Our innate immune system distinguishes microbes from self by detecting conserved pathogen-associated molecular patterns1. However, these are produced by all microbes, regardless of their pathogenic potential. To distinguish virulent microbes from those with lower disease-causing potential the innate immune system detects conserved pathogen-induced processes2, such as the presence of microbial products in the host cytosol, by mechanisms that are not fully resolved. Here we show that NOD1 senses cytosolic microbial products by monitoring the activation state of small Rho GTPases. Activation of RAC1 and CDC42 by bacterial delivery or ectopic expression of SopE, a virulence factor of the enteric pathogen Salmonella, triggered the NOD1 signalling pathway, with consequent RIP2 (also known as RIPK2)-mediated induction of NF-κB-dependent inflammatory responses. Similarly, activation of the NOD1 signalling pathway by peptidoglycan required RAC1 activity. Furthermore, constitutively active forms of RAC1, CDC42 and RHOA activated the NOD1 signalling pathway. Our data identify the activation of small Rho GTPases as a pathogen-induced process sensed through the NOD1 signalling pathway.

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We would like to thank S.-P. Nuccio for providing PCR primers for the construction of the bacterial strains. This work was supported by Public Health Service Grants AI044170 and AI076246. A.M.K. is supported by the American Heart Association Grant 12SDG12220022.

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  1. Department of Medical Microbiology and Immunology, School of Medicine, University of California at Davis, One Shields Avenue, Davis, California 95616, USA

    • A. Marijke Keestra
    • , Maria G. Winter
    • , Josef J. Auburger
    • , Simon P. Fräßle
    • , Mariana N. Xavier
    • , Sebastian E. Winter
    • , Anita Kim
    • , Victor Poon
    • , Mariëtta M. Ravesloot
    • , Julian F. T. Waldenmaier
    • , Renée M. Tsolis
    •  & Andreas J. Bäumler
  2. Proteomics Core Facility, UC Davis Genome Center, University of California at Davis, Davis, California 95616, USA

    • Richard A. Eigenheer


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A.M.K. contributed to the experimental design, performed experiments and contributed to Figs 1b–f, 2a–e, g, 3a–c, f, g and Supplementary Figs 1, 2d, 3a, b, d, 4a–c, 7 and 8. M.G.W. performed experiments, constructed bacterial strains and contributed to Figs 1a, e, f, 3d, g and Supplementary Figs 2a, b, e, h, g, e, 4c, 5b and 8. J.J.A. constructed expression plasmids and contributed to Figs 2f and 3c and Supplementary Fig. 7. S.P.F. constructed expression plasmids and contributed to Fig. 2g and Supplementary Figs 2f, 3c and 6a, b. M.N.X. contributed to Fig. 1e, f and Supplementary Fig. 5a. S.E.W. constructed bacterial strains, contributed to Supplementary Fig. 5b and critically read the manuscript. A.K. constructed expression plasmids and contributed to Supplementary Figs 2c and 7. V.P. constructed bacterial strains. M.M.R. contributed to Supplementary Fig. 2d. J.F.T.W. constructed expression plasmids. R.A.E. performed mass spectrometry. A.M.K., A.J.B. and R.M.T. provided financial support for the study and contributed to the experimental design. A.M.K. and A.J.B. were responsible for the overall study design and for writing the manuscript.

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The authors declare no competing financial interests.

Corresponding author

Correspondence to Andreas J. Bäumler.

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