(V) 24: Viral Entry

Question 1

Concept of entry

Answer 1

1. ATTACHMENT: recognition of host receptor
2. mechanisms to RELEASE genome (ex. in cytoplasm)

Question 2

Viruses and passive diffusion

Answer 2

Viruses are too LARGE to be passively transported through the host cell membrane

Question 3

Random electrostatic interactions

Answer 3

They occur but will NOT initiate infectious cycle

• non-specific

Question 4

How do viruses find the “right” receptor?

Answer 4

• specific interactions btwn a virus and its receptor will initiate the infectious cycle
• opened the door to the OBLIGATE INTRACELLULAR parasite
• RELEASE of genome “inside” the host

Question 5

Susceptible cell

Answer 5

functional receptor for virus

Question 6

Permissive cell

Answer 6

allows the virus to REPLICATE

Question 7

resistant cell

Answer 7

no receptor

Question 8

Co-receptor

Answer 8

second receptor (sometimes needed)
- specific to humans and animals

Very, very essential for many viruses

NOT in yeast and plants

Question 9

Host cell receptors

Answer 9

they are functional receptors in humans (NOT just there for the virus)

they are all SURFACE MEMBRANE PROTEINS

in some cases, the same receptor can recognize more than a single virus and vice versa

Question 10

Host cell receptors and SARS-CoV2

Answer 10

Angiotensin-converting enzyme 2 receptor (ACE2) is used by SARS-CoV2
- has a role in controlling b.p. and CV system

Question 11

Modes of attachment depends on…

Answer 11

outer shell of the virus

Non-enveloped viruses: outside of shell is hard protein
Enveloped viruses: outer surface is a membrane

Question 12

Picornaviridae general structure

Answer 12

non-enveloped virus w/ icosahedral symmetry

ss (+) RNA = “ready” to go

Viral proteins: 60 VP1 (membrane insertion), 60 VP2, 60 VP3
- 60 faces of 3 proteins each

Ex. Poliovirus
- naked virion: release their genomes at plasma memb
- injects RNA directly across plasma memb. after conformational change in capsid proteins

Question 13

Host cell entry for non-enveloped virus

Answer 13

Naked virion: viruses can RELEASE their genomes at the plasma memb

Ex.
Poliovirus INJECT RNA DIRECTLY across plasma membrane after conformational change in capsid proteins

• hydrophobic N-termini of VP1 insert into membrane
• creation of a channel

Poliovirus Receptor (PVR): can interact w/ VP1 (conformational change to create PORE on susceptible cell) = (+) ssRNA is injected in plasma memb.

VP4 is buried

Question 14

Host cell entry for enveloped viruses

Question 15

Influenza virus

Answer 15

(-) RNA and HAS an ENVELOPE

Hemagglutinin = viral envelope glycoprotein
- binds cell host receptors (sialic acid)

Sialic acid (sugar) = host cell receptor

Question 16

How is specificity of viruses determined?

Answer 16

Sugar linkages determine specificity
- orientations determine stereochemistry + specificity of sialic acid’s interactions w/ hemagglutinin

2-3 and 2-6 linkages are possible
- human influenza prefers 2-6 linkages
- avian influenza prefers 2-3 linkages

Question 17

Human influenza viruses and linkages

Answer 17

Human influenza viruses preferentially prefer 2-6 linkages

Question 18

Avian influenza viruses and linkages

Answer 18

Avian influenza viruses prefer 2-3 linkages

Question 19

Avian influenza viruses and humans

Answer 19

• sialic acid receptors w/ 2-6 linkages in upper AND lower tract = bind human influenza virus
• sialic acid receptors w/ 2-3 linkages in LOWER tract = bind avian influenza virus

Very deep breath of many virions is needed to travel all the way to lungs to find susceptible cells for avian flu

Question 20

SARS-Cov2 and receptors

Answer 20

SARS-Cov2 has evolved (ex. Omicrom) to interact w/ upper respiratory tract receptors
- opposed to original virus that interacted w/ lower tract

Question 21

How do viruses enter the cell?

Answer 21

Via common cellular mechanisms - endocytosis
- phagocytosis
- pinocytosis
- receptor-mediated endocytosis (MOST viruses)

• transport vesicles “move” along tracks that are powered by molecular motors
• ACTIVE TRANSPORT applies to virus trafficking inside the cell

Question 22

Release of virus

Answer 22

a TRIGGER releases virus or genomic contents in the cytoplasm

Question 23

Locations of genomic “release”

Answer 23

• release can occur at the CELL SURFACE
• release can occur at an EARLY ENDOSOME
• release can occur at a LATE ENDOSOME

pH is different btwn locations = becomes more acidic as you go further way from plasma membrane

Question 24

Reoviruses

Answer 24

• NON-enveloped example for entry
• onion virus = many layers

1. attachment
2. receptor-mediated endocytosis
3. disassembly (cysteine proteases)
   - cathespin = molecular scissors (HUMAN = host enzyme)
4. membrane penetration

Early endosome = pH causes loss of capsid
Late endosome = pH causes holes in late endosome

Question 25

Cathepsin

Answer 25

MOLECULAR SCISSORS

HUMAN host enzyme/protein

• more active in acidic environment

Question 26

Paramyxoviridae

Answer 26

Measles = HAS an envelope (lipid)

• fusion events are regulated by viral proteins
• viral fusion proteins will “behave” differently depending on specific pH
• different viral fusion proteins respond differently to different pH
• CO-RECEPTOR is needed (often interacts w/ fusion receptor)

Question 27

Influenza virus membrane fusion

Answer 27

When acidity increases, viral protein stretches (conformation changes) to mediate fusion event
- brings host membrane closer to viral envelope
- co-receptor like interaction

Question 28

Requirement for TWO host receptors

Answer 28

Ex. HIV (ENVELOPE virus)

• Host receptor: CD4/CCR5
• viral protein: gp120
• gp41: viral fusion protein

PROXIMITY of viral and cellular membranes = fusion

Question 29

Ebola Uptake

Answer 29

Niemann-Pick C1 (NPC1) cholesterol transporter: essential for Ebola virus infection in LATE ENDOSOMES
- already working receptor in humans