Class 18: Assembly and Release

Question 1

What is a chaperone?

Answer 1

A protein that facilitates the folding of other polypeptide chains
OR
The assembly of multimeric proteins
OR
The formation of macromolecular assemblies (such as chromatin)

Question 2

What is a scaffolding protein?

Answer 2

A viral protein that is required for assembly of an icosahedral protein shell BUT is absent from mature virions

Question 3

What is a procapsid?

Answer 3

A closed protein-only structure into which viral genomes are inserted

Precursor to a capsid or nucleocapsid

Question 4

What is packaging?

Answer 4

Incorporation of the viral genome during assembly of virus particles

Question 5

What are packaging signals?

Answer 5

Nucleic acid sequence or structural feature directing incorporation of a viral genome into a virus particle

Question 6

How are viral genomes distinguished from cellular RNA and DNA during encapsidation?

Answer 6

Packaging signals

Question 7

Viral genomes are packaged by one of two mechanisms, what are they?

Answer 7

Conjunction with concerted encapsidation
OR
Following assembly sequential assembly of a protein shell

Question 8

How are viruses assembled/formed?

Answer 8

1. Individual protein subunits
2. Single protein - covalently linked in a polyprotein precursor
3. Some are assisted by chaperone or scaffolding proteins

Question 9

T/F Viruses may be released as free particles or spread from cell to cell without exposure to the extracellular milieu.

Answer 9

TRUE

Question 10

Sort the hypothetical pathway of virus particle assembly.

Answer 10

1. Formation of individual structural units
2. Assembly of the protein shell
3. Selective packaging of the nucleic acid genome and other essential virion components
4. Acquisition of an envelope
5. Release from host cell
6. Maturation of virus particles

Question 11

Which technique would be best utilized to study viral assembly and exit within an infected cell?

Answer 11

Electron Microscopy

Question 12

What are the smallest wavelengths of light that humans can see?

Answer 12

400 nm

Question 13

How big is coronavirus (SARS-CoV-2)?

Answer 13

100 nm

Question 14

What is a scanning electron microscope (SEM)?

Answer 14

Electrons scan the surface of a sample and records what bounces back

Question 15

What is a transmission electron microscope (TEM)?

Answer 15

Electrons pass through a sample and a series of electromagnetic lenses providing a cross section of the sample

Question 16

What are three ways to make viral sub-assemblies?

Answer 16

1. Individual protein molecules
   - Multiple proteins coming together
2. Polyprotein precursor
   - Different regions of the proteins; 1 protein
   - RNA translated to big protein, then chopped up to make particular regions
3. Assisted Assembly (Viral/Cellular Chaperones)
   - help with proper protein folding

Question 17

What is a chaperone?

Answer 17

Proteins that assist with proper protein folding

Question 18

What is sequential encapsidation?

Answer 18

Genome inserted into preformed protein shell

Poliovirus, Herpesvirus assembly

Question 19

What is concerted encapsidation?

Answer 19

Assembly of viral particles in association with viral genome (so same time)

Influenza virus assembly

Question 20

What are the irreversible reactions of sequential encapsidation?

Answer 20

1. Proteolytic cleavage
2. Formation of 14S pentamers

Happens in Poliovirus assembly

Question 21

What are the twists of sequential encapsidation?

Answer 21

1. Addition of viral scaffolding proteins
   - transient intermediate structure
2. Bigger, complex virus = more stabilization needed

Happens in Herpesvirus assembly