Lec 5: Attachment & Entry

Question 1

general the process of virus replication: (7)

Answer 1

(AETTGAE):

1. Attachment of a virion to a cell
2. Entry into the cell
3. Transcription of virus genes into messenger RNA molecules (mRNAs)
4. Translation of virus mRNAs into virus proteins
5. Genome replication
6. Assembly of proteins and genomes into virions
7. Exit of the virions from the cell.

Question 2

Not all of the seven steps… (2)

Answer 2

• are relevant to all viruses,
  
  • depending on type of genetic info they are (+/-)
• occur in the same order, and some have an additional step

Question 3

attachment and entry of viruses into cells; mainly on the mechanisms used by

Answer 3

animal viruses

Question 4

Receptors and co-receptors are…

that include… (2)

Answer 4

…cell surface molecules, usually glycoproteins, with a wide range of functions that include:

• acting as receptors for chemokines and growth factors;(chemokines, small proteins guide the migration of cells)
• mediating cell-to-cell contact and adhesion

Question 5

The recognition of a receptor by a virion is

Answer 5

highly specific

Question 6

Sometimes a second type of…

Answer 6

…cell surface molecule (a co-receptor) is needed in order for virus to infect a cell.

Question 7

The virus-receptor binding causes a…

Answer 7

…conformational change in the virus protein that enables it to bind to the co-receptor

Question 8

for attachment, what is more important? (DNA or protein)

Answer 8

protein!

• Protein binding = conformational change
• Diff protein structure = completely different action

Question 9

How to prove a receptor is a virus binding cell surface protein? (4)

Answer 9

• mAb (monoclonal antibody)
• Soluble derivatives of the molecule block virus binding/infectivity.
• The normal ligand for the molecule blocks virus binding/infectivity.
• Introduction of the gene encoding the molecule into virus-resistant cells, and expression of that gene, makes those cells susceptible to infection.

Question 10

Monoclonal antibody (mAb) recognizes:

Answer 10

single epitope

Question 11

Polyclonal antibody (pAb) recognizes:

Answer 11

multiple epitopes

Question 12

epitope =

Answer 12

part of antigen that’s recognized by immune system antibodies, B-cells, or T-cells

Question 13

B cells =

Answer 13

makes antibody

Question 14

epitope aka

Answer 14

“antigenic determinant”

Question 15

mAb can help determine

Answer 15

viral specificity

Question 16

Ab can bind to

Answer 16

virus surface receptor and block viral binding to cell

Question 17

Drugs can bind to

Answer 17

virus AND cell to block viral binding
- however, virus surface protein is more likely to undergo mutation quicker than cell, t.f. its more efficient to design a drug that would bind to the cell instead of the virus

Question 18

(animal viruses)

Each virion has

Answer 18

multiple sites that can bind to receptors

Question 19

(animal viruses)

Each site is made up of

Answer 19

regions of one or more protein molecules

Question 20

(animal viruses)
The virus attachment sites of naked viruses are on
- sometimes… (2)

Answer 20

the capsid surface (ex: picornaviruses)

• sometimes within depressions (poliovirus)
• sometimes on ridges (ex: foot & mouth disease)

Question 21

Binding to receptors induces…

Answer 21

major structural changes in the virion

Question 22

(virus attachment sites within depressions)

canyons on the virion surface, virus attachment sites located in…

Answer 22

…pockets at the canyon bases.
- The canyons are too narrow for access by antibodies — the virus attachment sites are protected from the host’s immune surveillance

Question 23

Some virion surface proteins that bear the virus attachment sites are able to…

Answer 23

bind strongly to red blood cells of various species and cause them to clump, a phenomenon known as haemagglutination.

Question 24

haemagglutinins =

Answer 24

The viral proteins responsible for haemagglutination

Question 25

What causes the binding of a virus attachment site to a receptor? (3)

Answer 25

• hydrogen bonds,
• ionic attractions and
• van der Waals forces (No covalent bonds)

Question 26

(Attachment of virions to receptors)

• Initially, a virion is…
• more…

Answer 26

• …weakly bound to a cell at only one or a few receptors = reversible attachment
• …virus attachment sites to bind to more receptors = irreversible.

Question 27

Entry of animal viruses into cells: (2)

Answer 27

• may enter either at the cell surface (fuse directly with the plasma membrane)
• may cross the endosomal membrane

Question 28

Some enveloped viruses require…

So, endoscope provides…

Answer 28

an acidic pH for fusion to occur and are unable to fuse directly with the plasma membrane.
- Endosome provides the acid environment.

Question 29

Entry of NAKED viruses into cells: (2)

Answer 29

• through a pore in the plasma membrane
  OR
• endocytosis (more common)

Question 30

Entry of ENVELOPED viruses into cells: (2)

Answer 30

• by membrane fusion at the cell surface AND by endocytosis,
  OR
• ONLY by endocytosis

Question 31

(Intracellular transport)

Once in the cell, the virus (or its genome) may have to be…

• Most RNA viruses…
• Retroviruses, also RNA viruses…
• Most DNA viruses…

Answer 31

…delivered to a particular location, such as the nucleus.

• …replicate in cytoplasm (flu virus exemption)
• …need nucleus
• …need nucleus with a few exemptions

Question 32

motor proteins: (2)

Answer 32

kinesin = draws cargo to + side
dyne = draws cargo to - side

Question 33

Genome uncoating =

Answer 33

The complete or partial removal of the capsid to release the virus genome.

Question 34

Genome uncoating can take place: (4)

Answer 34

• at the cell surface, (the capsid remaining on the exterior surface of the cell)
• within the cytoplasm
• at a nuclear pore
• within the nucleus

Question 35

successful entry of a virion into a cell is not always

Answer 35

followed by virus replication

Question 36

The host’s intracellular defenses (such as lysosomal enzymes) may…
In some cases the virus genome may…

Answer 36

…inactivate infectivity before or after uncoating

…initiate a latent infection rather than a complete replication cycle

Question 37

Like animal viruses, phages bind specifically to

Answer 37

receptors and coreceptors

Question 38

On the cell side, some of the molecules are on…

On phage sides, many of the virus attachment sites are on…

Answer 38

…the surface of the host cell wall, some are on the surface of other structures (pili, flagella or capsules).
…particular virion structures, such as the tail fibres of phage T4.

Question 39

For most animal and plant viruses…

Answer 39

the entire virion (or at least the nucleocapsid) enters the host cell.