title: Pinpointing retrovirus entry sites in cells expressing alternatively spliced receptor isoforms by single virus imaging.
abstract: BACKGROUND
  The majority of viruses enter host cells via endocytosis. Current knowledge of viral entry pathways is largely based upon infectivity measurements following genetic and/or pharmacological interventions that disrupt vesicular trafficking and maturation. Imaging of single virus entry in living cells provides a powerful means to delineate viral trafficking pathways and entry sites under physiological conditions.
  
  
  RESULTS
  Here, we visualized single avian retrovirus co-trafficking with markers for early (Rab5) and late (Rab7) endosomes, acidification of endosomal lumen and the resulting viral fusion measured by the viral content release into the cytoplasm. Virus-carrying vesicles either merged with the existing Rab5-positive early endosomes or slowly accumulated Rab5. The Rab5 recruitment to virus-carrying endosomes correlated with acidification of their lumen. Viral fusion occurred either in early (Rab5-positive) or intermediate (Rab5- and Rab7-positive) compartments. Interestingly, different isoforms of the cognate receptor directed virus entry from distinct endosomes. In cells expressing the transmembrane receptor, viruses preferentially entered and fused with slowly maturing early endosomes prior to accumulation of Rab7. By comparison, in cells expressing the GPI-anchored receptor, viruses entered both slowly and quickly maturing endosomes and fused with early (Rab5-positive) and intermediate (Rab5- and Rab7-positive) compartments.
  
  
  CONCLUSIONS
  Since the rate of low pH-triggered fusion was independent of the receptor isoform, we concluded that the sites of virus entry are determined by the kinetic competition between endosome maturation and viral fusion. Our findings demonstrate the ability of this retrovirus to enter cells via alternative endocytic pathways and establish infection by releasing its content from distinct endosomal compartments.
authors.forename: Sergi
authors.initials: S
authors.lastname: Padilla-Parra
authors.forename: Mariana
authors.initials: M
authors.lastname: Marin
authors.forename: Naoyuki
authors.initials: N
authors.lastname: Kondo
authors.affiliation: Division of Pediatric Infectious Diseases, Emory University Children's Center, Atlanta, GA 30322, USA. gmeliki@emory.edu.
authors.forename: Gregory B
authors.initials: GB
authors.lastname: Melikyan
mesh_terms: D001692:Biological Transport; D002460:Cell Line; D003593:Cytoplasm / Q000378:metabolism / Q000502:physiology / Q000821:virology; D011992:Endosomes / Q000378:metabolism / Q000502:physiology / Q000821:virology; D058851:GPI-Linked Proteins / Q000378:metabolism; D057809:HEK293 Cells; D006801:Humans; D008565:Membrane Proteins / Q000378:metabolism; D020033:Protein Isoforms; D011956:Receptors, Cell Surface / Q000378:metabolism*; D012190:Retroviridae / Q000378:metabolism / Q000502:physiology*; D053586:Virus Internalization; D020691:rab GTP-Binding Proteins / Q000378:metabolism; D020696:rab5 GTP-Binding Proteins / Q000378:metabolism; D000090802:rab7 GTP-Binding Proteins
chemical_list: D058851:GPI-Linked Proteins; D008565:Membrane Proteins; D020033:Protein Isoforms; D011956:Receptors, Cell Surface; D000090802:rab7 GTP-Binding Proteins; C000716463:rab7 GTP-binding proteins, human; D020691:rab GTP-Binding Proteins; D020696:rab5 GTP-Binding Proteins