A large number of bacterial pathogens, including Shigella, Salmonella, Bordetella, Pseudomonas, and pathogenic E. coli that are pathogenic for humans, other animals including insects or nematodes, and plants are equipped with a special protein- export apparatus called a type III secretion system (TTSS) or an injectisome. The injectisome is a highly sophisticated nanomachine that has specifically evolved to allow bacteria to deliver proteins into eukaryotic cells. The TTSS enables these pathogens to inject virulence proteins (known as effectors) directly into the cytoplasm of the eukaryotic host cells they infect. Many of these type III translocated effectors mimic eukaryotic factors and are capable of subverting key host cellular processes to the benefit of the pathogen during infection. Over the past decade, significant progress has been made in understanding the structure, assembly and the mode of operation of TTSS. The cytosolic components, the principal structural building proteins of the injectisome, from the basal body embedded in the inner and outer bacterial membrane to the tip of the needle protruding from the cell surface, have been extensively characterized. Virulence factors (effectors, needle proteins and translocators) form tight complexes with cognate chaperones in the cytosol and are subsequently targeted specifically to an ATPase protein located at the base of the injectisome. Powered by ATP, the effector is then translocated through the needle and is secreted in the eukaryotic cell. Fundamental questions about the functional mechanisms underpinning these processes remain unaddressed. We propose to use an integrated approach combining structural, dynamic, thermodynamic, kinetic, biochemical and in vitro and in vivo functional assays to provide insight into the early events of the secretion process that involve the recognition and binding of virulence factors (effectors and translocators) by cognate chaperones and the targeting of these substrates to the ATPase. We have extensively characterized over the last 3 years TTS protein components from the enteropathogenic Escherichia coli (EPEC), a prototype for TTSS and the major cause of infantile diarrhea and child mortality worldwide. The specific aims are designed to provide atomic-resolution insight into (i) the mechanisms of specific interaction between TTS chaperones and virulence factors, (ii) the structural and dynamic properties of the ATPase, (iii) the "recognition" or "secretion" signal, and (iv) the specific targeting of chaperone-substrate complexes to the ATPase.