An introduction to retrovirus biology

Question 1

Describe the general structure of retroviruses.

Answer 1

1. Genome : ssRNA x2 (pseudo dsRNA)
2. Encapsulated
3. Lipid membrane (deriving from the previous cell before budding)
4. Can be transmitted vertically and horisontally, meaning that the virus can infect somatic cells (horisontal transfer) and germ cells (vertical transfer).

Question 2

What are the three core genes of retroviruses?

Answer 2

GAG: Capsid proteins
POL: RT, integrase, protease
ENV: envelope proteins (receptors etc)

Question 3

List the steps of the retroviral cell cycle (6 steps).

Answer 3

1. Infection / viral uncoating
2. Reverse transcription
3. Integration
4. Transcription / splicing / translation
5. Viral assembly / budding
6. Maturaiton

Question 4

What happens during the first step of the retroviral cell cycle: Infection / viral uncoating?

Answer 4

Fusion of viral lipid membrane and cell membrane via the viral receptor interacting with a cell surface receptor.

Question 5

What happens during the second step of the retroviral cell cycle: Reverse transcription?

Answer 5

ssRNA –> ssDNA –> dsDNA.

In order to synthesize dsDNA from ssDNA, RNAse H and host tRNAs are utilized.

Question 6

What happens during the third step of the retroviral cell cycle: Integration?

Answer 6

The genetic material is transferred into the nucleus. This is a hard step of the cell cycle as the nucleus is tightly regulated. Some viruses await nuclear membrane dissociation during the anaphase (in the metaphase), and transfer over the membrane is also available.

The integration is facilitated by the integrase enzyme, some viruses integrate randomly, and others integrate at specific locations.

After integrations, retroviruses can lay latent for a long time.

Question 7

What happens during the fourth step of the retroviral cell cycle: Transcription / splicing / translation?

Answer 7

The integrated genetic material often contains multiple splice sites, leading to lots of gene products present in a very dense code.

The ssRNA (2x) are produced alongside capsid proteins, RT, and integrase.

Question 8

What happens during the fifth step of the retroviral cell cycle: Viral assembly / budding?

Answer 8

The capsid self-assembles when the individual capsid protein concentrations are high enough. The ssRNA, integrase, and RT will host packaging tags and are thus mangled into the capsid.

Question 9

What happens during the sixth step of the retroviral cell cycle: Maturation?

Answer 9

The encapsulated virus needs to structure itself after leaving the extracellular matrix before being infectious again.

The gag is packaged as a polyprotein which is remodelled inside of the capsid by protease. This is the core of maturation.

Question 10

A: Name four different categories of retroviral drugs.
B: Explain HAART.

Answer 10

A: RT inhibitors, protease inhibitors, maturation inhibitors, cell surface receptor inhibitors (like CCR5 for HIV).

B: HAART (Highly active anti-retroviral therapy) is a good treatment as HIV doesn’t get the chance to develop resistance. It includes
- 2x NRTI (Nucleoside RT inhibitor)
- 1x PI (protease inhibitor)
- 1x NNRTI (non-nucleoside RT inhibitor)