P-216 - The sorting platform is a dynamic substrate docking site of the type III secretion system from the plant pathogen Xanthomonas euvesicatoria

The Gram-negative bacterium Xanthomonas euvesicatoria (Xe) causes bacterial spot disease in pepper and tomato plants, and employs a type III secretion (T3S) system to translocate effector proteins into plant cells. The T3S system is a complex nanomachine which is conserved in plant- and animal-pathogenic bacteria. As was shown in animal pathogens, the T3S system from Xe likely contains a cytoplasmic wheel-like sorting platform which consists of the central ATPase HrcN connected via HrcL to peripheral pod-like complexes composed of HrcQ and its C-terminal alternative translation initiation product HrcQ_C. Both HrcQ and HrcL contribute to HrcN stability, indicating that a preassembled HrcQ-HrcL complex associates with and stabilizes the ATPase. Fluorescence microscopy analysis showed that the sorting platform dynamically associates with the membrane-spanning secretion apparatus under T3S-permissive conditions when bacteria are cultivated at a low external pH. Components of the sorting platform likely act as substrate docking sites and interact with type III effector proteins and a complex of the secreted regulator HpaA and the T3S chaperone HpaB. We propose that T3S activation at acidic pH triggers the association of the sorting platform with the T3S system, thus leading to the export of HpaA. This, in turn, liberates HpaB which subsequently binds to and promotes the translocation of type III effectors.