No abstract

Type IV secretion (T4S) systems form the most common and versatile class of secretion systems in bacteria, capable of injecting both proteins and DNAs into host cells. T4S systems are typically composed of 12 components that form two major assemblies: the inner membrane complex embedded in the inner membrane and the core complex embedded in both the inner and outer membranes. Here we present the 3.3 Å resolution cryo-electron microscopy model of the T4S system core complex from Xanthomonas citri , a phytopathogen that utilizes this system to kill bacterial competitors. An extensive mutational investigation was performed to probe the vast network of protein-protein interactions in this 1.13 MDa assembly. This structure expands our knowledge of the molecular details of T4S system organization, assembly and evolution.

The molecular chaperone binding protein (BiP) participates in the constitutive function of the endoplasmic reticulum (ER) and protects the cell against stresses. In this study, we investigated the underlying mechanism by which BiP protects plant cells from stress-induced cell death. We found that enhanced expression of BiP in soybean (Glycine max) attenuated ER stress-and osmotic stress-mediated cell death. Ectopic expression of BiP in transgenic lines attenuated the leaf necrotic lesions that are caused by the ER stress inducer tunicamycin and also maintained shoot turgidity upon polyethylene glycol-induced dehydration. BiP-mediated attenuation of stress-induced cell death was confirmed by the decreased percentage of dead cell, the reduced induction of the senescence-associated marker gene GmCystP, and reduced DNA fragmentation in BiPoverexpressing lines. These phenotypes were accompanied by a delay in the induction of the cell death marker genes N-RICH PROTEIN-A (NRP-A), NRP-B, and GmNAC6, which are involved in transducing a cell death signal generated by ER stress and osmotic stress through the NRP-mediated signaling pathway. The prosurvival effect of BiP was associated with modulation of the ER stress-and osmotic stress-induced NRP-mediated cell death signaling, as determined in transgenic tobacco (Nicotiana tabacum) lines with enhanced (sense) and suppressed (antisense) BiP levels. Enhanced expression of BiP prevented NRP-and NAC6-mediated chlorosis and the appearance of senescence-associated markers, whereas silencing of endogenous BiP accelerated the onset of leaf senescence mediated by NRPs and GmNAC6. Collectively, these results implicate BiP as a negative regulator of the stress-induced NRP-mediated cell death response.