Lecture 5 - Viral Attachment and Entry

Question 1

Phases of a viral replication curve

Answer 1

1) Eclipse period
2) Latent period
3) Extracellular release

Question 2

Eclipse period

Answer 2

When viral proteins are being synthesised within a host cell.
Involves the uncoating phase

Question 3

Latent period

Answer 3

Period before extracellular release of virus.

Includes the eclipse period and the synthetic phase (when virions are assembled)

Question 4

How do the growth curves for enveloped and naked viruses differ?

Answer 4

Latent period for enveloped viruses is much shorter.

Enveloped infectious virus forms after budding from membrane, whereas naked infectious virus forms within host cell.

Question 5

How is viral infection of a cell a random event?

Answer 5

A virus has no motility. Attaches to host cell as a result of random collision

Question 6

What does ‘multiplicity of infection’ describe?

Answer 6

The number of virions required to infect a cell

Question 7

Generic stages of a viral life cycle
1)
2)
3)
4)
5)

Answer 7

1) Attachment
2) Uncoating
3) Replication
4) Assembly
5) Release

Question 8

Is the virion permanently stuck together?

Answer 8

No

Question 9

How can viruses enter a cell?

Answer 9

By endocytosis or membrane fusion

Question 10

How do viruses attach to host cell?

Answer 10

Very specifically.

Normally attach to a cell membrane protein or carbohydrate

Question 11

What is the only host cell mechanism that can take in viruses?

Answer 11

Receptor-mediated endocytosis

Question 12

Why can’t a virus enter a cell via phagocytosis or pinocytosis?

Answer 12

These functions operate on much larger objects than viruses

Question 13

Fusion strategies used by viruses
1)
2)
3)

Answer 13

1) Uncoating at plasma membrane (EG: measles)
2) Uncoating within endosomes
3) Uncoating at nuclear membrane (EG: adenovirus)

Question 14

How does a virus enter a cell via endocytosis?
1)
2)

Answer 14

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

Question 15

Entry and uncoating of adenovirus
1)
2)
3)
4)

Answer 15

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

Question 16

Entry and uncoating of poliovirus
1)
2)
3)

Answer 16

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

Question 17

Example of Reoviridae

Answer 17

Rotavirus

Question 18

Family that rotavirus belongs to

Answer 18

Reoviridae

Question 19

Entry and uncoating of reoviruses
1)
2)
3)
4)

Answer 19

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

Question 20

Rotavirus entry and uncoating
1)
2)

Answer 20

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

Question 21

Virus that binds to CD155 using a ‘canyon’ shape

Answer 21

Poliovirus

Question 22

Virus that escapes endosome using penton dissociation form capsid

Answer 22

Adenovirus

Question 23

Virus that needs low pH to form an infectious subviral particle

Answer 23

Reoviridae (EG: rotavirus)

Question 24

Virus that binds to sialic acid

Answer 24

Influenza

Question 25

Where is sialic acid found?

Answer 25

Terminal carbohydrate on many cell-surface carbohydrate chains

Question 26

Influenza entry and uncoating
1)
2)
3)

Answer 26

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

Question 27

Examples of viruses that can enter a cell via direct fusion

Answer 27

1) Paramyxoviruses (EG: measles)

2) Rotavirus (VP4 has fusion activity if activated by protease trypsin)

Question 28

Measles hypothesised methods of entry
1)
2)
3)
4)

Answer 28

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

Question 29

How can HIV nonspecifically bind to cells?

Answer 29

Using lectin receptors

Question 30

Generic HIV cell entry
1)
2)
3)
4)

Answer 30

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

Question 31

How is it determined whether a virus enters a cell via endocytosis or fusion?

Answer 31

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

Question 32

Protein that carries vesicles away from nucleus

Answer 32

Dynein

Question 33

Protein that carries vesicles towards nucleus

Answer 33

Kinesin