The sorting platform in the type III secretion pathway: From assembly to function

Plant PI4P is required for bacteria to translocate type-3 effectors

Type-3 effectors (T3E) of phytopathogenic Gram-negative bacteria fulfill a virulent role, causing disease, or an avirulent role, inducing immunity, following their translocation into plant cells. This study aimed to validate the hypothesis that bacterial T3E translocation requires lipidic compounds in plant cell membranes. Based on genetic, molecular, and biochemical assays, we determined that phosphatidylinositol 4-phosphate (PI4P) associated with plant cell membranes is essential for the translocation of T3E by bacterial pathogens. Replicate experimental data revealed that PI4P cooperates with the type-3 translocase HrpF to facilitate the translocation of effectors TAL and Xop from Xanthomonas oryzae and Hop from Pseudomonas syringae into the cells of Oryza sativa and Nicotiana benthamiana, respectively. Genetic and molecular analyses confirmed that, once translocated into plant cells, the distinct effectors induce disease or immunity. Combined genetic and pharmacological analyses revealed that when PI4P content is suppressed via genetic or pharmacological measures, the T3 effector translocation is considerably suppressed, resulting in serious inhibition of bacterial infection. Overall, these findings demonstrate that cooperative functioning of HrpF-PI4P is conserved in bacterial effectors and plants.

Cytosolic sorting platform complexes shuttle type III secretion system effectors to the injectisome in Yersinia enterocolitica

Bacteria use type III secretion injectisomes to inject effector proteins into eukaryotic target cells. Recruitment of effectors to the machinery and the resulting export hierarchy involve the sorting platform. These conserved proteins form pod structures at the cytosolic interface of the injectisome but are also mobile in the cytosol. Photoactivated localization microscopy in Yersinia enterocolitica revealed a direct interaction of the sorting platform proteins SctQ and SctL with effectors in the cytosol of live bacteria. These proteins form larger cytosolic protein complexes involving the ATPase SctN and the membrane connector SctK. The mobility and composition of these mobile pod structures are modulated in the presence of effectors and their chaperones, and upon initiation of secretion, which also increases the number of injectisomes from ~5 to ~18 per bacterium. Our quantitative data support an effector shuttling mechanism, in which sorting platform proteins bind to effectors in the cytosol and deliver the cargo to the export gate at the membrane-bound injectisome.