Lecture 5 - Viral Attachment and Entry Flashcards
Phases of a viral replication curve
1) Eclipse period
2) Latent period
3) Extracellular release
Eclipse period
When viral proteins are being synthesised within a host cell.
Involves the uncoating phase
Latent period
Period before extracellular release of virus.
Includes the eclipse period and the synthetic phase (when virions are assembled)
How do the growth curves for enveloped and naked viruses differ?
Latent period for enveloped viruses is much shorter.
Enveloped infectious virus forms after budding from membrane, whereas naked infectious virus forms within host cell.
How is viral infection of a cell a random event?
A virus has no motility. Attaches to host cell as a result of random collision
What does ‘multiplicity of infection’ describe?
The number of virions required to infect a cell
Generic stages of a viral life cycle 1) 2) 3) 4) 5)
1) Attachment
2) Uncoating
3) Replication
4) Assembly
5) Release
Is the virion permanently stuck together?
No
How can viruses enter a cell?
By endocytosis or membrane fusion
How do viruses attach to host cell?
Very specifically.
Normally attach to a cell membrane protein or carbohydrate
What is the only host cell mechanism that can take in viruses?
Receptor-mediated endocytosis
Why can’t a virus enter a cell via phagocytosis or pinocytosis?
These functions operate on much larger objects than viruses
Fusion strategies used by viruses
1)
2)
3)
1) Uncoating at plasma membrane (EG: measles)
2) Uncoating within endosomes
3) Uncoating at nuclear membrane (EG: adenovirus)
How does a virus enter a cell via endocytosis?
1)
2)
1) Invagination of a clathrin-coated pit, forming an endosome)
2) Acidification of endosome causes conformational change in virion that leads to fusion or genome release into cytoplasm
Entry and uncoating of adenovirus 1) 2) 3) 4)
1) Penton fibre of adenovirus binds to cell adenovirus receptor (CAR), which are integrins or IG-like molecules
2) Endocytosis
3) Acidification leads to penton release from virion, which forms pores where the pentons released from.
4) Endosome is burst, modified capsid is transported to the nuclear membrane pore via the cellular microtubule network
Entry and uncoating of poliovirus
1)
2)
3)
1) Viral capsid proteins VP1, VP2 and VP3 form a ‘canyon’ area. CD155 binds to canyon.
2) CD155 binding causes VP1 to undergo conformational change, where VP1 hydrophobic domain forms a pore in the plasma membrane
3) VPg and polio RNA genome enter host cell through pore
Example of Reoviridae
Rotavirus
Family that rotavirus belongs to
Reoviridae
Entry and uncoating of reoviruses 1) 2) 3) 4)
1) Outer capsid proteins bind to target cell
2) Receptor-mediated endocytosis
3) Acidification of the endosome leads to proteolytic cleavage of outer membrane, leading to the infectious subviral particle (ISVP).
4) ISVIP modified capsid proteins mediate endosomal escape
Rotavirus entry and uncoating
1)
2)
1) Rotavirus is a Reovirus, so it’s outer capsid needs low-pH proteolytic modification. This occurs in the low pH of the stomach
2) Once the infectious subviral particle (ISVP) is formed, it can infect target cells
Virus that binds to CD155 using a ‘canyon’ shape
Poliovirus
Virus that escapes endosome using penton dissociation form capsid
Adenovirus
Virus that needs low pH to form an infectious subviral particle
Reoviridae (EG: rotavirus)
Virus that binds to sialic acid
Influenza
Where is sialic acid found?
Terminal carbohydrate on many cell-surface carbohydrate chains
Influenza entry and uncoating
1)
2)
3)
1) Haemagglutinin binds to sialic acid, leading to receptor-mediated endocytosis
2) Acidification of endosome leads to revealing of haemagglutinin fusion protein
3) Further acidification brings together endosomal membrane and viral envelope, resulting in endosome escape
Examples of viruses that can enter a cell via direct fusion
1) Paramyxoviruses (EG: measles)
2) Rotavirus (VP4 has fusion activity if activated by protease trypsin)
Measles hypothesised methods of entry 1) 2) 3) 4)
SINGLE RECEPTOR MODEL
1) Measles haemagglutinin binds to CD150
2) Binding leads to conformational changes in fusion peptide, which binds to the plasma membrane
CO-RECEPTOR MODEL
3) Measles haemagglutinin binds to CD150
4) Fusion protein binds to CD46
How can HIV nonspecifically bind to cells?
Using lectin receptors
Generic HIV cell entry 1) 2) 3) 4)
1) Non-specific attachment to lectin receptor
2) CD4 attachment with gp120
3) gp120 coreceptor binding (CCR5)
4) Binding of gp120 to CCD5 leads to gp41 heptad repeat domain 2 (HR2) coils around HR1, bringing HIV and cell membrane together
How is it determined whether a virus enters a cell via endocytosis or fusion?
1) Expose cells to virus in the presence of a weak base (EG: chloroquine, methylamine)
2) This prevents acidification of endosome, which would prevent viruses infecting cells via endocytosis
Protein that carries vesicles away from nucleus
Dynein
Protein that carries vesicles towards nucleus
Kinesin
