Virus Entry

Question 1

What is virus entry

Answer 1

Virus entry is an essential step in the viral life cycle that allows the virus to establish infection within a host cell

Question 2

What is the purpose of virus-cell attachment

Answer 2

Virus-cell attachment enriches viruses on the cell surface and stabilizes them through multiple receptor-virus interactions

Question 3

What happens after a virus binds to cell surface receptors

Answer 3

After receptor binding, viruses can:
1) Fuse directly at the plasma membrane and uncoat

2) Be internalised through endocytosis and macropinocytosis, remaining in vesicles inside the cell

Question 4

How do viruses escape from endosomes into they cytosol

Answer 4

Viruses escape their envelope with the vesicle membrane by either:

1) Fusing their envelop with the vesicle membrane

2) Disrupting or breaking the vesicle membrane

Question 5

Where are the internalised viruses trafficked inside the cell

Answer 5

Internalised viruses are transported to specific endosomal pathways such as:

• The late endosome/lysosome pathway
• The retrograde trafficking pathway to the trans Golgi network

Question 6

What are the steps of virus entry

Answer 6

1) Binding

2) Lateral diffusion

3) Signaling

4) Internalisation

5) Vesicular transport

6) Membrane Penetration

7) Intra-cytosolic transport - via microtubules

8) Nuclear import

9) Uncoating

Question 7

What kinds of molecules can serve as viral receptors or attachment factors

Answer 7

Proteins

Glycoproteins

Glycolipids

These molecules are found on the cell surface and may be abundant or cell-type specific, influencing which cells a virus can infect.

Question 8

Can viruses use more than one receptor or attachment factor

Answer 8

Yes

Viruses often use:
- Multiple receptors/attachment factors, sometimes depending on the cell type.

• A primary receptor for main binding and a co-receptor to facilitate entry.

Question 9

What mediates the attachment of viruses to host cells

Answer 9

Electrostatic interactions primarily mediate attachment. These are influenced by:

• Ionic strength
• pH

Presence of specific ions
This attachment is often reversible, allowing viruses to detach if conditions aren’t favorable.

Question 10

How do non-enveloped viruses attach to host cells

Answer 10

Non-enveloped viruses use surface structures (such as capsid spikes or knobs) to attach to cellular receptors.

Adenoviruses use fiber proteins projecting from their capsids to engage receptors like CAR

Question 11

How do enveloped viruses attach to host cells

Answer 11

Enveloped viruses use envelope glycoproteins to bind host receptors. These glycoproteins are embedded in the viral envelope and mediate both attachment and fusion with host membranes.

Question 12

What are the structures found on the surface of host cells

Answer 12

Attachment factors

Receptors

Question 13

How do viral receptors on the cell surface interact with a virus

Answer 13

Viral receptors interact directly with the virus

These interactions are often multi-valent, meaning multiple binding sites contribute to the overall attachment

This increases the binding stability, even if individual interactions are weak

Question 14

What is the nature of viral receptor binding

Answer 14

Viral binding to receptors is typically:

Highly specific (the virus recognizes particular receptor structures).

Low affinity at a single binding site, but strengthened by multivalent interactions.

Question 15

What happens once the virus is stably attached to the cell surface

Answer 15

The virus induces signals in the host cell.

These signals prepare the cell for viral entry and may involve:

• Cytoskeletal rearrangements
• Changes in receptor conformation
• Activation of endocytic pathways

Question 16

How does receptor binding promote viral entry

Answer 16

Once bound, the virus can enter the cell via:

• Endocytosis/macropinocytosis – the cell engulfs the virus into a vesicle.
• Membrane fusion – the viral envelope merges with the cell membrane, releasing the genome inside.

Question 17

What are the virus attachment factors

Answer 17

Attachment factors are non-essential host cell surface molecules that help viruses bind loosely to the cell surface.

They are not required for entry but play a supportive role in concentrating the virus near entry receptors.

Question 18

What are the binding characteristics of viral attachment factors

Answer 18

The interactions between viruses and attachment factors are:

Non-specific

Low-affinity

These weak, reversible interactions help retain the virus near the cell long enough to engage specific receptors.

Question 19

What are some examples of common virus attachment factors

Answer 19

Heparan sulfate proteoglycans (HSPGs)

Carbohydrates

Sialic acids

Question 20

What role does influenza virus hemagglutinin (HA) play in host cell entry

Answer 20

Influenza virus HA (hemagglutinin) is an envelope glycoprotein that binds to sialic acid-containing receptors on the host cell surface.

This multivalent binding increases attachment strength and stability.

Question 21

What is receptor clustering in influenza virus entry

Answer 21

HA binding to multiple sialic acid receptors causes receptor clustering on the cell surface.

This clustering can bring together receptors and co-receptors, forming a platform that facilitates signaling and virus uptake.

Question 22

How does receptor clustering by HA trigger host cell signaling

Answer 22

Clustered receptors can activate intracellular signaling pathways, such as:

• Receptor Tyrosine Kinases (RTKs) like EGFR (Epidermal Growth Factor Receptor)
• Voltage-gated calcium channels, like Cav1.2

These signals help remodel the cytoskeleton, open endocytic pathways, and prepare the cell for virus internalization.

Question 23

What does cellular uptake mean in the context of influenza virus entry

Answer 23

After HA binding and receptor signaling, the virus is taken into the cell by endocytosis.

