Lecture 18: viruses and their hosts

Question 1

what are the different types of viruses

Answer 1

non enveloped
enveloped

Question 2

what are the different types of host cells for viruses

Answer 2

eukaryotic
prokaryotic/archaeal

Question 3

how do the different host cells impact viruses ability to infect and multiply

Answer 3

animal cell: one barrier = plasma membrane
plant cell/bacterial cell: two barriers = plasma membrane and cell wall

–> for infection viruses ned to navigate the cell barriers

Question 4

the life cycle of most viruses

Answer 4

1. attachment - virion attaches to host cell surface
2. entry - virion breaches the host cell barrier (s)
3. synthesis - of viral components in host cell compartment (s)
4. assembly - of viral progeny in host cell compartment(s)
5. release - viral progeny breaches the host cell barrier(s)

Question 5

step 1: attachment

Answer 5

• this requires an interaction between specific molecules present on the surface of the virion and host cell
  receptor = the molecule on host cell surface
  receptor-binding protein = the molecule on virion surface is a protein called..
• present on the capsid of non-enveloped viruses
• present on the envelope of enveloped viruses

Question 6

what are examples of what a receptor could be

Answer 6

• protein molecule eg: CD4 for HIV, ACE2 for SARS-CoV-2
• carb molecule eg: sialic acid for influenza virus, lipopolysaccharide for bacteriophages
  –> can be unique to one virus or common to multiple viruses (eg: sialic acid for influenza virus and adenovirus)
• can have a co-receptor eg: CCR5 for HIV

Question 7

function of a receptor

Answer 7

• for a virus may not be present in all hosts OR on all tissues
• determines the host and tissue tropism of a virus
• for plant viruses has not been identified yet

Question 8

attachment of a non-enveloped virion

Answer 8

receptor binding protein on capsid interacts with the receptor

Question 9

attachment of an enveloped virion

Answer 9

receptor binding protein on envelope interacts with the receptor

Question 10

step 2: entry

Answer 10

the goal is to deliver the viral genome into host cell

Question 11

active mechanisms for entry

Answer 11

1. fusion - virion surface fuses with host cell surface
2. endocytosis - whole virion is internalised into host cell
3. direct injection = just viral genome enters host cell

Question 12

passive mechanisms for entry

Answer 12

whole virion enters damaged host cell

Question 13

entry by fusion with example

Answer 13

• mainly enveloped viruses use this mechanisms
  eg: HIV
• viral envelope spikes bind to receptors on surface of host cell
• lipid bilayer of viral envelope fuses with host cell membrane
• nucleocapsid is released into the cytoplasm

Question 14

entry by endocytosis

Answer 14

both enveloped and non-enveloped viruses use this mechanism

Question 15

entry by endocytosis of an enveloped virion example

Answer 15

Influenza virus
- viral envelope spikes bind to receptors on the cells surface
- binding to the receptor triggers receptor mediated endocytosis
- increased acidity allows nucleocapsid to escape from the endosome and enter the cytoplasm

Question 16

entry by endocytosis of non-enveloped virion example

Answer 16

adenovirus
- capsid proteins bind to receptors on cell surface and trigger receptor-mediated endocytosis
- nucleic acid extruded from the endosome into the cytoplasm

Question 17

entry by direct injection example

Answer 17

Bacteriophage T4
- tail attaches to the cell wall, tail has a small pore
- viral genome is injected into cell through tail

Question 18

example of entry by passive mechanism

Answer 18

Plant viruses
- enter passively via a wound that:
–> breaches the cell wall and plasma membrane of plant cell
–> allows virion to enter the cell
- cause of wound = insects, animals, and weather elements

Question 19

step 3. synthesis

Answer 19

once released into the host cell, viral genome must:
- replicate to make genome copies for progeny
- transcribe into mRNA
- translate into mRNA
- translate mRNA into viral proteins

Question 20

synthesis: replication of viral genome

Answer 20

• depends on the type (RNA or DNA) and configuration (single-stranded or double stranded) of the viral genome
• occurs in the nucleus or cytoplasm of eukaryotic host cells, and cytoplasm of prokaryotic host cells

Question 21

synthesis: transcription of viral genome

Answer 21

–> also depends on the type and configuration of the viral genome:
- ds/ssDNA and dsRNA - must make mRNA
- ssRNA - some must make mRNA, some directly act as mRNA
- occurs at the site of the genome replication

Question 22

synthesis: translation of viral genome

Answer 22

• for this, viruses hijack the host cell translation machinery
• occurs like the translation of host mRNA
• occurs universally in the cytoplasm of all host cell types
• viruses make two types of proteins
  1. structural proteins
  2. structural proteins

Question 23

step 4: assembly of progeny virions

Answer 23

• a complex and tightly regulated process
• occurs in the cytoplasm, nucleus and/or plasma membrane of host cells
• structural viral proteins sequentially interact with each other, then with viral genome to assemble a progeny virion
• viruses exploit some host proteins as chaperones to assemble the virion

Question 24

what happens to non-structural proteins during assembly of progeny virions

Answer 24

• non structural proteins are not assembled into a virion
• they remain in host cell and:
• promote the translation of viral proteins over host proteins
• help virus to evade the host innate immune defences

Question 25

step 5: release of progeny virions from host cell (main mechanisms)

Answer 25

1. lysis, common in non-enveloped viruses 2. budding, common in enveloped viruses (keep you alive as long as you need) 3. cell-to-cell release, common in plant viruses (communication between cells)

Question 26

release of progeny virions from host cell: release by lysis

Answer 26

- viral proteins are required for this event eg: Bacteriophage T4, requires two viral proteins 1. Holin - creates holes in bacterial cell plasma membrane 2. T4 lysozyme - dissolves the bacterial cell wall

Question 27

release of progeny virions from host cell: release by budding

Answer 27

eg: influenza virus - hemagglutinin - neuraminidase = enzyme to release neuron particle - envelope is derived from host cell membrane - produces budding virion - releases free infectious virion with envelope

Question 28

release of progeny virions from host cell: cell-to-cell release

Answer 28

plant viruses- through plasmodesmata, a specialized channel connecting plant cells