[2a] Penetration & Uncoating (Enveloped)

• Penetration is the entry of viruses into cells
• Uncoating is the removal of the envelope and capsid
• Upon initial entry of virus, infectious virions cannot be recovered from the cell

  • Due to recent initiation of replication phase
  • Also called the eclipse phase
• Phase where infectious virions cannot be recovered from the cell

  Eclipse phase
• For enveloped viruses, removal of envelope can occur either in:

  Plasma membrane or endosomes
• Enveloped Virus Uncoating:

  1. Viral membrane can fuse with the plasma membrane of host
  2. Virus can be taken up via endocytosis if it occurs in endosomes (viral membrane fuses with the endosomal membrane)
• The nucleocapsid is released into the cytoplasm
• Direct fusion with plasma membrane:

  1. Binding of receptor leads to fusion of viral envelope with plasma membrane of host
  2. Fusion is promoted by fusion protein
• Facilitates fusion of viral envelope with host membrane

  Fusion protein
• This protein is only present in enveloped viruses

  Fusion protein
• Fusion protein properties

  1. Only present in enveloped viruses
  2. Hydrophobic sequence (allows insertion in bilayer)
  3. Normally hidden in initial conformation of receptor
  4. Must become exposed for fusion to take place
• Endocytosis of virion by host cell:

  1. Fusion at endosomal membranes
  2. Host plasma cell surrounds virion and forms a vesicle
  3. Low pH causes virion envelope to fuse with endosome membrane
  4. Nucleocapsid released into cytoplasm
• Two types of endocytosis:

  1. Clathrin-mediated
  2. Caveolin-mediated
• Sometimes a protease is required to reveal the fusion protein
• Hemagglutinin spikes in Influenza cause fusion by hairpinning action
• Conserved family of fusion proteins

  Class 1 Fusion Proteins
• Ebola Penetration:

  1. Taken up into endosome, cleavage of mucin and glycan caps, bind to receptor
  2. Binding triggers fusion between viral and endosomal envelope
  3. Binding to npc1 protein
• Dengue Penetration:

  1. Makes use of Class 2 fusion proteins
  2. Fusion proteins are parallel to envelope
  3. Low pH causes conformational changes to produce fusion-active forms
• 2 Pathways for Fusion:

  1. Cathepsin-mediated activation (endosome)
  2. TMPRSS2-mediated activation (plasma membrane)
• Can prevent acidification of the endosome, preventing proteolysis of cathepsin

  Hydroxychloroquine