Lec 12-Virus Entry Flashcards
Entry mechanisms of virus
All viruses have diff ones, they are selective
Do all viruses follow common steps in their replication cycle?
Yes
Steps of virus replication cycle
- Attachment—entry
- Penetration—entry
- Uncoating—entry
- Transcription
- Translation
- Genome replication—maturation
- Assembly—maturation
- Release—maturation
Can viruses diffuse across PM?
No, they need mechanisms to bypass it
How do we know where viruses can enter body?
Dictated by location in body of virus receptor. Where the virus is infecting determines the disease—there is a connection between virus tropism and the disease it causes
4 examples of variation in viral tropism
- Influenza—respiratory
- Zika—placenta and neurons
- Rabies—muscle and nervous sys
- HIV—blood cells
Where do attachment, penetration, and uncoating occur for bacteriophages?
Bacterial cell surface—virus touches down on surface, genome is injected into cell through helical tail
2 steps of bacteriophage entry
- Initial, reversible (skimming cell surface until it finds correct position) binding to bacterial cell surface via 6 long tail fibres
- Irreversible binding via short tail fibres which interact with receptors on bacterial cell surface
Steps of T4 bacteriophage binding to E. coli receptors
- Long tail fibres recognize outer membrane protein C or LPS of E. coli (reversible)
- After at least 3 long tail fibres have bound, conform change in baseplate occurs, and short tail fibres extend and bind to core region of host cell LPS (irrev)
- Contraction of tail sheath and penetration of outer memb
- T4 lysozyme degrades peptidoglycan layer
- Inner memb is degraded
- Phage DNA is delivered into cytoplasm
Locations of steps of viral infection of eukaryotic cells
- Attachment @ cell surface
- Penetration @ cell surface or internal memb
- Uncoating @ cytoplasm
Initial attachment of virus to eukaryotic cells
Reversible electrostatic interactions, stable attachment=irrev tight interactions w receptors
How do viruses penetrate eukaryotic hosts?
Membrane fusion at cell surface or receptor mediated endocytosis
What triggers virus release into cytoplasm
pH drop
Path for viral infection of eukaryotic cells: icosahedron
- Free virus
- Interacts with PM
- Endocytosis
- Endosome is formed
- pH drop of endosome as it matures
- pH drop triggers penetration thru endosome membrane
- Virus released into cytoplasm
Path for viral infection of eukaryotic cells: enveloped #1 (without endosome)
- Enveloped virus
- Attachment to cell surface
- Fusion of envelop with PM
- Release of envelop contents into cytoplasm
**called penetration
Path for viral infection of eukaryotic cells: enveloped #2 (with endosome)
- Enveloped virus
- Attaches to cell surface
- Endocytosis into endosome
- pH drop triggers release of virus by fusing with endosome membrane
- Virus released into cytoplasm
Are virus receptors diverse?
No
What are virus receptors made up out of?
Could be proteins, sugars, lipids
What receptor does influenza bind to?
Sialic acid (sugar)
What receptor does vesicular stomatitis bind to?
Negatively charged membrane lipids
What do viral receptors help determine?
Host range of virus. If cell doesn’t have a receptor for a virus, virus can’t penetrate, cell can’t get infected by it
What is a VAP?
Virus attachment protein
Do viruses have receptors? What do they use to interact with hosts?
They don’t have receptors, they use VAPs to interact with host receptors
Can different viruses bind to the same receptor? Give an example
Yes. Eg: Adenovirus and Caxsackievirus B3 both bind to the same receptor—these 2 viruses are completely unrelated other than their receptor, they have both evolved specificity for this kind of receptor and there is some overlap in the types of cells they infect
Can viruses bind to more than one receptor for entry? Give 2 examples
- Yes.
- Eg: HIV binds to CD4 (primary receptor) and CCR5 (co-receptors) receptors. Gp120 is its VAP. Viruses bind to receptors first, then co-receptors after.
- Eg: HSV has 4 diff VAPs one one virus cell that interact with 5 diff receptors on host—not always simple!
