Legionella pneumophila,the causative agent of Legionnaires' disease,has been recognized as a major health problem responsible for an estimated number of 15 000-30 000 cases of severe pneumonia per year in Germany alone.Despite of the high clinical relevance,many aspects of the intracellular life cycle of Legionella,especially details on interactions with host cells,are not well understood.Structural information on virulence proteins helps unravel basal pathogenicity mechanisms and is a prerequisite for the rational development of effective drug molecules.Here we discuss structures of three important virulence proteins ofLegionella that have been determined in our laboratory.The structure of the macrophage infectivity potentiator (Mip) protein of Legionella pneumophila is the first of a novel subgroup within the family of FK506-binding protein (FKBP) peptidyl-prolyl cis/trans isomerases.On the basis of the Mip structure,promising antibacterial agents are being designed.Recently,structures of two equally exciting Legionella proteins have been reported.The ferrous iron transport protein FeoB is a transmembrane protein responsible for Fe2+ aquisition after entry of the pathogen into the host cell.The structure of the cytoplasmic domain of FeoB provides insights into the family of prokaryotic G proteins and allows a detailed comparison with structures of related FeoBs.Furthermore,the characterization of DegQ,a periplasmatic chaperone-protease involved in protein quality control represents an intriguing example of how enzymatic activity is regulated by oligomerization as well as by an intrinsic loop activation cascade,depending on subtle conformational rearrangements.