4. Learning Objectives
• Viral Life Cycle Overview
• Overview of virus replication
• Mechanisms used by animal viruses to gain entry into host
cells
• Evidence that a cell surface molecule is a virus receptor
• Two endocytic mechanism
• Initial stages of SV40 Internalization Via Caveolae
5. Figure 1. Pathways of virus entry into cells. (A) Enveloped viruses
can bind to cell surface receptors and directly fuse with the plasma
membrane. Virus particles can also be internalized via endocytosis, with
escape to the cytosol occurring either from the (B) early endosome or
(C) late endosome and lysosome. The acidic environment and
proteolytic enzymes in these compartments are required for fusion and
cytosol entry by different viruses.
Figure 2. Methods for viral genome release into host cells. (A) Certain
viruses, including rhinoviruses, expand to form pores in the endosome
through which the viral genome can escape. (B) Influenza and other
viruses induce fusion of the virion envelope with the endosomal
membrane, releasing the viral genome. (C) Many viruses, including
reoviruses, maintain a partially intact capsid in the cytosol that acts as a
“home base” for replication.
Figure 3. Several viruses must transport their genomes into the nucleus for viral
transcription or replication to occur. Influenza genome segments are transported through
the nuclear pore into the nucleus. Herpesvirus capsids are transported along microtubules
to the nuclear pore, where uncoating occurs. Adenovirus capsids disassemble at the nuclear
pore, and the viral DNA is transported into the nucleus. Other viruses, including hepatitis
B virus, are small enough that the entire capsid may pass through the nuclear pore.
Figure 4. Nucleocapsid formation and formation of mature virions. After
the production viral structural proteins, nucleocapsids are assembled in the
cytoplasm and followed by budding into the lumen of the endoplasmic
reticulum (ER)–Golgi intermediate compartment. Virions are then released
from the infected cell through exocytosis.
Viral Life Cycle Overview
7. Overview of virus replication
• The aim of a virus is to replicate itself, and in order to
achieve this aim it needs to enter a host cell, make copies of
itself and get the new copies out of the cell.
• Seven steps:
• 1. Attachment of a virion to a cell
• 2. Entry into the cell
• 3. Transcription of virus genes into messenger RNA
molecules (mRNAs)
8. • 5. Genome replication
• 6. Assembly of proteins and genomes into virions
• 7. Exit of the virions from the cell.
9. Mechanisms used by animal viruses to gain entry into
host cells
• Cell receptors and co-receptors
1. A virion attaches via one or more of its surface proteins to
specific molecules on the surface of a host cell.
2. These cellular molecules are known as receptors.
3. Some viruses need to bind to a second type of cell surface
molecule (a co-receptor) in order to infect a cell.
10. Cont.
• Receptors and co-receptors are cell surface molecules,
usually glycoproteins.
• Functions of Receptors and co-receptors include
1. Acting as cell signaling mediator for chemokines and
growth factors.
2. Mediating cell-to-cell contact and adhesion.
11. Some examples of cell receptors, virus proteins involved in
attachment and fusion proteins
13. Evidence that a cell surface molecule is a virus receptor
1. If one of the antibodies blocks virus binding and infectivity,
then this is strong evidence that the corresponding antigen is
the receptor.
2. Soluble derivatives of the molecule block virus
binding/infectivity.
3. The normal ligand for the molecule blocks virus
binding/infectivity.
14. A. Virus attachment sites
• The virus attachment sites of some
naked viruses are on specialized
structures, such as the fibre and
knobs of adenoviruses and the spikes
of rotaviruses
15. The poliovirus receptor is the glycoprotein CD155
1. CD155 is a member of the immunoglobulin super family of
molecules with three immunoglobulin-like domains.
2. The virus attachment site is located in the outermost domain.
3. CD155 is found only in humans and some other primate
species.
4. Transgenic mice expressing CD155 were developed.
5. These animals were found to be susceptible to infection with
all three serotypes of poliovirus and they have been used in
studies of replication and pathogenesis.
16. B. Attachment of virions to receptors
• The forces that bind a virus attachment site to a receptor
include
1. Hydrogen bonds,
2.Ionic attractions
3.Van der Waals forces.
• No covalent bonds are formed between virions and
receptors.
1.At this bind, the attachment is bind, and the virion may
detach.
2.If sufficient receptors bind, then the attachment to the cell
17. C. Entry of animal viruses into cells
• Viruses may either:
1. Cross the plasma membrane at the cell surface directly
2. Or they may cross the membrane through Endosome
3.This process (endocytosis) is used by cells for a variety of
functions, including nutrient uptake and defense against
pathogens.
4.There are a number of endocytic mechanisms and most
animal viruses hi-jack one or more of these mechanisms in
order to gain access to their host cells.
18. Two endocytic mechanism
I. Clathrin mediated endocytosis
II. Caveolin-mediated endocytosis
• Clathrin and caveolin are proteins that are involved in
endosome formation.
• When virion binds to a region of the plasma membrane the
coat protein clathrin or caveolin protein molecules force
the membrane to bend around the virion.
