Structure of Shigella IpgB2 in Complex with Human RhoA

Abstract

A common theme in bacterial pathogenesis is the manipulation of eukaryoticcells by targeting the cytoskeleton. This is in most cases achievedeither by modifying actin, or indirectly via activation of key regulatorscontrolling actin dynamics such as Rho-GTPases. A novel group ofbacterial virulence factors termed the WXXXE family has emerged asguanine nucleotide exchange factors (GEFs) for these GTPases. Theprecise mechanism of nucleotide exchange, however, has remained unclear.Here we report the structure of the WXXXE-protein IpgB2 from Shigellaflexneri and its complex with human RhoA. We unambiguously identifyIpgB2 as a bacterial RhoA-GEF and dissect the molecular mechanismof GDP release, an essential prerequisite for GTP binding. Our observationsuncover that IpgB2 induces conformational changes on RhoA mimickingDbI- but not DOCK family GEFs. We also show that dissociation ofthe GDP·Mg2+ complex is preceded by the displacement of the metalion to the α-phosphate of the nucleotide, diminishing its affinityto the GTPase. These data refine our understanding of the mode ofaction not only of WXXXE GEFs but also of mammalian GEFs of the DH/PHfamily.