Viruses I

Question 1

describe the structure of a virus

Answer 1

• components:
  
  • genome: either DNA or RNA
  • capsomere: viral (protein) subunits that assemble into capsid
  • nucleocapsid: genome assembled into the capsid
  • virus specific glycoproteins
  • envelope: outer layer that originates from the host membranes and covers the capsid to maintain aqueous solution
    
    • tegument - a cluster of proteins that line the space between the envelope and nucleocapsid

Question 2

name the functions of the viral capsid

Answer 2

• capsid proteins have various functions:
  
  • define tissue or species specific transmission by interaction with host receptors to facilitate the host cell entry
  • interact with the viral nuclei acid for packaging/assembling the virus
  • enzymatic functions
  • assist in viral and/or host gene regulation
  • evade/block host immune system

Question 3

name the 3 structures of the viral capsid

Answer 3

capsid is helth together by non-covalent, reversible hydrophobic or hydrogen bonds

capsomeres are compactly folded proteins which fold only one way

• icosahedral symmetry: the amount of nucleic acid that can be packaged is limited by the size of the particle; icosahedral capsids form independently of the genome
• helical symmetry: there is no theoretical limit to the amount of nucleic acid that can be packaged; helical capsids form around the genome
• complex symmetry: some viruses do not exhibit cubic or helical symmetry

Question 4

describe an icosahedral capsid

Answer 4

• few proteins assemble in a basic promoters
  
  • the protomers assemble into pentamers
• the icosahedral capsid has a complex 5-3-2 axes of symmetry, e.g. poliovirus

Question 5

describe the helical capsid

Answer 5

• helical capsids appear as rod-like, filamentous structures (may be rigid or flexible depending on the specific virus)
• capsomers bind to the viral genome in a regular fashion; no empty helical capsids can form

Question 6

describe a retrovirus capsid

Answer 6

• retroviruses like HIV possess a distinctive cone-shaped capsid structure

Question 7

describe a triple shell capsid

Answer 7

• some viruses have more than one layer in their capsid
  
  • e.g. Rotaviruses are non-enveloped (naked) viruses with a triple shell virion: outer capsid, intermediate capsid and inner capsid
  • the genome has 11 different dsRNA segments wrapped inside the capsid
  • the capsid is composed of structural and non-structural proteins

Question 8

describe some viral proteins

Answer 8

• peplomers (spike-like viral glycoproteins) attach to receptors on host cells are on the outermost surface area: capsid or envelope
  
  • typical for some (e.g. rabies virus)
• viral attachment proteins (VAP): facilitate host cell entry
  
  • hemaglutinin (HA) on influenza virus which binds erythrocytes
  • VAP on Epstein-Barr virus (EBC) binds C3d receptor (CR2) on B-cells
• virus specific genome polymerases or transcription factors
• matrix proteins line inside the envelope to facilitate assembly with the nucleocapsid, also known as tegument

Question 9

describe the viral envelope

Answer 9

• viral envelope is a lipoprotein membrane derived from host membrane and can be:
  
  • cell plasma membrane (HIV; most of the enveloped viruses)
  • nuclear or other internal (endoplasmic reticulum) membranes (herpes virus)
• the viral envelope is poor in host cell proteins, but rich in virus specific glycoproteins
• spike-like formations with viral origin (peplomers)

Question 10

describe enveloped viruses

Answer 10

• viral nucleocapsid is surrounded by a lipoprotein membrane derived from host cell membrane (e.g. measles virus) or nuclear (herpes virus) or other internal membrane and contains virus specific glycoproteins
• they are sensitive to inactivation by detergents, drying, acid, heat
• they are usually transmitted by secretions, large droplets, blood or sexual contact e.g. measles virus
• enveloped viruses are usually spherical or pleomorphic in shape except rhabdovirus (rabies, bullet shaped) and poxvirus (smallpox, complex)

Question 11

describe non-enveloped (naked) viruses

Answer 11

• viral nucleocapsid is naked
• non-enveloped (naked) viruses are tough and relatively resistant to inactivation by detergents, drying, acid and heat
• they are usually transmitted by fomites, small droplets or fecal/oral route
• they are released from infected cells by lysis e.g. poliovirus, adenovirus

Question 12

describe polymerase enzymes

Answer 12

present within the genome in some viruses and involved in transcription and replication of viral genome

Question 13

describe matrix proteins

Answer 13

• present between the nucleocapsid and envelope
• stabilize the organization of viral glycoprotein and the lipid envelope
• directs the viral genome to intracellular sites of viral assembly
• facilitate viral budding

Question 14

describe defective (satellite) virus

Answer 14

• defective (satellite) viruses cannot replicate without a helper virus
  
  • up to 100 times more defective viruses are produced than normal viruses (100:1) due to genome mutations
• example is hepatitis D virus (HDV)
  
  • a very small ssRNA virus that can only infect in the presence of hepatitis B virus (HBV)
  • HDV shows replication similarities with viroids in plants
  • HDV produces an antigen termed “delta antigen” (HDsAg)
  • HDV utilizes HBV surface antigen (HBsAg) for entry into host cell

Question 15

describe prions

Answer 15

• prions are infectious proteins:
  
  • that have no genome (RNA or DNA)
  • that adopt a modified structure/folding compared to the non-infectious folding of the same protein
• first identified as “transmissible spongiform encephalopathies” causing:
  
  • loss of motor control
  • dementia
  • paralysis
  • encephalitis; widespread neuronal loss