Once encapsulated, the trapped bacteria subsequently die (Silva et al., 2002). Cytotoxic factors targeting immunocytes are good candidates for the mediators of immunosuppression (Clarke, 2008). Plu1961/Plu1962 caused death of CF-203 cells via necrosis. Further studies on the necrotic and apoptotic activities of Plu1961/Plu1962 against insect hemocytes will be necessary to elucidate its role in immunosuppression. Confocal

microscopy revealed that Plu1961/Plu1962 caused a notable decrease in cellular tubulin of CF-203 cells. Microtubule, one of the principal components of cytoskeleton, is critical to cell shape, cell movement, intracellular transport of organelles, and the separation of chromosomes during mitosis (Archuleta

et al., 2011). As a result, microtubule is a prime target for pathogens and their virulence factors. Mouse macrophages treated with Bacillus anthracis lethal toxin (LT) induced INCB024360 datasheet a notable decrease in the level of cellular tubulin and altered stability of the microtubule network (Chandra et al., 2005). Treatment of human colonocytes with Clostridium difficile toxin A resulted in tubulin deacetylation and subsequent microtubule depolymerization (Nam et al., 2010). Assembly of the two components www.selleckchem.com/products/torin-1.html is essential for binary toxins to exhibit their cytotoxicity (Schleberger et al., 2006). However, the stage at which the assembly of the binary toxin components occurs is debatable. Previous study suggested that intoxication by binary toxins initially involved specific, receptor-mediated binding of ‘B’ component to a targeted cell as monomers that form homoheptamers on the cell surface. The ‘B’ heptamer–receptor complex then acts as a docking platform that subsequently translocates the enzymatic ‘A’ component into the cytosol. Once inside the cytosol, ‘A’ component can inhibit normal cell functions (Barth et al., 2004).

It was reported that at low toxin concentrations, complex formation might enhance the efficiency of the binary toxin (Kaiser et al., 2006). Our data demonstrated that ID-8 when co-expressed in the same cytoplasm, Plu1961 and Plu1962 could interact with each other and form a complex. This could in part explain the observation that Plu1961 and Plu1962 mixed in vitro did not affect the growth of mammalian cells, but while co-expressed in the same cytoplasm, Plu1961/Plu1962 exhibited cytotoxic effect against B16, 4T1, and HeLa cells. In conclusion, we have identified XaxAB-like binary toxin from P. luminescens TT01, which exhibits highly injectable toxicity against insect larvae. Plu1961/Plu1962 mixture could cause rapid cell necrosis when applied to insect midgut CF-203 cells. However, co-expression in the same cytoplasm is essential for Plu1961/Plu1962 to exhibit cytotoxic activity against mammalian cells. The biological role of Plu1961/Plu1962 in the infection process needs further study.