Lecture 5: 7 Stages of Viral Attachment

Question 1

What are the basic 7 stages?

Answer 1

1. Attachment/Adsorption
2. Penetration & entry
3. Genome uncoating/early protein production
4. Viral genome replication
5. Production of virus late proteins
6. Assembly
7. Exit & release

Question 2

What is required of the cell for penetration & entry of the virus/viral genome?

Answer 2

A hydrated surface, not dry or waxy surfaces

Question 3

Why is the production of virus early proteins necessary?

Answer 3

Needed to produce proteins in preparation for genome replication afterwards. ie. Polymerase

Question 4

What are nascent virions?

Answer 4

Virions produced for immediate release, freshly produced to be immediately pathogenic

Question 5

What is the difference between early and late gene expression?

Answer 5

Early expression occurs before the original genome is replicated and is responsible for producing replication-related proteins. Late expression occurs after genome replication, is responsible for producing capsid or envelope proteins, and is produced from the REPLICATED GENOME.

Question 6

Why might scaffolding proteins be needed during the assembly of nascent virions (step 6)?

Answer 6

These are proteins that bring together two or more proteins into a favorable reactive position to assemble them. Scaffolding proteins assemble. They assist with capsid or envelope assembly.

Question 7

How do cytocidal viruses cause cytolysis?

Answer 7

Upon exit & release, these cells will burst with nascent virions, killing the host cell.

Question 8

What is the cytopathic effect?

Answer 8

The cytopathic effect are the changes on the host cell surface’s appearance that show up during viral infection.

Question 9

What are the necessary steps of attachment?

Answer 9

1. A COLLISION must occur btw the host cell and virion
2. Attachment protein on virion and glycoprotein on host cell BINDING

Question 10

Define avidity

Answer 10

The strength of binding between attachment protein and receptor molecules that is further increased by multiple points of interaction.

Question 11

Define co-opted

Answer 11

To be used for multiple functions. In our situation, a receptor can have its own function AND be used as a virus binding site. A virus can use various surface receptors, acid residues, and glycoproteins on the host cell to bind; especially a cell-cell adhesion molecule. (ie. A large variety of proteins are available to be potentially co-opted by a virus as its receptors).

Question 12

How are viral diseases and cell receptors related?

Answer 12

The cell-type on which receptor is found may be consistent with the type of viral disease.

Question 13

What type of molecule is common as a surface receptor?

Answer 13

Cell-cell adhesion molecules (ie. ICAM)

Question 14

How do cell-cell adhesion molecules relate to antibodies?

Answer 14

Cell-cell adhesion molecules tend to be members of the immunoglobulin-superfamily and contain multiple immunoglobulin-like domains.

Question 15

What is the Ig fold and why is it important to know?

Answer 15

Ig domain predates the evolution of vertebrates. It’s likely that Ig-like genes were present in invertebrates, diversifying and duplicating as we evolved.

Question 16

How did the neutralizing activity of antibodies evolve?

Answer 16

Ig domains likely started as surface receptors for blocking viral infection acting as rudimentary antibodies.This selectively advantageous neutralizing activity likely evolved further, possibly some protease cleaved it to allow it to float in solution.

Question 17

How are viruses important in the evolution of rudimentary antibodies to soluble antibodies?

Answer 17

The existence of viruses likely drove this evolution of the adaptive immune system.

Question 18

How does fusion-mediated entry work? What kind of virus (enveloped or non-enveloped) would use this method?

Answer 18

Enveloped. Viral lipid envelope binds to host plasma membrane via spike-receptor binding, in a process called adsorption. Then, their membranes would fuse and the viral capsid would enter the host cell, leaving the viral spikes and envelope on the host membrane.

Question 19

How does receptor-mediated endocytosis work? What kind of virus (enveloped or non-enveloped) would use this method?

Answer 19

Non-enveloped/naked capsid. Clathrin-coated pit forms, triggered by virion collision with cell-cell adhesion molecule (ie. ICAM). Endocytotic vesicle forms, and becomes acidified. The capsid is partially broken down, and the endosome is lysed, releasing the viral genome into the host cell cytoplasm.

Question 20

How are pre-lysosomal proteases activated?

Answer 20

Acidification of the endosome exposes parts of the virus capsid protein that makes capsid porous to virus genome (genome can exit the capsid).
Exposes portions of virus capsid proteins comprising endosomolytic peptides and may activate pre-lysosomal protease (Cathespin) cleaves the endosomolytic peptides from capsid.

Question 21

What are endosomolytic peptides? How do they lead to endosomal lysis? How do they help with viral genome extraction into the cytoplasm?

Answer 21

These proteins destabilize the endosome’s membrane so the endosome can break down. Virus’s genome can be released into the cytosol.

Question 22

How does exosome-mediated entry work? (Know generally)

Answer 22

Virus infects secreted exosomes from a donor cell. Exosomes are uptaken by recipient cell, thus transferring the virus to the recipient cell.

Question 23

How do enveloped viruses trigger membrane fusion?

Answer 23

Enveloped viruses can trigger lipid mixing by isomerizaton of the attachment protein/receptor complex. This occurs in step 1.

Question 24

What is isomerization? When does it occur?

Answer 24

When a compound is transformed into a different isomeric form, changing its configuration/structure. It is used by enveloped viruses to begin membrane fusion. Also used by enveloped virus to exit an endocytic vesicles, dumping its contents into the host cytoplasm.

Question 25

What are the four major steps of attachment protein isomerization?

Answer 25

1. CONFORMATION CHANGE in receptor upon virus binding
2. Virus binds CO-RECEPTOR
3. FUSION PEPTIDE projects into host cell membrane
4. Attachment protein PULLS 2 membranes together.

Question 26

How does HIV enter its host cell? [NOT SURE IF NEED TO KNOW]

Answer 26

Its attachment protein, consisting of gp41/gp120, binds to CD4 on host cell, inducing a conformational change in gp120. V3 loops are exposed, which binds the CCR5 co-receptor. Fusion peptide projects into host membrane, then attachment protein isomerizes to a trimer of hairpins to promote membrane fusion.

Question 27

What is a syncytium? How is this involved in viral attack?

Answer 27

Viruses can spread directly to neighboring cells, fusing the cells together and creating a deformed, dysfunctional clump of fused cells.

Question 28

How do viruses move toward the nucleus after it enters the cell?

Answer 28

Hijacking microtubule-based molecular motors like kinesins, and dyneins

Question 29

What viruses use penetration? Why is it necessary?

Answer 29

Bacteriophages. Bacteria can have thick cell walls, so the bacteriophage will inject only the genome.

Question 30

How can bacteriophages bind to bacterial cell walls?

Answer 30

Bacteriophages can bind glyco/lipoproteins, amino acids, sugar transport proteins, or F (sex) pili to attach to bacterial cell wall.

Question 31

Describe how a bacteriophage can inject its genome into a bacterial cell?

Answer 31

1. Receptor (feet) binds outer cell wall
2. Tail pins lower to strengthen viral attachment.
3. Pilot protein penetrates outer cell wall.
4. Lysozyme degrades inner cll wall. Pilot protein disassembles.
5. Genome is injected into both cell walls.