Characterization of Type Three Secretion System Translocator Interactions with Phospholipid Membranes

Three Proteins (Hpa2, HrpF and XopN) Are Concomitant Type III Translocators in Bacterial Blight Pathogen of Rice

Type III (T3) proteic effectors occupy most of the virulence determinants in eukaryote-pathogenic Gram-negative bacteria. During infection, bacteria may deploy a nanomachinery called translocon to deliver T3 effectors into host cells, wherein the effectors fulfill their pathological functions. T3 translocon is hypothetically assembled by bacterial translocators, which have been identified as one hydrophilic and two hydrophobic proteins in animal-pathogenic bacteria but remain unclear in plant pathogens. Now we characterize Hpa2, HrpF, and XopN proteins as concomitant T3 translocators in rice bacterial blight pathogen by analyzing pathological consequences of single, double, and triple gene knockout or genetic complementation. Based on these genetic analyses, Hpa2, HrpF, and XopN accordingly contribute to 46.9, 60.3, and 69.8% proportions of bacterial virulence on a susceptible rice variety. Virulence performances of Hpa2, HrpF, and XopN were attributed to their functions in essentially mediating from-bacteria-into-rice-cell translocation of PthXo1, the bacterial T3 effector characteristic of transcription factors targeting plant genes. On average, 61, 62, and 71% of PthXo1 translocation are provided correspondingly by Hpa2, HrpF, and XopN, while they cooperate to support PthXo1 translocation at a greater-than-95% extent. As a result, rice disease-susceptibility gene SWEET11, which is the regulatory target of PthXo1, is activated to confer bacterial virulence and induce the leaf blight disease in rice. Furthermore, the three translocators also undergo translocation, but only XopN is highly translocated to suppress rice defense responses, suggesting that different components of a T3 translocon deploy distinct virulence mechanisms in addition to the common function in mediating bacterial effector translocation.

Plant Aquaporins in Infection by and Immunity Against Pathogens - A Critical Review

Plant aquaporins (AQPs) of the plasma membrane intrinsic protein (PIP) family face constant risk of hijack by pathogens aiming to infect plants. PIPs can also be involved in plant immunity against infection. This review will utilize two case studies to discuss biochemical and structural mechanisms that govern the functions of PIPs in the regulation of plant infection and immunity. The first example concerns the interaction between rice Oryza sativa and the bacterial blight pathogen Xanthomonas oryzae pv. oryzae (Xoo). To infect rice, Xoo uses the type III (T3) secretion system to secrete the proteic translocator Hpa1, and Hpa1 subsequently mediates the translocation of T3 effectors secreted by this system. Once shifted from bacteria into rice cells, effectors exert virulent or avirulent effects depending on the susceptibility of the rice varieties. The translocator function of Hpa1 requires cooperation with OsPIP1;3, the rice interactor of Hpa1. This role of OsPIP1;3 is related to regulatory models of effector translocation. The regulatory models have been proposed as, translocon-dependent delivery, translocon-independent pore formation, and effector endocytosis with membrane protein/lipid trafficking. The second case study includes the interaction of Hpa1 with the H2O2 transport channel AtPIP1;4, and the associated consequence for H2O2 signal transduction of immunity pathways in Arabidopsis thaliana, a non-host of Xoo. H2O2 is generated in the apoplast upon induction by a pathogen or microbial pattern. H2O2 from this source translocates quickly into Arabidopsis cells, where it interacts with pathways of intracellular immunity to confer plant resistance against diseases. To expedite H2O2 transport, AtPIP1;4 must adopt a specific conformation in a number of ways, including channel width extension through amino acid interactions and selectivity for H2O2 through amino acid protonation and tautomeric reactions. Both topics will reference relevant studies, conducted on other organisms and AQPs, to highlight possible mechanisms of T3 effector translocation currently under debate, and highlight the structural basis of AtPIP1;4 in H2O2 transport facilitated by gating and trafficking regulation.

Using disruptive insertional mutagenesis to identify the in situ structure-function landscape of the Shigella translocator protein IpaB

Bacterial type III secretion systems (T3SS) are used to inject proteins into mammalian cells to subvert cellular functions. The Shigella T3SS apparatus (T3SA) is comprised of a basal body, cytoplasmic sorting platform and exposed needle with needle "tip complex" (TC). TC maturation occurs when the translocator protein IpaB is recruited to the needle tip where both IpaD and IpaB control secretion induction. IpaB insertion into the host membrane is the first step of translocon pore formation and secretion induction. We employed disruptive insertional mutagenesis, using bacteriophage T4 lysozyme (T4L), within predicted IpaB loops to show how topological features affect TC functions (secretion control, translocon formation and effector secretion). Insertions within the N-terminal half of IpaB were most likely to result in a loss of steady-state secretion control, however, all but the two that were not recognized by the T3SA retained nearly wild-type hemolysis (translocon formation) and invasiveness levels (effector secretion). In contrast, all but one insertion in the C-terminal half of IpaB maintained secretion control but were impaired for hemolysis and invasion. These nature of the data suggest the latter mutants are defective in a post-secretion event, most likely due to impaired interactions with the second translocator protein IpaC. Intriguingly, only two insertion mutants displayed readily detectable T4L on the bacterial surface. The data create a picture in which the makeup and structure of a functional T3SA TC is highly amenable to physical perturbation, indicating that the tertiary structure of IpaB within the TC is more plastic than previously realized.