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Newcastle Disease Virus | 53
(A) F (1-116) F (117-553)
2
1
85 191 366 447 471 541

NH 2 HRC FP HRA HRB c TM CT COOH
CT
S S
Signal sequence
HRA HRB TM CT
(142-170) (466-491) (501-521) (522-553)
(B)
Sialic acid
receptors
Cell
membrane
Receptor HN-F
binding binding

Viral
membrane
F trimer HN, 4-heads down F trimer HN, 4-heads up F trimer HN, 4-heads up
(Pre-fusion)

F HRA Refolding HRA HRA 6HB
activation HRB linker formation
6HB 6HB
HRB HRB
HRB

Pre-hairpin intermediate Fusion intermediate Post-fusion

Figure 2.6 Schematics of the Newcastle disease virus fusion (F) protein and the mechanism of membrane fusion. (A) Schematic diagram
showing important domains and features of the Newcastle disease virus F protein: The position of the signal sequence, the transmembrane
domain (TM), the cleavage site, the hydrophobic fusion peptide, the heptad repeats (HRA, HRB and HRC) and cytoplasmic tail (CT) are
indicated. The sites used for addition of N-linked carbohydrate (lollipop) are also indicated with reference to the putative protein. Position
117 indicates the amino acid position for cleavage-activation. The amino acid sequences for major domains are indicated in the parenthesis.
(B) Schematic model depicting the mechanism of paramyxovirus membrane fusion caused by haemagglutinin-neuraminidase (HN) and
F proteins. Fusion is initiated by binding of HN protein to cell surface receptor, which allows F protein to interact with HN protein. This
interaction causes rearrangement of prefusion F protein leading to fusion peptide insertion into the cell membrane and refolding into a
post-fusion form through a series of intermediates, eventually causing membrane merger.

virus or infected cell. The F1 subunit has two heptad repeat (HR) Waning et al., 2004). Also, tyrosine-containing signals in the CTs
motifs: HRA is immediately C-terminal to the fusion peptide, and of membrane proteins have been associated with protein target-
HRB is immediately N-terminal to the TM domain. NDV F pro- ing (Weise et al., 2010). The functional role of the NDV F protein
tein possesses a third heptad repeat region (HRC) located in the CT in viral replication and pathogenicity has been investigated
F2 subunit that plays a role in modulating fusogenicity. HR motifs using reverse genetics. It has been shown that truncation of 2 and
mediate protein–protein interactions by enabling the formation 4 C-terminal aa and substitution of the two conserved tyrosine
of coiled coils. Crystal structure analysis of F proteins of NDV residues (Y524 and Y527) in the CT domain resulted in hyper-
and other paramyxoviruses revealed that HRA and HRB peptides fusogenic phenotype with increased pathogenicity (Samal et al.,
have strong affinity and assemble to form a highly stable six-helix 2013). It was also shown that substitution of tyrosine at position
bundle and that this assembly is tightly coupled to membrane 527 by alanine in a low virulence strain resulted in a hyperfu-
fusion (Chen et al., 2001; Luque and Russell, 2007; Swanson et sogenic virus with increased replication and immunogenicity
al., 2010). The function of the F2 subunit is unknown. (Manoharan et al., 2016).
NDV F protein has structural and functional characteristics Several studies have shown that the TM domain of paramyxo-
highly related to class I fusion proteins of other paramyxoviruses virus F proteins could also modulate fusion activity (Smith et al.,
(Lamb and Parks, 2013). While the structural features in the 2012; Webb et al., 2017). It was reported that the TM domain of
ectodomains of the F protein can have major impact on fusion, the NDV F protein is important for the structure and function of
the CT domain of type I fusion proteins has also been implicated the F protein (Gravel et al., 2011).
in fusion activity (Bagai and Lamb, 1996; Tong et al., 2002; The F glycoprotein of NDV contains six potential N-linked