SopE and SopE2 from Salmonella typhimurium activate different sets of RhoGTPases of the host cell

The bacterial enteropathogen Salmonella typhimurium employs a specialized t ype III secretion system to inject toxins into host cells, which trigger si gnaling cascades leading to cell death in macrophages, secretion of pro-inf lammatory cytokines, or rearrangements of the host cell cytoskeleton and th us to bacterial invasion. Two of the injected toxins, SopE and the 69% iden tical protein SopE2, are highly efficient guanine nucleotide exchange facto rs for the RhoGTPase Cdc42 of the host cell. However, it has been a puzzle why S. typhimurium might employ two toxins with redundant function. We hypo thesized that SopE and SopE2 might have different specificities for certain host cellular RhoGTPases. In vitro guanine nucleotide exchange assays and surface plasmon resonance measurements revealed that SopE is an efficient g uanine nucleotide exchange factor for Cdc42 and Rac1, whereas SopE2 was int eracting efficiently only with Cdc42, but not with Rac1. Affinity precipita tion of Cdc42(.)GTP and Rac1(.)GTP from lysates and characteristic cytoskel etal rearrangements of infected tissue culture cells confirmed that SopE is highly efficient at activating Cdc42 and Rac1 in vivo, whereas SopE2 was e fficiently activating Cdc42, but not Rac1. We conclude that the translocate d effector proteins SopE and SopE2 allow S. typhimurium to specifically act ivate different sets of RhoGTPase signaling cascades.