TI - A Bacterial Cytotoxin Identifies the RhoA Exchange Factor Net1 as a Key Effector in the Response to DNA Damage DNA Damage Activates Net1 . AB - Background Exposure of adherent cells to DNA damaging agents , such as the bacterial cytolethal distending toxin ( CDT ) or ionizing radiations ( IR ) , activates the small GTPase RhoA , which promotes the formation of actin stress fibers and delays cell death . The signalling intermediates that regulate RhoA activation and promote cell survival are unknown . Principal Findings We demonstrate that the nuclear RhoA-specific Guanine nucleotide Exchange Factor ( GEF ) Net1 becomes dePHOSphorylated at a critical inhibitory site in cells exposed to CDT or IR . Expression of a dominant negative Net1 or Net1 knock down by iRNA prevented RhoA activation , inhibited the formation of stress fibers , and enhanced cell death , indicating that Net1 activation is required for this RhoA -mediated responses to genotoxic stress . The Net1 and RhoA -dependent signals involved activation of the Mitogen-Activated Protein Kinase p38 and its downstream target MAPK -activated protein kinase 2 . Significance Our data highlight the importance of Net1 in controlling RhoA and p38 MAPK mediated cell survival in cells exposed to DNA damaging agents and illustrate a molecular pathway whereby chronic exposure to a bacterial toxin may promote genomic instability .