Gram-negative pathogens exert virulence to a significant degree by the secretion of toxins, termed effectors, through the type III secretion system. One effector from Salmonella sp., SpvC, was found to have a highly novel enzymatic function, irreversibly dephosphorylating an activating threonine residue on MAP kinase proteins via a lyase activity (phosphothreonine lyase), leaving a double bond between the alpha and beta carbon (Science. 2007 315:1000-3.). We have solved the structure of the homologue of SpvC from Shigella flexneri, termed OspF, to 2.7 Å, from residue 23 to the C-terminus. This structure shows a very similar overall fold to Salmonella enterica SpvC, to which it shares 64% sequence identity. The principal difference involves a change in direction in the most N-terminal helix, which folds around the structure in SpvC but which remains straight in OspF. The overall significance of this finding has yet to be determined. Although 2 molecules of OspF can be found in an asymmetric unit in crystals, the protein is predicted to be monomeric.