Solution structure of the Legionella pneumophila Mip-rapamycin complex

We have solved the solution structure of free Mip77–213 and the Mip77–213-rapamycin complex by NMR spectroscopy. Mip77–213 showed the typical FKBP-fold and only minor rearrangements upon binding of rapamycin. Apart from the configuration of a flexible hairpin loop, which is partly stabilized upon binding, the solution structure confirms the crystal structure. Comparisons to the structures of free FKBP12 and the FKBP12-rapamycin complex suggested an identical binding mode for both proteins.The structural similarity of the Mip-rapamycin and FKBP12-rapamycin complexes suggests that FKBP12 ligands may be promising starting points for the design of novel Mip inhibitors. The search for a novel drug against Legionnaires' disease may therefore benefit from the large variety of known FKBP12 inhibitors.The Gram-negative pathogen Legionella pneumophila infects phagocytic cells such as various freshwater protozoa and human alveolar macrophages [1]. The bacteria enter the human lung via aerosols generated by man-made water systems, and cause severe and often fatal human pneumonia particularly in immunocompromised patients. One major virulence factor contributing to infection is the macrophage infectivity potentiator (Mip) protein. L. pneumophila strains lacking Mip or expressing a mutant of Mip with low PPIase activity were significantly attenuated in a guinea pig infection model [2]. The protein contributes to the disintegration of lung tissue and subsequent dissemination of the bacteria within the body. Transwell assays support the idea that Mip enables the bacteria to transmigrate across a barrier of lung epithelial cells and extracellular matrix [3].Mip is a basic 22.8 kDa surface protein (pI 9.8) localized at the outer membrane of the bacteria. Cross-linking experiments revealed that it forms homodimers [4,5]. Mip belongs to the FK506 binding protein (FKBP) family exhibiting peptidyl-prolyl cis/trans isomerase activity (PPIase, EC 5.2.1.8), and is in this respect a homolog