Rapid escape of the dot/icm mutants of Legionella pneumophila into the cytosol of mammalian and protozoan cells

Maëlle Molmeret, Marina Santic', Rexford Asare, Reynold A. Carabeo, Yousef Abu Kwaik

Research output: Contribution to journalArticlepeer-review

28 Citations (Scopus)


The Legionella pneumophila-containing phagosome evades endocytic fusion and intercepts endoplasmic reticulum (ER)-to-Golgi vesicle traffic, which is believed to be mediated by the Dot/Icm type IV secretion system. Although phagosomes harboring dot/icm mutants are thought to mature through the endosomal-lysosomal pathway, colocalization studies with lysosomal markers have reported contradictory results. In addition, phagosomes harboring the dot/icm mutants do not interact with endocytosed materials, which is inconsistent with maturation of the phagosomes in the endosomal-lysosomal pathway. Using multiple strategies, we show that the dot/icm mutants defective in the Dot/Icm structural apparatus are unable to maintain the integrity of their phagosomes and escape into the cytoplasm within minutes of entry into various mammalian and protozoan cells in a process independent of the type II secretion system. In contrast, mutants defective in cytoplasmic chaperones of Dot/Icm effectors and rpoS, letA/S, and letE regulatory mutants are all localized within intact phagosomes. Importantly, non-dot/icm L. pneumophila mutants whose phagosomes acquire late endosomal-lysosomal markers are all located within intact phagosomes. Using high-resolution electron microscopy, we show that phagosomes harboring the dot/icm transporter mutants do not fuse to lysosomes but are free in the cytoplasm. Inhibition of ER-to-Golgi vesicle traffic by brefeldin A does not affect the integrity of the phagosomes harboring the parental strain of L. pneumophila. We conclude that the Dot/Icm transporter is involved in maintaining the integrity of the L. pneumophila phagosome, independent of interception of ER-to-Golgi vesicle traffic, which is a novel function of type IV secretion systems.
Original languageEnglish
Pages (from-to)3290-3304
Number of pages15
JournalInfection and Immunity
Issue number7
Early online date16 Apr 2007
Publication statusPublished - Jul 2007


  • Macrophages
  • Microscopy, Confocal
  • Animals
  • Legionella pneumophila
  • Carrier Proteins
  • Humans
  • Phagosomes
  • Membrane Proteins
  • Acanthamoeba
  • Bacterial Proteins
  • Epithelial Cells
  • Cytosol
  • Time Factors
  • Mutation
  • U937 Cells


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