3-4 Virus: Infectious cycle Flashcards
The infectious cycle
Steps
Summary
Do boundaries between steps exist?
- Attachment
- Entry
- Uncoating and Translation
- Replication
- Assembly
- Release
Summary
1. Attachment: Viral proteins on the capsid or phospholipid envelope interact with specific receptors on the host cellular surface. This specificity determines the host range (tropism) of a virus.
- Entry / Penetration: The process of attachment to a specific receptor can induce conformational changes in viral capsid proteins, or the lipid envelope, that results in the fusion of viral and cellular membranes. Some viruses can also enter the host cell through receptor-mediated endocytosis.
- Uncoating: The viral capsid is removed and degraded by viral enzymes or host enzymes releasing the viral genomic nucleic acid or nucleocapsid.
- Replication: After the viral genome has been uncoated, transcription or translation of the viral genome is initiated. It is this stage of viral replication that differs greatly between DNA and RNA viruses and viruses with opposite nucleic acid polarity. This process culminates in the de novo synthesis of viral proteins and genome.
- Assembly: After de novo synthesis of viral genome and proteins, which can be post- transrciptionally modified, viral proteins are packaged with newly replicated viral genome into new virions that are ready for release from the host cell. This process can also be referred to as maturation.
- Virion release: There are two methods of viral release: lysis or budding.
Steps to facilitate the study but not such artificial boundaries occur irl
Where do viral infections begin?
3 different ones
They begin at exposed epithelial surfaces
apical (top, presented to outside)
basal (bottom, prestented to inside)
lateral (side to side cell contacts)
What’s the difference between affinity and avidity?
What’s the characteristics of viral binding?
Affinity is the strength of an specific interaction between a single receptor and single virus binding protein, its the sum of attractive and repulsive forces between the 2 components
Avidity is a measure of the overall binding of receptor with ligand, involving multiple binding site, so its influenced by how many binding sites exist on receptor and ligand
Characteristics of viral binding
The interaction between receptor and viral ligand is low affinity reaction, but the combination if several receptors with viral ligand leads to high avidity reaction
Attachement
How is it mediated?
Which host cell molecules exist? (3)
What does it faciliate? (6)
What does entry receptors do? (3)
Attachment to host cell is mediated by virion proteins binding to specific host surface molecules:
- Glycoproteins
- Glycolipids
- Carbohydrates like sialic acid and heparan sulfate
Despite 1. low affinity, these receptors 2. facilitate adhesions and therefore 3. concentrate the virus near it’s entry 4. triggering signaling pathways. It can also 5. carry the virus to a specific organ. But the 6. ahesion alone does not trigger entry, that’s what entry receptors do
Entry receptors trigger endocytosis or membrane fusion, have a higher affinity, but they dont function as viral receptor, they are misused by the virus
Glycoprotein and different viruses
Rules and examples
Same glycoprotein can be receptor to multiple viruses, often ig-superfamily
CAR for CoxA and Adenovirus
DC-SIGN
Related viruses can bind different receptors
SARS (ACE2) and MERS (DPP4)
One virus might require multiple receptors
HCV
Viral glycoproteins can bind different receptors
HSV-1
HIV entry
Attachment via
Entry Mechanism (7)
Attachment via
Heparan Sulfate Proteoglycan
Entry
1. env gene codes for gp160
2. processed by host cell protease in gp 120 ans gp41
3. gp 120 binds to CD4
4. conformational change exposes co receptor binding site
5. binding of co receptors CCR5 and CXCR4
6. gp41 non covalently bound to gp120 is conformationally changed
7. Insertion of fusion peptide into host cell membrane, resulting in fusion of viral envelope with host cell membrane
Influenza
Attachment and involved (viral) proteins
Sialic acid 2 forms
Attachment
HA trimer binds sialic acid
NA cleaves sialic acid
M1 matrix
M2 ion channel
bound alpha 2,3 (avian) or alpha 2,6 (human) to Galactose
cleaved by Neuraminidase
Unspecific mechanism for uptake of macromolecules
2 examples and mechanism + how big?
example of viruses
Phagocytosis
particle broken down to simpler substances engulfed by plasma membrane
-> Pseudopodia (1-2µm)
Pinocytosis
ingested substances can be absorbed
-> Invagination (0,1-0,2µm)
example of viruses
Adenovirus
Specific uptake of macromolecules
2 examples and subtypes in general
Receptor mediated endocytosis
cell absorbs metaboilites, hormones, proteins, viruses by receptor specific inward budding of plasma membrane
Clathrin mediated
Caveolin mediated
others
Fusion with plasma membrane
enveloped viruses only
Fusion with plasma membrane
which viruses
Defintion
Mechanism (when active, which formation, mechanism)
only enveloped viruses
Defintion
Merging of 2 initially seperate lipid bilayers with result of mixing of 2 distict aqueous compartments
Mechanism
Viral fusion proteins need to hide and display the fusion peptide at the right moment to avoid fusion in wrong compartments
Active in trimieric conformation, some natively trimeric, some dimers on virion that are converted to trimers upon activation
- fusion peptide
- conformational change due to low pH induces insertion of fusion peptide into host cell membrane
- multiple event occur
- another conformational change leads to host cell membrane notches
- followed by hairpining and
- membrane fusion
Examples of different viruses and sequence
Dengue Virus, Mosquito borne viruses, Tick borne, Powassan
-> quite conserved sequence
Fusion protein
classes and example
classes, formation, facts and example
class 1:
alpha-helices
trimer
Influenza
class 2:
dimer
parallel to membrane, ß-sheet
Flaviviridae
class 3:
trimer
ß-sheet
Herpesviridae
Flaviviridae
examples
4 facts
example of fusion protein
examples
many mosquitp borne viruses like West-Nil or Dengue, Yellow Fewer Virus, tick born encephalitis
+ssRNA 10-11k, size 50nm, symetry, electron microscopy
fusion protein
class 2, dimer, tick born encephalitis E protein
Trafficking
Why necessary?
most common type and mechanism
Which viruses mostly use it?
In the cell, simple diffusion is not sufficient because the cytoplasmic enviroment is highly packed
strcutures above 20nm like viruses require an energy dependent motility to travel, therefore most viruses utilize molecular motors like microtubules (most) or actin skeleton (rare)
Microtubuli
Dynein motors on microtubule move towards minus end (nucleus)
Kinesin motors on microtubules move towards positive end (periphery)
Receptor for motor molecule required for e.g. cargo proteins
Which viruses?
Mostly used by viruses that target their genomes to nucleus
Also neutrotropic viruses to enter nearby nerve endings and their viral genome must be moved to cell bodies by aonomal transport (retrograde = towards center)
Nuclear transport
mechanism
via nuclear pore complexes (4000 present per cell, highly efficient)
Protein with NLS is recognized and transported to complex by importin
Then protein + alpha subunit of Importin translocated through nuclear pore complex in energy requiring step
Viral capsid
function
metastability
Mechanism
protects genome
delivers/releases the genome (metastable)
optional
binds to host cell receptors
involved in mebrane fusion
trafficking inside cell
Metastability
Have not attained minimum free energy conformation, this conformatio is only gained when unfavorable energy barrier is reached
Followed by irreversible conformational changes during attachment and entry
