The role of chaperones in bacterial type III secretion

The type III secretion system is used by many Gram-negative bacterial pathogens to deliver a set of virulence proteins directly into the host cell cytosol. These virulence proteins, called effectors, allow the bacteria to evade the host immune system. Secretion of several effectors requires association with specific chaperones in the bacterial cytosol prior to secretion. Yersinia pseudotuberculosis injects six effector proteins, called Yops, into professional phagocytes to disable phagocytosis and enable proliferation within the lymphatic system. YopE is a required effector that acts as a Rho GTPase-activating protein and disrupts the actin cytoskeleton, effectively arresting phagocytosis. SycE is the specific chaperone for YopE and is required for YopE stability in the bacterial cytosol. The X-ray crystallographic structures of SycE alone and bound to the chaperone-binding domain of YopE are presented here. SycE forms a compact, globular homodimer with a novel fold that provides hydrophobic binding sites for YopE. This fold and the cognate effector binding sites are found to be conserved among several sequence unrelated but structurally homologous chaperones. Despite structural conservation, the binding of YopE to SycE is specific and arises from a large number of hydrogen bonds by nonconserved residues. While in complex with SycE, the catalytic domain of YopE is found to be completely active, possess a separate guanidine-induced unfolding transition, and be proteolytically susceptible, indicating that the chaperone's influence on YopE is limited solely to the chaperone-binding domain. The binding of YopE to its chaperone is found to be remarkably similar to another effector-chaperone structure from Salmonella and may be representative of a universal binding mode. Five N-terminal amino acids in particular are chemically and positionally equivalent. We postulate that these five residues may provide a three-dimensional secretion signal important for targeting the effector to the type III secretion apparatus.