Yersinia enterocolitica type III secretion: Evidence for the ability to transport proteins that are folded prior to secretion

Abstract

BackgroundPathogenic Yersinia species (Y. enterocolitica, Y. pestis, Y. pseudotuberculosis) share a type three secretion system (TTSS) which allows translocation of effector proteins (called Yops) into host cells. It is believed that proteins are delivered through a hollow needle with an inner diameter of 2–3 nm. Thus transport seems to require substrates which are essentially unfolded. Recent work from different groups suggests that the Yersinia TTSS cannot accommodate substrates which are folded prior to secretion. It was suggested that folding is prevented either by co-translational secretion or by the assistance of specific Yop chaperones (called Sycs).ResultsIn this study we have fused YopE secretion signals of various length to the mouse dihydrofolate reductase (DHFR) in order to analyse the DHFR folding state prior to secretion. We could demonstrate that secretion-deficient as well as secretion-competent YopE-DHFR fusions complexed to SycE can be efficiently purified from Yersinia cytosol by affinity chromatography using methotrexate-agarose. This implies the folding of the DHFR fusion moiety despite SycE binding and contradicts the previously presented model of folding inhibition by chaperone binding. Secretion-deficient YopE-DHFR fusions caused severe jamming of the TTSS. This observation contradicts the co-translational secretion model.ConclusionsWe present evidence that the Yersinia TTSS is familiar with the processing of transport substrates which are folded prior to secretion. We therefore predict that an unfoldase is involved in type III secretion.

Crystal Structure of the Yersinia enterocolitica Type III Secretion Chaperone SycT

“…This suggests that it is the chaperone that directs the effector toward the ATPase. This finding is in line with our recently presented model of type III secretion, predicting that TTSS ATPases act as unfoldases using the TTSS chaperones to encounter the secretion substrates (21,24,43). After displacement of the chaperone by the ATPase, the chaperone-binding site of the effector, lacking tertiary structure, is distinguished as an ideal starting point for unraveling the effector.…”

“…A on the conformation of their substrates is restricted to the binding sites identified close to the N termini (9,14,17,21,24). Here, for the first time, an interaction between the catalytic domain of an effector, YopT, and its specific chaperone, SycT, was revealed.…”

“…ComplexesWork from several groups suggests that the influence of class I TTSS chaperones on the folding of their substrates is restricted to the binding site generally encompassing about 50 -100 amino acid residues (9,14,17,21,24). To understand the functioning of these chaperones, high resolution structural data on complete type III effectors in complex with their respective chaperones are required.…”

“…The effector domain is wrapped around the chaperone dimer in an expanded form retaining secondary structure elements. Work from several groups further suggests that the C-terminally located effector domains are folded and catalytically active in the presence of their respective chaperones (9,14,21,24). Only recently, the structure of the secretable regulatory TTSS component YopN from Yersinia in complex with the heterodimeric chaperone SycN/YscB was presented (17).…”

Cross-talk between Type Three Secretion System and Metabolism in Yersinia

“…To give an idea of the metabolic requirements, Yops are secreted to the culture supernatant in 10-mg amounts per liter of culture within 2 h after calcium depletion of the medium. Furthermore, post-translationally secreted substrates need to be unfolded by a T3SS-specific ATPase prior to secretion (11)(12)(13)(14). In addition, T3SS-dependent transport of Yops requires the proton motive force (15).…”