Manipulation of small Rho GTPases is a pathogen-induced process detected by Nod1

Our innate immune system distinguishes microbes from self by detecting conserved pathogen-associated molecular patterns (PAMPs) (1). However, all microbes produce PAMPs, regardless of their pathogenic potential. To distinguish virulent microbes from ones with lower disease-causing potential the innate immune system detects conserved pathogen-induced processes (2), 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 a Salmonella virulence factor, SopE, triggered the Nod1 signaling pathway with consequent Rip2-mediated induction of NF-κB-dependent inflammatory responses. Similarly, activation of the Nod1 signaling pathway by peptidoglycan required Rac1 activity. Furthermore, constitutively active forms of Rac1, Cdc42 and RhoA activated the Nod1 signaling pathway. Our data identify activation of small Rho GTPases as a pathogen-induced process sensed through the Nod1 signaling pathway (Fig. S1).