Present address: National Institutes of Health, Bethesda, MD 20892, USA.
Triggering Ras signalling by intracellular Francisella tularensis through recruitment of PKCα and βI to the SOS2/GrB2 complex is essential for bacterial proliferation in the cytosol
Article first published online: 5 JUL 2010
© 2010 Blackwell Publishing Ltd
Volume 12, Issue 11, pages 1604–1621, November 2010
How to Cite
Al-Khodor, S. and Abu Kwaik, Y. (2010), Triggering Ras signalling by intracellular Francisella tularensis through recruitment of PKCα and βI to the SOS2/GrB2 complex is essential for bacterial proliferation in the cytosol. Cellular Microbiology, 12: 1604–1621. doi: 10.1111/j.1462-5822.2010.01494.x
- Issue published online: 5 JUL 2010
- Article first published online: 5 JUL 2010
- Received 16 March, 2010; revised 25 May, 2010; accepted 28 May, 2010.
Intracellular proliferation of Francisella tularensis is essential for manifestation of the fatal disease tularaemia, and is classified as a category A bioterrorism agent. The F. tularensis-containing phagosome (FCP) matures into a late endosome-like phagosome with limited fusion to lysosomes, followed by rapid bacterial escape into the cytosol. The Francisella pathogenicity island (FPI) encodes a type VI-like secretion system, and the FPI-encoded IglC is essential for evasion of lysosomal fusion and phagosomal escape. Many host signalling events are likely to be modulated by F. tularensis to render the cell permissive for intracellular proliferation but they are not fully understood. Here we show that within 15 min of infection, intracellular F. tularensis ssp. novicida triggers IglC-dependent temporal activation of Ras, but attached extracellular bacteria fail to trigger Ras activation, which has never been shown for other intracellular pathogens. Intracellular F. tularensis ssp. novicida triggers activation of Ras through recruitment of PKCα and PKCβI to the SOS2/GrB2 complex. Silencing of SOS2, GrB2 and PKCα and PKCβI by RNAi has no effect on evasion of lysosomal fusion and bacterial escape into the cytosol but renders the cytosol non-permissive for replication of F. tularensis ssp. novicida. Since Ras activation promotes cell survival, we show that silencing of SOS2, GrB2 and PKCα and βI is associated with rapid early activation of caspase-3 within 8 h post infection. However, silencing of SOS2, GrB2 and PKCα and βI does not affect phosphorylation of Akt or Erk, indicating that activation of the PI3K/Akt and the Erk signalling cascade are independent of the F. tularensis-triggered Ras activation. We conclude that intracellular F. tularensis ssp. novicida triggers temporal and early activation of Ras through the SOS2/GrB2/PKCα/PKCβI quaternary complex. Temporal and rapid trigger of Ras signalling by intracellular F. tularensis is essential for intracellular bacterial proliferation within the cytosol, and this is associated with downregulation of early caspase-3 activation.