Backbone resonance assignment of the homodimeric, 35 kDa chaperone CesT from enteropathogenic Escherichia coli.

Enteropathogenic Escherichia coli (EPEC) is a human pathogen causing diarrhoea (Nataro and Kaper, 1998). As many Gram-negative bacterial pathogens, it uses a type III secretion system (TTSS) to deliver virulence effector proteins into the eukaryotic cell. Many TTSS effectors have a specific chaperone which is required for secretion. The TTSS chaperones share only limited sequence similarity, tend to form homodimers, are highly acidic and are predicted to contain a C-terminal amphipathic a-helix (Wattiau et al., 1996) . Their function is not clearly understood and seems to be versatile. Best known is their role in preventing aggregation or degradation of effector molecules. There is also evidence indicating a regulatory role in the release of effectors (Isberg and Duménil, 2001). The TTSS chaperone CesT is known to be specific for the translocated intimin receptor (Tir) and the mitochondrial associated protein (Map) (Creasey et al., 2003). The crystal structure of CesT with 2.8 Å resolution reveals a domain-swapped homodimer. As all other solved crystal structures of TTSS chaperones do not show a domain swap, the question arises whether it is a crystallographic artefact or facilitates the chaperone–effector interaction (Luo et al., 2001). The backbone assignment of the 35.4 kDa homodimer CesT is a fundamental step to address this question by NMR and to further understand the mechanism of the TTSS machinery. Furthermore it forms the basis for a structural characterization of the CesT/Tir and CesT/Map complexes.