Page 35 - TBE_Book_V2_2019

Chapter 2a: Virology

prM protein is a glycosylated precursor of the host cell. It is also believed to be a main
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membrane protein M. The carboxyl terminus determinant of TBEV virulence. The three-
of C protein serves as an internal signal dimensional structure of the E protein was
sequence element leading the structural studied at the resolution of 2.0 Å by X-ray
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protein prM into the membrane of the crystallography (Figure 5). Comparison of the
endoplasmic reticulum. The viral protease crystal structure of E protein and the structure
NS2B-NS3 cleaves this signal sequence, of E protein in the virion observed by
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releasing the N-terminus of prM protein. The cryoelectron microscopy revealed root-mean-
prM protein shows a chaperone-like activity square deviations (RMSD) of 1.7 Å for the
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119
during the envelope protein E folding. The N- corresponding Cα atoms. The most
terminus of the pr is mainly hydrophilic and, in important difference is in the positioning of
TBEV, contains a single N-linked glycosylation domains I–III relative to each other. Whereas
site that appears to have an important role in the crystal structure the domains I, II, and III
during virion assembly and release. 31,39,56 Six are arranged in a line, in the virion the tip of
cysteine residues, all disulfide-bridged, are domain II is bent 15 Å towards the virus
highly conserved. The C-terminal region membrane (Figure 5A). 119 Such a bending of
contains an ectodomain and 2 potential the ectodomain in the virion prevents
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membrane-spanning domains. The cleavage induction of premature membrane fusion
of prM into pr and M occurs in the Golgi mediated by the E protein. 119 The structure of
complex and is mediated by furin or a furin- TBEV E protein was found to be highly similar
like enzyme 57,58 leading to a conversion from to E1 glycoprotein from a distantly related
immature to mature fusogenic and fully virus, Semliki Forest virus (family Togaviridae).
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infectious viral particles (Figure 3). The pr These proteins were defined as class II virus
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fragment is then secreted. A conserved fusion proteins, distinct from previously
region in the prM protein is a critical characterized class I fusion proteins such as
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molecular determinant for the assembly and hemagglutinin of influenza virus.
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secretion of the virus. The M-protein
The protein forms 2 monomers anchored in
consists of an N-terminal loop and three the membrane by their distal parts at
helices (Figure 5B). The first helix is situated as physiological pH. After virus uptake by
a perimembrane and the last two as trans-
receptor-mediated endocytosis into host cells,
membranes; however, the M-protein is not
acidic pH in endosomes triggers irreversible
exposed at the surface of the viral particle due
changes in the E protein structure including its
to its small size and close association with viral re-arrangement to trimeric forms. This leads
the envelope membrane. 119 Two M-proteins to the initiation of the fusion process
together with two E-proteins form a compact
between the viral and endosomal mem-
heterotetramer, which is the main building 62
brane. Conserved histidines in the E protein
block of the virion, formed by head-to-tail
function as molecular switches and, by their
dimerization of two E-M heterodimers (Figure
119 protonation at acidic pH, control the fusion
5C). 63
process.
The E protein contains the major viral
Each E protein monomer is composed of 3
antigens and is the main target for neutralizing
domains (I- III). Domain I is located in the
antibodies (although antibodies directed central part of the protein. It is formed by 8
against prM/M and NS1 also induce some antiparallel beta sheets, contains the N-
protective immunity). Moreover, the E protein terminus of the protein, 2 disulfide bridges,
is responsible for specific binding to a cellular
and an N-glycosylation site. The function of E
receptor and penetration of the virus into the
protein glycosylation was investigated using
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