Can related viruses in the same family bind different receptors?
Yes
What is the receptor-coreceptor system?
Virus entry can require more than one receptor
Sialic acid is the generic term for…
N or O substituted derivatives of neuraminic acid, a monosaccharide with a 9 carbon backbone
Where are sialic acids found
Animal tissues, mostly in glycoproteins
Where in the cell do sialic acids occur
Ends of sugar chains connected to cell surfaces and soluble proteins. Proteins can have a lot or a little sialic acid, but it will always be there
Steps of influenza virus attachment
- HA envelope glycoprotein attaches to cell surface sialic acid.
- Receptor mediated endocytosis
- pH in endosomes decreases (7–>5)
- Low pH triggers conform change in HA, exposing hydrophobic fusion peptide
- Fusion of viral and cell membs
- Influenza virus genome delivered into host cell
(Photo annotated on GN for assistance)
Sialic acid is common on cell surfaces. Why is this relevant?
Virus will affect diff kinds of cells using sialic acid as its receptor
Do different virus strains bind more/less strongly to different types of sialic acid strains? Give an example
Yes. In chickens, GI tract is lined with sialic acid with an alpha 2,3 linkage. In humans, we have alpha 2,3 and alpha 2,6 receptors. 2,3 is deeper in lungs and 2,6 is closer to airway. If we were exposed to birds they could make us sick, but the virus would have to get deeper into lungs to access 2,3 receptors
What is essential for uncoating of influenza?
Acidification of core (caused by H+ protons from endosomes being pumped into viral core)
What does amantadine do?
Drug that plugs M2 channel that pumps protons into virus
What is the effect of amantadine?
Virus cant acidify core anymore
Why dont we give people amantadine?
It can evolve. We put selective pressure on influenza virus which can cause 1 AA change in channel which allows for amantadine resistance
What is uncoating?
Completion of virus entry
When is virus entry complete?
When viral genome is delivered to cellular compartment where replication takes place
Where does replication take place in influenza?
Nucleus
What happens after uncoating?
Each of 8 genome segments traverses nuclear pore. Each segment is attached to polymerase enzyme. Polymerase starts copying RNA in nucleus
What mediates attachment and fusion of human coronavirus?
Spike glycoprotein
How many diff human coronaviruses so far?
7, each with their own cellular receptors
Coronavirus entry pathways—similar or diff?
Despite having diff receptors, entry pathways are similar
What are the spike protein domains?
SARS-CoV-2 spike proteins have S1 domain and S2 domain
1. Internal RBD (receptor binding domain) that binds sugars
2. RBD that binds ACE2
3. Hydrophobic fusion peptide—allows for fusion of COVID virus in the same way as influenza
What are the conformations of spike?
Closed: unable to interact w receptors
Open: able to interact w receptors
Job of proteases:
Determine fusion site and entry route
How are spikes activated?
Cut by host cell protease on cell surface
What does proteasing spike do?
Enables fusion/penetration
What do host protease inhibitor drugs do? Give 2 examples
Block fusion step of virus entry
Eg: Paxlovid inhibits viral protease
Eg: Remdesivir inhibits viral RNA-dependent RNA polymerase
3 opportunities for protease cleavage:
- Virus producing human cell: Inside cell that is producing SARS-CoV-2 , proteases in host can cleave
- Cell surface proteases: If spike didnt get cut by proteases in 1, proteases on target cell will cut spike
- Virus targeted human cell: If spike didnt get cut by 1 and 2, upon endocytosis, endosome matures and gets access to lysosomal proteases (eg. Cathepsin) which can process spike
Viruses have different dependencies on __
Different proteases
Pathway of viral entry for SARS-CoV-2
Can do either endosomal fusion or surface fusion—same as influenza ones
When is entry of SARS-CoV-2 complete?
When viral genome is delivered where replication takes place
Where does COVID replicate? Why there?
Cytoplasm, because its genome can be directly translated by host protein synthesis machinery (which is in the cytoplasm) to initiate infection
What is attachment?
Process by which VAPs bind to host cell surface receptors to initiate entry
