Cross-talk between type three secretion system and metabolism in Yersinia

Pathogenic yersiniae utilize a type three secretion system (T3SS) to inject Yop proteins into host cells in order to undermine their immune response. YscM1 and YscM2 proteins have been reported to be functionally equivalent regulators of the T3SS in Yersinia enterocolitica. Here, we show by affinity purification, native gel electrophoresis and small angle x-ray scattering that both YscM1 and YscM2 bind to phosphoenolpyruvate carboxylase (PEPC) of Y. enterocolitica. Under in vitro conditions, YscM1, but not YscM2, was found to inhibit PEPC with an apparent IC(50) of 4 mum (K(i) = 1 mum). To analyze the functional roles of PEPC, YscM1, and YscM2 in Yop-producing bacteria, cultures of Y. enterocolitica wild type and mutants defective in the formation of PEPC, YscM1, or YscM2, respectively, were grown under low calcium conditions in the presence of [U-(13)C(6)]glucose. The isotope compositions of secreted Yop proteins and nine amino acids from cellular proteins were analyzed by mass spectrometry. The data indicate that a considerable fraction of oxaloacetate used as precursor for amino acids was derived from [(13)C(3)]phosphoenolpyruvate by the catalytic action of PEPC in the wild-type strain but not in the PEPC(-) mutant. The data imply that PEPC is critically involved in replenishing the oxaloacetate pool in the citrate cycle under virulence conditions. In the YscM1(-) and YscM2(-) mutants, increased rates of pyruvate formation via glycolysis or the Entner-Doudoroff pathway, of oxaloacetate formation via the citrate cycle, and of amino acid biosynthesis suggest that both regulators trigger the central metabolism of Y. enterocolitica. We propose a "load-and-shoot cycle" model to account for the cross-talk between T3SS and metabolism in pathogenic yersiniae.

Yersinia enterocolitica type III secretion chaperone SycH. Recombinant expression, purification, characterisation, and crystallisation

All pathogenic Yersinia species (Y. enterocolitica, Y. pestis, and Y. pseudotuberculosis) share a type three secretion system (TTSS) that allows translocation of effector proteins into host cells. Yersinia enterocolitica SycH is a chaperone assisting the transport of the effector YopH and two regulatory components of the TTSS, YscM1 and YscM2. We have recombinantly expressed SycH in Escherichia coli. Purification of tag-free SycH to near homogeneity was achieved by combining ammonium sulfate precipitation, anion exchange chromatography, and gel filtration. Functionality of purified SycH was proven by demonstrating binding to YopH. SycH crystals were grown that diffracted to 2.94A resolution. Preliminary crystallographic data and biochemical findings suggest that SycH forms homotetramers. SycH may therefore represent a novel class of TTSS chaperones. In addition, we found that YopH was enzymatically active in the presence of SycH. This implies that the function of the secretion chaperone SycH is not to keep YopH in a globally unfolded state prior to secretion.

Recombinant Yersinia enterocolitica YscM1 and YscM2: homodimer formation and susceptibility to thrombin cleavage

Pathogenic Yersinia species (Y. enterocolitica, Y. pestis, and Y. pseudotuberculosis) make use of a virulence plasmid-encoded type three secretion system (TTSS) to inject effector proteins into host cells. Y. enterocolitica YscM1 (LcrQ in Y. pestis and Y. pseudotuberculosis) and its homologue YscM2 are regulatory components of the TTSS that are also secreted by this transport apparatus. YscM1 and YscM2 share 57% identity and are believed to be functionally equivalent. We have recombinantly expressed and purified YscM1 and YscM2 in Escherichia coli. After expression as glutathione S-transferase (GST) fusions purification to near homogeneity was achieved by glutathione-Sepharose affinity chromatography followed by PreScission protease treatment to cleave off GST and gel filtration on a Superdex 75 column. Such recombinant YscM1 and YscM2 bound efficiently to the specific chaperone SycH, indicating proper folding of the purified proteins. Gel filtration analyses revealed that both YscM1 and YscM2 formed homodimers. The YscM1 and YscM2 homodimers could be dissociated at high ionic strength, indicating that salt bridges essentially contribute to the dimerization. We further demonstrated that YscM1 and YscM2 are susceptible to thrombin cleavage.

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

Background

Pathogenic 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).

Results

In 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.

Conclusions

We 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.