_________________________________________________________________________
Introduction – infection cycle
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Cell entry of enveloped viruses
• Binding of viral glycoprotein with cell surface receptor lead to the
fusion of viral envelope with the plasma membrane
• Binding of viral glycoprotein with cell surface receptor triggers
receptor mediated endocytosis (receptor/clathrin/caveolae)
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Steven AC and Spear PG. Science VOL 313, 2006
Cell entry of enveloped viruses
receptor-triggered
fusion
low-pH mediated
fusion
_________________________________________________________________________
Nature Reviews Microbiology 6, 143-155 (February 2008)
Viral glycoprotein structure
• Viral surface glycoproteins essential for virus-cell membrane fusion
(e.g. HIV (gp41/gp120) & Influenza HA1/HA2)
structure of glycoprotein:
- external domain (host interaction)
- endodomain (internal part)
- transmembrane domain (spans the viral
envelope membrane; typically -helix)
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Nature Reviews Microbiology 6, 143-155 (February 2008)
Classes of virus fusion proteins
Class I
Class II
Class III
• glycoprotein B of herpes simplex virus (HSV) gB
• glycoprotein G of vesicular stomatitis virus (VSV)
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Classes of virus fusion proteins
• priming step of fusion proteins during budding process:
- proteolysis of precusrsor (e.g. HA0) or co-regulatory protein (e.g. p62)
• Trigger mechanisms (low pH or co-receptor binding) induce conformational change
of fusion protein to a extended intermediate due to altered energy profile
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Main mechanism of membrane
fusion
_________________________________________________________________________
Nature Reviews Microbiology 6, 143-155 (February 2008); Nat Struct Mol Biol. 2008 July, 15(7): 690–698.
Class I membrane-fusion proteins
(Influenza)
cell
virus
Folding-backHA2
Metastable
conformational
Loop-to-helix
bridge
transition,
change
protein
collapsing
and
trimer
translocation
HA1
with
todissociation
stable
transmembrane
of α-helical
fusion
upon
peptide
coiled
specific
domain,
towards
coiltrigger
hairpin-trimer
α-helical
cell
mechanism
membrane
coiled
and fusion
coil stalk
(HIV
and
peptide
anchoring
-> capping
CD4
and receptor;
buried
->
of pre-hairpin
transmembrane
fusion
Influenza
peptide
intermediate
-> domain
sialic acid
(high
-> &irreversible
low
energy)
pH) followed
hairpin fusion
by hemifusion
pore
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Nat Struct Mol Biol. 2008 July, 15(7): 690–698
Class II membrane-fusion proteins
(flavivirus and alphavirus)
fusion peptides
• folding with regulatory companion protein: flavivirus (p62); alphavirus (prM)
• β-sheet (red), fusion loop domain (yellow) and Ig-like (blue) domain connected with TM
• in metastable form: flavivirus (E)2 homodimer; alphavirus (E1-E2)3 heterodimer
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Nat Struct Mol Biol. 2008 July, 15(7): 690–698
Class II membrane-fusion proteins
(flavivirus)
cell
virus
+ binding
Flipping-over
Extended
Dimer
Metastable
dissociation
intermediate
E(arrows)
protein
upon
homodimer
of
and
specific
domain
fusiontrigger
III
peptide
(alphavirus:
leadmechanism
toanchoring
back-zipping
heterodimer)
(H
upon
ofinteraction
stem,
linked
atbringing
to
lowofpH)
domains
fusion
and
Itrimerization
loop
transmembrane
andand
II ->
transmembrane
pre-hairpin
to homotrimers
domain,
intermediate
domain
buried
upon
fusion
together
fusion
(high
peptide
peptide
->
energy)
irreversible
(black
contact
followed
circle)
hairpin
toby
target
hemifusion
fusion
membrane
pore
_________________________________________________________________________
Nat Struct Mol Biol. 2008 July, 15(7): 690–698
Class II membrane-fusion proteins
(alphavirus)
_________________________________________________________________________
Class III membrane-fusion proteins
(VSV)
cell
virus
Flipping
G-protein
Post fusion
over
fusion
conformation
of fusion
trimer domain
with
of core
the(arrow)
subunit
domain
upon
(d)
(red)
and
specific
and
thefusion
trimer
trigger
peptide
(e)
mechanism
after
(black
back-folding
(pH)
circle)
and
held
fusion
and away
up-zipping
peptide
fromcontact
target
of transmembrane
to
membrane
target membrane
domain bringing
-> extended
the membranes
intermediatetogether
_________________________________________________________________________
Nat Struct Mol Biol. 2008 July, 15(7): 690–698
Important factors that contribute to
virus membrane fusion
• cholesterol and sphingolipid content of target cell membranes
• cooperative interaction of fusion peptides to cluster rings of five or
six protein homotrimers
-> enhancement of initial fusion pore formation (´dome like´)
• requirement of trimers for membran fusion: - Influnenza -> 8-9
- HIV -> 1 !!
_________________________________________________________________________
Summary I
_________________________________________________________________________
Dev Cell. 2008 Jan;14(1):11-21. Review.
Summary II
Class III (VSV G)
reversible trimer formation
upon pH change
α-helix + β-sheet
Trimer of hairpins
No proteolytic process
No companion
Internal loop at fusion
protein
_________________________________________________________________________
Modified from Nat Rev Microbiol. 2006 Jan;4(1):67-76. Review.
Thank you for your attention
_________________________________________________________________________
Classes of virus fusion proteins
Class I
• Found in othomyxoviruses, paramyxoviruses,
retrovoruses, filoviruses and coronaviruses.
• Form trimers with triple coiled-coil stem which in the postfusion
state gives a distinctive six-helix bundle.
• The active fusogenic form is obtained through a proteolytic
cleavage which reveals the hydrophobic fusion peptide.
Class II
• Found in flaviviruses and alphaviruses
• Initially are present in form of dimers but at low pH the dimers
dissociate exposing the fusion loops. The fusion loops insert into
host membrane which triggers trimerization of the glycoproteins.
Class III
• Glycoprotein B of Herpes Simplex Virus (HSV) gB
• Glycoprotein G of Vesicular Stomatitis Virus (VSV)
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