Type III secretion chaperones ShcS1 and ShcO1 from Pseudomonas syringae pv. tomato DC3000 bind more than one effector

The hrp-type III secretion (TTS) system is a key pathogenicity factor of the plant pathogen Pseudomonas syringae pv. tomato DC3000 that translocates effector proteins into the cytosol of the eukaryotic host cell. The translocation of a subset of effectors is dependent on specific chaperones. In this study an operon encoding a TTS chaperone (ShcS1) and the truncated effector HopS1' was characterized. Yeast two-hybrid analysis and pull-down assays demonstrated that these proteins interact. Using protein fusions to AvrRpt2 it was shown that ShcS1 facilitates the translocation of HopS1', suggesting that ShcS1 is a TTS chaperone for HopS1' and that amino acids 1 to 118 of HopS1' are required for translocation. P. syringae pv. tomato DC3000 carries two shcS1 homologues, shcO1 and shcS2, which are located in different operons, and both operons include additional putative effector genes. Transcomplementation experiments showed that ShcS1 and ShcO1, but not ShcS2, can facilitate the translocation of HopS1' :: AvrRpt2. To characterize the specificities of the putative chaperones, yeast two-hybrid interaction studies were performed between the three chaperones and putative target effectors. These experiments showed that both ShcS1 and ShcO1 bind to two different effectors, HopS1' and HopO1-1, that share only 16% amino acid sequence identity. Using gel filtration it was shown that ShcS1 forms homodimers, and this was confirmed by yeast two-hybrid experiments. In addition, ShcS1 is also able to form heterodimers with ShcO1. These data demonstrate that ShcS1 and ShcO1 are exceptional class A TTS chaperones because they can bind more than one target effector.