Structural and biochemical characterization of the type III secretion chaperones CesT and SigE

Several Gram-negative bacterial pathogens have evolved a type III secretion system to deliver virulence effector proteins directly into eukaryotic cel ls, a process essential for disease. This specialized secretion process req uires customized chaperones specific for particular effector proteins. The crystal structures of the enterohemorrhagic Escherichia coli O157:H7 Tir-sp ecific chaperone CesT and the Salmonella enterica SigD-specific chaperone S igE reveal a common overall fold and formation of homodimers. Site-directed mutagenesis suggests that variable, delocalized hydrophobic surfaces obser ved on the chaperone homodimers are responsible for specific binding to a p articular effector protein. Isothermal titration calorimetry studies of Tir -CesT and enzymatic activity profiles of SigD-SigE indicate that the effect or proteins are not globally unfolded in the presence of their cognate chap erones.