Exam 2 - Lecture 5

Question 1

What is the “purpose” of viral replication (from the virus perspective) and why should we care about it?

Answer 1

The purpose of viral replication is for the virus to make more of itself and further infect a host cell

Viral replication leads to cell death and human disease, and viral treatments are almost always virus-specific, so it’s important to understand how viruses replicate

Question 2

Attachment
- What types of chemical bonds are used?

Answer 2

Noncovalent bonds (H bond, van der Waals, ionic)

Question 3

Attachment
- Know the general names of the proteins that interact (on virus and host cell), how specific the interactions are, and the concept of multi-step attachment

Answer 3

There are host cell receptors that interact with adhesins (adhesion proteins) on the virions
- these interactions are extremely specific in the tissue tropism and host range

Some viruses can use different receptors to infect different cell types (primary receptor and co-receptor); some viruses may use alternative receptors if the primary receptor is not available

Question 4

Attachment
- What is meant by tissue tropism and host range?

Answer 4

Tissue tropism: tissue type infected by the host

Host range: species infected by the virus

Question 5

Attachment
- Know the experiments that can be done to prove that a host protein is serving as a receptor

Answer 5

1) add an antibody specific for the suspected receptor and see if viral attachment is blocked
2) add extra amounts of the normal ligand for the receptor and see if viral attachment is blocked

3) add the receptor to a viral-resistant cell and see if the virus attaches
4) add a soluble form of the receptor and see if the virus attaches

Question 6

Attachment
- What are the different classes of host proteins/products used as receptors by viruses?

Answer 6

1) cell adhesion receptors
2) cytokine and growth factor receptors
3) glycoproteins containing sialic acid
4) bacterial products

Question 7

Attachment
- What specialized parts of cell membranes often contain high levels of receptors (for viruses)?

Answer 7

Lipid rafts, intracellular joints, and clathrin-coated pits

Question 8

What cellular/viral changes occur when adhesins bind to receptors?
Understand the concept of permissiveness (and its relationship to attachment)

Answer 8

1) binding often triggers a conformational change in the adhesin and receptor
2) changes to the receptor can trigger clathrin recruitment to the membrane
3) binding can activate signal transduction in the host cell

Question 9

How do some viruses access particularly challenging cell membranes (what strategies do they use?)

Answer 9

1) wait for damage to the tissue
2) encode an enzyme to help break through the barriers
3) physical force through using an injectisome

Question 10

Where are viral adhesins located on naked vs enveloped viruses?

Answer 10

Naked: on capsid in pockets, on the outer surface of the capsid, or on protruding spikes

Envelope: embedded in the envelope

Question 11

What is meant by entry?

Answer 11

viral entry: crossing the plasma membrane and gaining access to the cytosol

Question 12

Plasma membrane entry
- Know the mechanism by which naked viruses enter at the plasma membrane
- Same for enveloped viruses…what are fusion proteins and how do they work?

Answer 12

Naked viruses: enter the PM by directly injecting the genome through a pore created in the bilayer

Enveloped viruses: can fuse their envelope directly with the plasma membrane
- Fusion proteins in the viral envelope help pull the two membranes together

Question 13

Internal membrane entry
- How do the virions get into an endosome?
- What is special about endosomes that aids in viral entry? (what is inside of those endosomes)

Answer 13

Instead of entering directly into the cytosol, virions trigger an endocytosis event and are brought into an endosome as a result of attaching to the PM

pH of the endosome becomes acidic and allows for viral entry

Question 14

Internal membrane entry
- What are advantages to entering via internal membranes?

Answer 14

1) make it through the plasma membrane cortex
2) can use endosomal tracking to move more easily deep into the cytosol
3) better at hiding from the immune system

Question 15

Uncoating
- What is it and what are some mechanisms that are used?

Answer 15

Uncoating: the genome escapes the protein capsid shell

Mechanisms
- Some uncoat during entry at the PM, others in the endosome
- Some uncoat due to cytosolic enzymes degrading it
- Some relatively intact capsids expel the genome

Question 16

Uncoating
- Besides the genome, what else in the virion is “uncoated”?

Answer 16

Uncoating is also about releasing other viral proteins that may be needed to start the infection

Question 17

What is meant by intracellular transport?

Answer 17

Intracellular transport: the process of moving the genome to specific components within the host cell

Question 18

What is different about RNA vs DNA viruses in terms of where they go after entry and why? What are some exceptions to the RNA “rule”?

Answer 18

RNA - replicate completely in the cytosol, and do not need to go anywhere specific
- exceptions:
1) retroviruses, which must go to the nucleus
2) flu – its mRNAs need processing

DNA - must go to the nucleus bc DNA viruses use host enzymes to copy their genome

Question 19

In what forms do viruses move to the nucleus (e.g. naked genome vs…)?

Answer 19

1) some move as a mostly naked genome with some associated proteins
2) some move as the capsid is being degraded
3) some move within fully intact capsids, and then uncoat at/inside the nucleus

Question 20

How do viruses move in the cell (what cellular components do they use?)? Know the specifics.

Answer 20

Transport occurs along cytoskeletal filaments, usually by using motor proteins such as microtubules and microfilaments

Question 21

How do viruses enter into the nucleus? (in what forms)

Answer 21

• some go through the nuclear pore with the help of a few viral proteins
• in others, the capsid docks on the nuclear pore and injects the genome in
• some whole capsids go in
• or they wait for the cell to divide and sneak in as the new nucleus forms