Crystal Structure of the VgrG1 Actin Cross-linking Domain of the Vibrio cholerae Type VI Secretion System

Abstract

Vitamin E is the major lipid-soluble chain-breaking antioxidant in mammals and plays an important role in normal development and physiology. Deficiency (whether dietary or genetic) results in primarily nervous system pathology, including cerebellar neurodegeneration and progressive ataxia (abnormal gait). However, despite the widely acknowledged antioxidant properties of vitamin E, only a few studies have directly correlated levels of reactive oxygen species with vitamin E availability in animal models. We explored the relationship between vitamin E and reactive oxygen species in two mouse models of vitamin E deficiency: dietary deficiency and a genetic model (tocopherol transfer protein, Ttp ؊/؊ mice). Both groups of mice developed nearly complete depletion of ␣-tocopherol (the major tocopherol in vitamin E) in most organs, but not in the brain, which was relatively resistant to loss of ␣-tocopherol. F4-neuroprostanes, an index of lipid peroxidation, were unexpectedly lower in brains of deficient mice compared with controls. In vivo oxidation of dihydroethidium by superoxide radical was also significantly lower in brains of deficient animals. Superoxide production by brain mitochondria isolated from vitamin E-deficient and Ttp ؊/؊ mice, measured by electron paramagnetic resonance spectroscopy, demonstrated a biphasic dependence on exogenously added ␣-tocopherol. At low concentrations, ␣-tocopherol enhanced superoxide flux from mitochondria, a response that was reversed at higher concentrations. Here we propose a mechanism, supported by molecular modeling, to explain decreased superoxide production during ␣-tocopherol deficiency and speculate that this could be a beneficial response under conditions of ␣-tocopherol deficiency.␣-Tocopherol was first discovered as a micronutrient indispensable for reproduction in female rats (1). The view that ␣-tocopherol functions solely as a chain-breaking antioxidant has prevailed, reflecting a large body of accumulated experimental evidence showing that ␣-tocopherol can reduce lipid peroxidation in many models and the observation that ␣-tocopherol inhibits lipid peroxidation by scavenging peroxyl radicals faster (10 6 M Ϫ1 s Ϫ1 ) than they can react with other lipids (ϳ10 2 M Ϫ1 s Ϫ1 ) or proteins (2). Recently, however, a broad array of "nonantioxidant" cellular functions for ␣-tocopherol have emerged (reviewed in Refs. 3-8, but see Ref. 9 for the opposing view that supports a pure antioxidative function for ␣-tocopherol). The mechanism by which these "nonantioxidant" actions factor into the observed biological effects of ␣-tocopherol is not yet known. A systematic study of the relationship between levels of ␣-tocopherol and ROS 4 production is one approach to begin to differentiate the documented ability of ␣-tocopherol to act as a lipid peroxidation inhibitor and antioxidant versus its other potential activities in vivo.In the current study, we used two mouse models of vitamin E deficiency: 1) dietary deficiency (VED) in C57BL6 mice and 2) a genetic model, ␣-tocopherol tra...

Retrocyclins neutralize bacterial toxins by potentiating their unfolding

“…We assessed the effects of both RCs on unfolding of ACD, an actin cross-linking effector toxin, which is expressed as a part of larger toxins MARTX ( Vibrio and Aeromonas spp . ) [31] and valine-glycine repeat protein G1 (VgrG1) ( V. cholerae ) [32]. Previously we have demonstrated that similar to many other bacterial toxins, ACD from MARTX toxins of both V. cholerae and A. hydrophila (ACD Vc and ACD Ah respectively) have low thermodynamic stability and, at the physiological temperature of the human body, exist in equilibrium between fully folded and partially unfolded (molten globule) states [20].…”

H-NS Silencing of the Salmonella Pathogenicity Island 6-Encoded Type VI Secretion System Limits Salmonella enterica Serovar Typhimurium Interbacterial Killing

“…The type VI secretion system (T6SS) is required for the virulence of several Gram-negative pathogens. In Vibrio cholerae, the T6SS translocates VgrG1, a toxin that carries a domain responsible for actin cross-linking in eukaryotic cells (1)(2)(3)(4). However, the role of the T6SS is not limited to virulence toward eukaryotes; an increasing number of reports demonstrate that the T6SS is also involved in interbacterial intoxication (5)(6)(7)(8)(9)(10).…”

Promoter Swapping Unveils the Role of the Citrobacter rodentium CTS1 Type VI Secretion System in Interbacterial Competition

“…A few examples of T6SSs that have been studied are directly responsible for pathogenesis by the injection of toxins into eukaryotic cells that interfere with the cytoskeleton (15,(23)(24)(25)(26). In contrast, a growing number of T6SSs are shown to kill target bacterial cells by translocating antibacterial effectors directly into the periplasm or the cytoplasm of the prey upon cell-cell contact (27)(28)(29)(30)(31)(32)(33).…”