Modulation of host signaling by a bacterial mimic: structure of the Salmonella effector SptP bound to Rac1

Salmonella spp. utilize a specialized protein secretion system to deliver a battery of effector proteins into host cells. Several of these effecters s timulate Cdc42- and Rad-dependent cytoskeletal changes that promote bacteri al internalization. These potentially cytotoxic alterations are rapidly rev ersed by the effector SptP, a tyrosine phosphatase and GTPase activating pr otein (GAP) that targets Cdc42 and Rac1. The 2.3 Angstrom resolution crysta l structure of an SptP-Rac1 transition state complex reveals an unusual GAP architecture that mimics host functional homologs. The phosphatase domain possesses a conserved active site but distinct surface properties. Binding to Rad induces a dramatic stabilization in SptP of a four-helix bundle that makes extensive contacts with the Switch I and Switch II regions of the GT Pase.