endosomes

Examples of endosomes in the following topics:

• Attachment and Entry to the Host Cell

  • Second, once bound, it facilitates the entry of the viral genome into the target cells by causing the fusion of the host endosomal membrane with the viral membrane.
  • The cell membrane then engulfs the virus and the portion of the membrane that encloses it pinches off to form a new membrane-bound compartment within the cell called an endosome, which contains the engulfed virus.
  • The cell then attempts to begin digesting the contents of the endosome by acidifying its interior and transforming it into a lysosome.
  • This so-called "fusion peptide" acts like a molecular grappling hook by inserting itself into the endosomal membrane and locking on.
  • Then, when the rest of the HA molecule refolds into a new structure (which is more stable at the lower pH), it "retracts the grappling hook" and pulls the endosomal membrane right up next to the virus particle's own membrane, causing the two to fuse together.

• Diphtheria

  • This signals the cell to internalize the toxin within an endosome via receptor-mediated endocytosis.
  • Inside the endosome, the toxin is split by a trypsin-like protease into its individual A and B fragments.
  • The acidity of the endosome causes fragment B to create pores in the endosome membrane, thereby catalyzing the release of fragment A into the cell's cytoplasm.

• Replicative Cycle of Influenza A

  • It is known that virions converge to the microtubule organizing center, interact with acidic endosomes, and finally enter the target endosomes for genome release.
  • Once inside the cell, the acidic conditions in the endosome cause two events to happen:

• Endocytosis

  • Once the vesicle containing the particle is enclosed within the cell, the clathrin disengages from the membrane and the vesicle merges with a lysosome for the breakdown of the material in the newly-formed compartment (endosome).
  • When accessible nutrients from the degradation of the vesicular contents have been extracted, the newly-formed endosome merges with the plasma membrane and releases its contents into the extracellular fluid.
  • The endosomal membrane again becomes part of the plasma membrane.

• Replication of Double-Stranded DNA Viruses of Animals

  • They are able to be transported through the endosome (i.e., envelope fusion is not necessary).
  • Once the virus has successfully gained entry into the host cell, the endosome acidifies, which alters virus topology by causing capsid components to disband, which in turn destroys the endosome and allows the virion entry into the cytoplasm.

• Neutralization Reaction

  • It may interfere with virion binding to receptors, block uptake into cells, prevent uncoating of the genomes in endosomes, or cause aggregation of virus particles.

• Double-Stranded DNA Viruses: Adenoviruses

  • Next, a specialized motif in the penton base protein interacts with αv integrin, stimulating internalization of the adenovirus via clathrin-coated pits, resulting in entry of the virion into the host cell within an endosome.
  • Following internalization, the endosome acidifies, which alters virus topology, causing capsid components to disassociate.

• Antigenic Determinants and Processing Pathways

  • First, pathogens are phagocytized, and then endosomes within the cell break down antigens with proteases, which then combine with MHC II.

• Retroviral RNA Genome

  • The env gene serves three distinct functions: enabling the retrovirus to enter/exit host cells through endosomal membrane trafficking, protection from the extracellular environment via the lipid bilayer, and the ability to enter cells.

• Double-Stranded DNA Viruses: Pox Viruses

  • Following attachment to cell surfaces and fusion with the plasma or endosomal membrane, poxvirus replication is initiated by entry of the viral core into the cytoplasm, where all subsequent steps of the life cycle take place.