Question 24

Why must the influenza virus traffic to specific intracellular compartments after uptake

Answer 24

Influenza must reach acidic endosomal compartments where the low pH triggers conformational change in HA.

This allows HA to mediate membrane fusion, releasing the viral genome into the cytosol for replication.

Question 25

What are the 2 virus uptake mechanisms

Answer 25

1) Receptor mediated endocytosis 2) Receptor mediated signalling - fusion

Question 26

What are the steps to the endocytosis pathway

Answer 26

1) Attachment via Receptor-mediated endocytosis 2) Internalization into vesicles 3) Intracellular trafficking 4) Fusion within Endosomes 5) Penetration and Uncoating

Question 27

What does attachment via receptor-mediated endocytosis include

Answer 27

The virus is now enclosed in an endosome or pinosome (intracellular vesicle). This allows the virus to avoid immune detection and prepare for fusion/penetration from within the host.

Question 28

What does the intracellular trafficking include

Answer 28

The endocytic vesicle carrying the virus travels along the microtubule network, using motor proteins like dynein and myosin. The vesicle matures (early endosome → late endosome).

Question 29

What does fusion within endosome entail

Answer 29

Inside the endosome, pH drops (acidification), triggering conformational changes in viral proteins. This allows fusion of the viral envelope with the endosomal membrane. - pH-dependent fusion: Requires low pH to trigger. - pH-independent fusion: Does not require acidification.

Question 30

What does the penetration and uncoating of the virus entail

Answer 30

Viral genome is released into the cytosol via: - Fusion (for enveloped viruses). - Endosome rupture or pore formation (for some non-enveloped viruses). Uncoating = disassembly of the viral capsid to release the genome for replication.

Question 31

What are the steps for direct fusion pathway

Answer 31

1) Attachment and signaling 2) Membrane fusions at the surface 3) Penetration and uncoating

Question 32

What does the attachment and signaling step in the direct fusion include

Answer 32

The virus binds directly to receptors on the plasma membrane. This triggers signaling cascades, such as: - Receptor Tyrosine Kinases (RTKs) like EGFR. - Changes in Ca²⁺ levels via voltage-gated calcium channels (e.g., Cav1.2).

Question 33

What does membrane fusion at the surface in direct fusion entail

Answer 33

The virus fuses directly with the plasma membrane without being internalized into a vesicle. This is often pH-independent (doesn’t require acidification). The cytoskeletal actin cortex may disassemble locally to permit fusion

Question 34

What does the penetration and uncoating step entail in direct fusion of the virus

Answer 34

After fusion, the viral core is released directly into the cytosol. Capsid disassembly (uncoating) follows to expose the viral genome.

Question 35

How does Ca2+ contribute to viral entry

Answer 35

Calcium ions regulate membrane fusion, cytoskeletal dynamics, and endocytic vesicle formation. Virus-induced signaling can activate voltage-gated calcium channels (like Cav1.2), facilitating entry.

Question 36

What roles do actin and microtubules play in virus uptake

Answer 36

Actin filaments support receptor clustering and membrane remodeling during endocytosis. Microtubules and motor proteins like dynein and myosin help traffic endocytic vesicles toward the cell interior and nucleus.

Question 37

Why are ions like K+ important in viral entry

Answer 37

Changes in intracellular ion concentrations (e.g., K⁺ efflux) can activate viral entry or uncoating steps and influence endosome maturation.

Question 38

What is the role of host enzymes and proteases in virla

Answer 38

Host proteases cleave viral envelope proteins, activating fusion peptides or preparing the virus for uncoating.

Question 39

What roles do chaperones and disaggregation machinery play in viral uncoating?

Answer 39

These host factors help disassemble the viral capsid, releasing the genome into the cytosol. They may also help resolve aggregated protein complexes formed during entry.

Question 40

What does “cross-talk with the cytosol” refer to in virus entry?

Answer 40

Once in the cytosol, viruses interact with host signaling pathways that regulate translation, immunity, and replication. This cross-talk helps them establish infection and avoid detection.

Question 41

What is the difference between the virus entry mechanisms of enveloped viruses and non-enveloped viruses?

Answer 41

Enveloped viruses fuse their lipid bilayer with the host cell membrane, releasing their genome into the cell. Non-enveloped viruses penetrate the host cell by disrupting the membrane, forming pores, or other non-fusion methods.

Question 42

How does the virus initially attach to the host cell in the case of enveloped viruses?

Answer 42

The virus has glycoproteins (GP) on its surface. These glycoproteins bind to specific receptors on the host cell surface, referred to as Cue. This binding is crucial for viral entry into the host cell.

Question 43

What happens during the hemifusion stage of viral entry for enveloped viruses?

Answer 43

After the virus binds to the host cell, the viral envelope fuses with the outer leaflet of the host cell membrane, forming hemifusion. At this stage, the outer membrane leaflets of both the virus and host cell merge, but the inner leaflets remain separate.

Question 44

What occurs after hemifusion during the viral entry of enveloped viruses?

Answer 44

The fusion process completes, and the full fusion of the viral envelope with the host cell membrane occurs. This results in the viral genome being released into the host cell’s cytoplasm for replication and viral assembly.

Question 45

How do non-enveloped viruses penetrate the host cell membrane?

Answer 45

Non-enveloped viruses create pores in the host cell membrane using structural proteins, allowing the viral genome to enter the host cell. These pores are formed by specific interactions between the viral capsid and the host membrane.