Identification of interactions among host and bacterial proteins and evaluation of their role early during Shigella flexneri infection

Microbiology (Reading). 2018 Apr;164(4):540-550. doi: 10.1099/mic.0.000637. Epub 2018 Feb 28.


Shigella species cause diarrhoea by invading and spreading through the epithelial layer of the human colon. The infection triggers innate immune responses in the host that the bacterium combats by translocating into the host cell cytosol via a type 3 secretion system bacterial effector proteins that interfere with host processes. We previously demonstrated that interaction of the Shigella type 3 secreted effector protein IcsB with the host protein Toca-1 inhibits the innate immune response microtubule-associated protein light-chain 3 (LC3)-associated phagocytosis, and that IcsB interaction with Toca-1 is required for inhibition of this host response. Here, we show that Toca-1 in vitro precipitated not only IcsB, but also the type 3 secreted proteins OspC3, IpgD and IpaB. OspC3 and IpgD precipitation with Toca-1 was dependent on IcsB. Early during infection, most of these proteins localized near intracellular Shigella. We examined whether interactions among these proteins restrict innate host cell responses other than LC3-associated phagocytosis. In infected cells, OspC3 blocks production and secretion of the mature pro-inflammatory cytokine IL-18; however, we found that interaction of OspC3 with IcsB, either directly or indirectly via Toca-1, was not required for OspC3-mediated restriction of IL-18 production. These results indicate that interactions of the host protein Toca-1 with a subset of type 3 effector proteins contribute to the established function of some, but not all involved, effector proteins.

Keywords: IcsB; IpaB; OspC3; Shigella flexneri; Toca-1; pathogenesis.

Publication types

  • Research Support, N.I.H., Extramural

MeSH terms

  • Bacterial Proteins / genetics
  • Bacterial Proteins / metabolism*
  • Carrier Proteins / genetics
  • Carrier Proteins / metabolism*
  • Cell Line
  • Cytoplasm / metabolism
  • Dysentery, Bacillary / metabolism
  • Dysentery, Bacillary / microbiology*
  • Epithelial Cells / metabolism
  • Epithelial Cells / microbiology
  • Gene Deletion
  • Gene Knockdown Techniques
  • Host-Pathogen Interactions
  • Humans
  • Interleukin-18 / analysis
  • Interleukin-18 / metabolism
  • Macrophages / metabolism
  • Macrophages / microbiology
  • Protein Binding
  • Shigella flexneri / genetics
  • Shigella flexneri / metabolism
  • Shigella flexneri / physiology*
  • Type III Secretion Systems / genetics
  • Type III Secretion Systems / metabolism*


  • Bacterial Proteins
  • Carrier Proteins
  • FNBP1L protein, human
  • IL18 protein, human
  • Interleukin-18
  • Type III Secretion Systems