Virus cell biology

• what is a virus

  Infectious agent, consisting of a segment of nucleic acid (DNA or RNA), surrounded by a protein coat
• features of viruses

  • 20-300 nm in size (~1000x smaller than a human cell)
  • They carry RNA or DNA (latter double-or single-stranded)
  • They encode 2-200 proteins
  • Viruses infect animals, plants, fungi and bacteria (the latter two are called mycophages and bacteriophages)
  • Viruses are NOT alive (they do not have metabolic activity) and cannot self-replicate.
  • A virus infects a host cells and use them to make copies of itself
  • Outside the host cell, the virus particle is called virion
• what does the capsid do

  • The protein coat (capsid) contains of few proteins (capsomers) that self assemble
  • The capsid protects the RNA/DNA of the virus
  • The capsid determines the shape of the virus particle (e.g. helical or icosahedral)
• what is the smallest virus known to infect humans

  hepatitis delta virus
• what does hepatitis delta virus need to cause infection

  hepatitis B
• what hepatitis delta antigens form the capsid

  L-DAg, S-DAg
• which proteins does hep delta borrow from hep B

  L-HBsAg, M-HBsAg, S-HBsAG
• naked virus features

  • Protein coat (Capsid)
  • Nucleic acid (RNA or DNA)
• example of naked virus

  Adenovirus - causes cold, pneumonia, bronchitis
• enveloped virus features

  • Protein coat (Capsid)
  • Nucleic acid (RNA or DNA)
  • Biomembrane (lipids from the host cell)
  • Envelope proteins (virus encoded)
• example of enveloped virus

  Human Immunodeficiency Virus (HIV) - causes acquired immunodeficiency syndrome (AIDS)

  Hepatitis Delta Virus - causes severe hepatitis
• complex virus features

  • Protein coat (Capsid)
  • Nucleic acid (RNA or DNA)
  • Biomembrane (lipids from the host cell)
  • Injection tail (virus encoded)
• examples of complex viruses

  T4 Phage (infects bacteria)
• how do viruses enter host cells

  • endocytosis
  • membrane fusion
  • injection
• endocytosis of viruses

  The virus triggers clathrin-mediate endocytosis and enters the cell as a "Trojan Horse"
• membrane fusion of viruses

  Enveloped virus particles bind to receptors in the plasma membrane of host and forces membrane fusion
• injection of viruses

  Bacteriophages inject their genetic information and leave the rest of the virus particle behind
• how are viruses released from host cell

  • viral budding
  • cell lysis
  • lysosome/secretory vesicle trafficking
• viral budding

  The virus particle assembles at the plasma membrane and "buds off" into the extracellular space
• cell lysis of virus

  The virus disintegrates the cell, thereby releasing virus particles from the cytoplasm
• Lysosome/Secretory vesicle trafficking

  The virus assembles inside the organelles and is released by secretory vesicles at the plasma membrane
• examples of enveloped viruses

  • influenza
  • herpes simplex 1
  • ebola
  • HIV
  • vaccinia
  • SARs
• life cycle of bacteriophage

  • The virus attaches to surface receptors
  • The tail contracts and the core needle pinches the cell
  • The content of the head (DNA/RNA) is released into the cell
  • Viral proteins are transcribed that disrupt the bacterial metabolism and degrade the genomic DNA
  • Viral DNA is transcribed into mRNA
  • Viral DNA is replicated
  • Viral proteins are produced (incl. the capsid proteins, lytic enzymes etc.)
  • (in case of T4: >134 proteins)
  • The complex virus particle self-assembles
  • (proteins “know” how to assemble)
  • 50-100 phages assemble in the infected cell
  • Lytic viral proteins (lysozyme) disintegrate the cell and release the phages
• what is a viroid

  An infectious small single-stranded circular RNA molecule, not surrounded by a protein coat, that causes plant disease
• features of viroids

  • Viroids are ~8-times smaller than viruses
  • Viroids enter via wounds (e.g. infection by aphids)
  • They use host RNA-polymerase II to amplify
  • They can produce small RNA fragments that deregulate the defence of the plant cell
  • Viroids come in 2 families and 32 different "species"
  • Some viroids (not all!) are plant pathogenic
• who discovered viroids

  Identified in 1971 by Theodor Otto Diener when he investigated potato spindle tuber disease (PSTV)
• viroid RNA secondary structure

  • Part of the viroid RNA can fold into an enzyme-like structure (= ribozyme)
  • Inhibition of plant defence
  • Replication
  • Disease