Page 147 - Avian Virology: Current Research and Future Trends

140 | Liu et al.

M protein in the assembly and budding phases of the viral life cycle (Wang,
While S protein is a key defining feature for CoVs, they are surpris- J. et al., 2009). The A159 and K160 in the M protein were found
ingly not the most abundant structural protein found in CoVs. The to be essential for binding to actin, and recombinant viruses
most abundant structural protein is the M protein, which ranges harbouring mutations in A159-K160 could no longer generate
from 25–30 kDa and accounts for ≈ 40% of the mass of virus par- infectious virions, although the genome could still be replicated
ticles (Stern et al., 1982). M protein monomer contains a small and transcribed normally (Wang, J. et al., 2009). This M protein–
N-terminal ectodomain and a large C-terminal endodomain and actin interaction was supported by the observation that purified
is thought to give CoVs their shape (Machamer and Rose, 1987) IBV particles contain a certain amount of beta-actin (Kong et al.,
(Fig. 5.8). While the M protein is co-translationally inserted into 2010).
the ER, most coronavirus M proteins, including IBV, contain no
signal sequences (Kapke et al., 1988; Fung and Liu, 2018). Ecto- E protein
domain of IBV M protein is modified by N-linked glycosylation, Contrary to the abundance of the M protein, the E protein is a
which is different from the O-linked glycosylation observed in small polypeptide (8–12 kDa) found in limited amounts in the
the M proteins of murine coronavirus and bovine coronavirus L9 virion envelope (Liu and Inglis, 1991; Fung and Liu, 2018). Cur-
(Cavanagh, 1983). M protein is a polytopic membrane protein rent evidence supports the membrane topology of IBV E protein
that is embedded within the envelope by three transmembrane as a transmembrane protein with an N-terminal ectodomain,
domains (Armstrong et al., 1984). The cytoplasmic tail of IBV a hydrophobic domain (HD) and a C-terminal endodomain
M protein is required for its interaction with the E protein (Lim (Fig. 5.9). The major function of the E protein is to facilitate virus
and Liu, 2001) and, similar to other CoVs, when overexpressed assembly and release (Liu et al., 2007; Ye and Hogue, 2007),
together, IBV M and E protein could support the formation of which is mediated by the physical interaction between E and M
virus-like particles (Lim and Liu, 2001; Corse and Machamer, (Lim and Liu, 2001). In fact, when coexpressed in cells, the E pro-
2003). Recent studies suggest that M protein exists as a dimer tein can relocate M to the same subcellular compartments that E
in the virion and adopts two different conformations to promote resides in (Lim and Liu, 2001). The six residues at the C-terminal
curvature of the virion (Neuman et al., 2011). It also helps to (RDKLYS) serves as the ER retention signal of E, and mutation
regulate the virion size and virus assembly through interactions of the fourth lysine to glutamine resulted in the accumulation of
with other structural proteins (Neuman et al., 2011). E in the Golgi apparatus (Lim and Liu, 2001).
Coimmunoprecipitation and immunofluorescence micros- As opposed to other structural proteins, deletion of the E
copy also revealed interactions between M protein and beta-actin protein is not always lethal. In fact, recombinant viruses with

Figure 5.8 Linear and folded representations of coronavirus membrane (M) protein. Important residues for the infectious bronchitis virus
(IBV) M protein are N and N (glycosylation sites) and A -K (Actin-binding). TM, transmembrane domain.
3 6 159 160