Ebola.....The threat

Replication of Ebola Virus published in "MICROBIOLOGY INFO AND NOTES"
Nov 23rd, 2014 Edition·
Replication of Ebola Virus
Ebola Virus do not replicate through any kind of cell
division; rather, they use a combination of host and
virally encoded enzymes, alongside host cell
structures, to produce multiple copies of viruses.
These then self-assemble into viral macromolecular
structures in the host cell. The virus completes a set
of steps when infecting each individual cell.
Following are the steps during the replication of
Ebola Virus :
1. Attachment
First of all, there is attachment of virus to host
receptors through GP glycoprotein which is
endocytosed into vesicles in the host cell. Host DC-
SIGN and DC-SIGNR play a role in virion attachment.
2. Viral Entry (Penetration)
The virion enters early endosomes by
Macropinocytosis or clathrin-mediated endocytosis.
A. Macropinocytosis
In this process, ruffled segments of the host’s
plasma membrane protrude outward from the cell and
form invaginations where the virus utilizes
glycoproteins in order to attach to the surface of the
plasma membrane. Macropinocytosis is a process in
which the Eukaryotic host cells form
macropinosomes, segments of plasma membranes
that extend out from the cell approximately 0.2-10
µm, in order to incorporate the virus into the cell. The
formation of macropinosomes occurs spontaneously,
as a result of the activation of various growth factors,
or simultaneously with the intake of cellular
molecules or extracellular fluid.
B. Clathrin-mediated endocytosis
Clathrin-mediated endocytosis is the other means by
which Ebolavirus enters the host cell. This process is
very similar to macropinocytosis in that the plasma
membrane forms invaginations that engulf the cell.
However, clathrin-mediated endocytosis is different
in that proteins on the surface of the host’s surface,
and in particular clathrin, facilitate the attachment of
the virus to the host’s cell surface. Glycoproteins are
still used to attach the virus to the cell surface, and
the NP-C1 cholesterol transporter still facilitates the
fusion of the virus with endosomes and lysosomes
and still allows the virus to escape into the
cytoplasm. Without the NPC1 cholesterol transporter,
Ebolavirus cannot leave the vesicle in order to
replicate and cause infection in other cells.
To penetrate the cell, the viral membrane fuses with
vesicle membrane, and the nucleocapsid is released
into the cytoplasm.
In some culture cells, GP glycoprotein can be
processed by host Cathepsin L andCathepsin B into
19kDa GP1. But this processing is not happening in
all cells or for all ebolavirus. 19kDA GP1 interacts
with host NPC1, which is highly expressed in
dendritic cells.
Fusion of virus membrane with the vesicle
membrane is triggered by either low pH orNPC1
binding.
3. Sequential Transcription
During transcription, the RNA genome is transcribed
into seven monocistronic mRNAs whose length is
determined by highly conserved start and stop
signals.
The transcription process begins with the binding of
the polymerase complex to a single binding site
located within the leader region of the genome. The
complex then slides along the RNA template and
sequentially transcribes the individual genes in their
3’ to 5’ order. Encapsidated, negative-sense genomic
ssRNA is used as a template for the synthesis (3′-5′)
of polyadenylated, monocistronic mRNAs and, using
the host cell’s ribosomes, tRNA molecules, etc., the
mRNA is translated into individual viral proteins.
4. Replication
As viral protein levels rise, a switch occurs from
translation to replication. Using the negative-sense
genomic RNA as a template, a complementary
+ssRNA is synthesized; this is then used as a
template for the synthesis of new genomic (-)ssRNA,
which is rapidly encapsidated
Replication presumably starts when enough
nucleoprotein is present to encapsidate neo-
synthetized antigenomes and genomes.
5. Budding
The newly formed nucleocapsids and envelope
proteins associate at the host cell’s plasma
membrane; budding occurs, destroying the cell.
These viruses recruit components of the cellular
ESCRT (endosomal sorting complex required for
transport) system to mediate host-assisted viral
budding. SCRT complexes are normally used by the
cell for biological functions involving membrane
remodeling, such as intraluminal vesicle formation,
autophagy or terminal stages of cytokinesis. The
ESCRT family consists of ESCRT-0, ESCRT-I,
ESCRT-II which are primarily involved in cargo
sorting and membrane deformation, and ESCRT-III
which cleaves the bud neck from its cytosolic face . In
the last step, vps4 disassembles the complex. The
budding reaction catalyzed by the ESCRT machinery
has reversed topology when compared with most
other budding processes in the cell, such as
endocytosis and formation of transport vesicles.
6. Release
Finally, the virion is released.