

Avian Influenza Virus | 7

have been described to alter the effects of PB1-F2 in mallard conserved in IAVs, but shorter C-terminal sequences of 41 amino
ducks infected with a HPAIV H5N1 (Marjuki et al., 2010). acid in length are also found, particularly in swine- and canine-
Thus, amino acid variations, both in terms of length as well as origin IAV strains, as well as in the swine-origin 2009 pandemic
identity, among PB1-F2 sequences may account for the differ- H1N1 virus (Shi et al., 2012). PA-X maintains the endonuclease
ent activities observed and its overall biological significance. domain found in PA but lacks the PB1-binding domain. PA-X
represses cellular gene expression by localizing in the nuclear
Segment 3: PA, PA-X, and N-terminal truncated compartment and degrading nascent host mRNAs (Khaperskyy
forms of PA et al., 2016). Recent studies demonstrated that basic amino acids
Segment 3 encodes two major products, PA and PA-X. PA is the within the first 15 amino acids of PA-X’s C-terminal domain are
third component in the polymerase complex and it is essential for required for these activities (Oishi et al., 2015; Hayashi et al.,
transcription and replication. PA contains two major domains, 2016). Just like PB1-F2, it is possible that PA-X effects on virus
the N-terminal 197 amino acids corresponds to the endonuclease replication and pathogenesis are strain- and host-dependent
domain (Yuan et al., 2009) and the C-terminal ~ 460 amino acids (Jagger et al., 2012; Gao et al., 2015a,b; Feng et al., 2016).
that contain the PB1 binding site (Zürcher et al., 1996; Guu et PA-N155 and PA-N182 are expressed from alternative initia-
al., 2008). The endonuclease activity is essential as part of the tion sites in the PA mRNA (Muramoto et al., 2013). These newly
cap-snatching mechanism during viral mRNA transcription, identified proteins are translated from the highly conserved
but it seems dispensable during replication. The structure and 11th and 13th in-frame AUG codons in the PA mRNA and are,
active-site arrangement of the endonuclease domain is similar therefore, N-terminally truncated forms of PA. The PA-N155
to members of the PD-(D/E)XK family of nucleases. Early stud- and PA-N182 proteins are detected in cells infected with IAV
ies indicated that PA can be phosphorylated by casein kinase II strains isolated from different host species, suggesting universal
(CKII)-like enzymes (Sanz-Ezquerro et al., 1998). In addition, expression of PA’s alternative forms. Neither one of these protein
the PA of at least some strains possesses proteolytic activity products show polymerase activity when expressed together with
(Sanz-Ezquerro et al., 1996); although it is not clear whether such PB1 and PB2. However, mutant viruses lacking the N-truncated
attribute is significant for polymerase activity (Perales et al., 2000; PAs replicated more slowly in cell culture and had lower patho-
Naffakh et al., 2001). Since PA’s proteolytic activity depends on genicity in mice than did wild-type virus (Muramoto et al., 2013).
an N-terminal domain that overlaps the recently discovered PA-X
protein (Jagger et al., 2012), PA’s role in proteolysis during virus Segment 4: haemagglutinin
infection needs further studies. PA contains two potential NLSs Segment 4 encodes the haemagglutinin (HA) glycoprotein and
between amino acids 124 to 139 and 186 to 247; however, these owes its name due to its ability to interact and agglutinate red
signals are recognized by HAX1, a cytoplasmic protein with blood cells (RBC) (reviewed extensively in Skehel and Wiley,
anti-apoptotic function that prevents PA’s nuclear translocation 2000; Gamblin and Skehel, 2010; Xiong et al., 2014; Byrd-Leotis
(Hsu et al., 2013). Thus, PA relies on co-expression of other viral et al., 2017). Antigenic differences led to the classification of the
proteins, particularly PB1, to reach the nuclear compartment HA into 16 subtypes (H1–H16) that are phylogenetically sepa-
(Nieto et al., 1992, 1994). The assembly of newly synthesized rated into two major groups: Group 1 contains H1, H2, H5, H6,
polymerase subunits into polymerase complexes is considered H8, H9, H11, H12, H13, and H16, and group 2 contains H3, H4,
a three-step process that involves binding of PA to PB1 in the H7, H10, H14, and H15. Recently, two unique IAV subtypes have
cytoplasm, followed by their migration to the nucleus via the been identified in bats, but because their HA lack of canonical
importin pathway, and subsequent binding to PB2 to complete properties they have been provisionally designated HL17 and
the heterotrimer (Hutchinson and Fodor, 2012; Hutchinson et HL18 due the (Sun et al., 2013; Zhu et al., 2013). The HA binds
al., 2014). PA can bind additional host factors. Through domains to cellular receptors on the cell surface and facilitates the fusion
spanning amino acids 493–512 and 557–574, PA binds human of the viral and endosomal membranes during virus entry. The
CLE (hCLE, and likely the chicken homologue CLE7). The PA– HA plays a major role in IAV’s host range and it is also the most
hCLE interaction impacts the host’s RNA polymerase II activity significant target of neutralizing antibodies during infection. The
to favour viral polymerase activity and virus production (Huarte HA is a single-pass type I transmembrane glycoprotein present as
et al., 2001; Pérez-González et al., 2006, 2014; Rodriguez et al., a homotrimer on the virus’ surface that extends ≈ 130 Å from the
2011). PA also interacts with the minichromosome maintenance membrane. The HA undergoes N-linked glycosylation at aspara-
(MCM) complex, proposed to function as a scaffold between the gine residues in the classical N-X-S/T consensus sequences.
nascent RNA chains and the viral polymerase (Kawaguchi and N-linked glycosylation is essential for the protein’s structural
Nagata, 2007). The biological significance of these interactions, integrity as well as to mask antigenic sites; between three and
particularly for replication in natural and non-natural avian hosts nine potential N-linked glycosylation sites have been described
deserves further attention. depending on subtype and origin of the virus. Each HA monomer
PA-X is a protein product of a second ORF in segment 3 pro- contains two subunits, HA1 and HA2, produced by the cleavage
duced via ribosomal frameshifting (Firth et al., 2012; Jagger et of the inactive precursor HA0. Disulfide bonds between the two
al., 2012). PA-X shares with PA the N-terminal 191 amino acids, subunits maintain the integrity of each monomer. Each monomer
while the C-terminal sequence, typically 61 amino acids long, is carries a transmembrane anchor and a small cytoplasmic tail.
derived via a + 1 frameshift of the colinear mRNA. PA-X is highly Three monomers bundle in coiled-coil structures to make the