ExoT belongs to the family of type 3 secretion system (T3SS) effector toxins in Pseudomonas aeruginosa, known to be one of the major virulence determinant toxins that cause chronic and acute infections in immuno-compromised individuals, burn victims and cystic fibrosis patients. Here, we report the X-ray crystal structure of the amino terminal fragment of effector toxin ExoT, in complex with full-length homodimeric chaperone SpcS at 2.1 Å resolution. The full-length dimeric chaperone SpcS has the conserved α-β-β-β-α-β-β-α fold of class I chaperones, the characteristic hydrophobic patches for binding effector proteins and a conserved polar cavity at the dimeric interface. The stable crystallized amino terminal fragment of ExoT consists of a chaperone binding domain and a membrane localization domain that wraps around the dimeric chaperone. Site-directed mutagenesis experiments and a molecular dynamics study complement each other in revealing Asn65, Phe67 and Trp88 as critical dimeric interfacial residues that can strongly influence the effector–chaperone interactions.
The atomic coordinates and structure factors of ExoT–SpcS complex (code 4JMF) have been deposited in the Protein Data Bank Japan (PDBj), Institute for Protein Research, Osaka University.