19. Clathrin-coated endosomes
1. Many viruses, such as adenoviruses, are
endocytosed at clathrin-coated regions of the
plasma membrane.
2. The virions end up in clathrin-coated endosomes,
from which the clathrin is soon lost.
3. An endosome may fuse with other vesicles such as
lysosomes, which have a pH of 4.8–5.0, thus
lowering the pH within the vesicle.
20. EM views - coated pit to coated vesicle
Coated
pits
coated
vesicles
Coated pits coated vesicles
23. ECB 15-19
Coated pit “pinching
off”
(dynamin)
budding uncoating
Pinching off the vesicles requires the protein dynamin
Assembly of coat causes pit to form due to 3D shape of clathrin coat
25. GTP
GDP + Pi
Clathrin uncoating
ATPase
Naked
transport
vesicle
Dynamin
ATP
ADP + Pi
Clathrin-coated vesicles are rapidly uncoated
By the “clathrin-uncoating ATPase” a
member of the HSP70 family of chaperones;
requires ATP hydrolysis
Naked transport vesicles targeted to
endosome…
Clathrin and adaptins recycled
See ECB figure 15-19
“Clathrin-coated pit”
To endosome…
Clathrin
Adaptin complexes
26. • Caveolins are proteins essential for the
formation and stability of caveolae (signaling
platforms).
• Caveolae rounded plasma membrane
invaginations of 50–80 nm in diameter.
Caveolin-mediated endocytosis
27. • After binding to the membrane, virus particles are
mobile until trapped in caveolae, which are linked to
the actin cytoskeleton (step 1).
• In the caveolae, SV40 particles trigger a signal
transduction cascade that leads to local protein
tyrosine phosphorylation and depolymerization of the
cortical actin cytoskeleton (step 2).
Initial stages of SV40 Internalization Via
Caveolae
28. • Actin monomers are recruited to the virus-loaded caveolae and an
actin patch is formed (step 3).
• Concomitantly, Dynamin is recruited to the virus-loaded caveolae
and a burst of actin polymerization occurs on the actin patch (step
4).
• Virus-loaded caveolae vesicles are now released from the
membrane and can move into the cytosol (step 5).
29. • After internalization, the cortical actin cytoskeleton returns to its
normal pattern (step 6).
32. Learning Outcomes
• Mechanisms used by animal viruses to gain entry into host
cells
• Evidence that a cell surface molecule is a virus receptor
• Initial stages of SV40 Internalization Via Caveolae
33. Recommended Literature
Replication of RNA virus | How RNA virus replicate |
Virology USMLE
Review of Medical Microbiology and Immunology
Fenner and White's Medical Virology (5th Edition) by Christopher
J. Burrell, Colin R. Howard, Frederick A. Murphy.
Human Virology (4th Edition) by Leslie Collier, John Oxford and Dr.
Paul Kellam.
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#14:CD155 is a member of the immunoglobulin super family of molecules with three immunoglobulin-like domains.
#15:Mice that have had DNA from another source put into their DNA. The foreign DNA is put into the nucleus of a fertilized mouse egg. The new DNA becomes part of every cell and tissue of the mouse.
#17:Endosome: which is a vesicle formed by part of the plasma membrane pinching off into the cytoplasm.
#18:Clathrin mediated endocytosis: a process by which cells absorb metabolites, hormones, proteins – and in some cases viruses – by the inward budding of the plasma membrane (invagination).
Caveolin-mediated endocytosis: Caveolin-mediated endocytosis of virus by host (kw:KW-1166)Virus internalization by the host cell via caveolae, which are specialized lipid rafts that form 50-70 nm flask-shaped invaginations of the plasma membrane.
#19:The primary function of endosomes relates to the transportation of extracellular material into the intracellular domain. Lysosomes, on the other hand, are primarily involved in the degradation of macromolecules
#21:The shortest possible line between two points on a sphere or other curved surface. Clathrin is a protein that plays a role in the formation of coated vesicles. Clathrin was first isolated by Barbara Pearse in 1976. It forms a triskelion shape composed of three clathrin heavy chains and three light chains. When the triskelia interact they form a polyhedral lattice that surrounds the vesicle.
#23:Dynamin= Self-polymerizing mechanoenzyme that triggers vesicle scission upon GTP hydrolysis.
#25:Molecular chaperones facilitate and regulate protein conformational change within cells. This encompasses many fundamental cellular processes: including the correct folding of nascent chains; protein transport and translocation; signal transduction and protein quality control.
#26:Lipid rafts and caveolae are microdomains of the plasma membrane enriched in sphingolipids and cholesterol, and hence are less fluid than the remainder of the membrane. Caveolae have an invaginated structure, while lipid rafts are flat regions of the membrane.
The invaginating structures involve cell membranes of two different cells, with the outward projection – the invaginating structure – from one cell being surrounded by the invaginated membrane of the other cell.
#27:SV40: simian virus 40, a polyomavirus that is found in both monkeys and humans.
Cortical actin plays a key role in cell movement and division, but has also been implicated in the organisation of cell surface receptors such as G protein-coupled receptors.
Depolymerization is defined as the process where a polymer breaks down into monomers or co-monomers due to oxidation at the chain
