Effector loading onto the VgrG carrier activates type VI secretion system assembly.

The type VI secretion system (T6SS) is used by many bacteria to engage in social behavior and can affect the health of its host plant or animal. Because activities associated with T6SSs are often costly, T6SSs must be tightly regulated. However, our knowledge regarding how T6SS assembly and contraction are regulated remains limited. Using the plant pathogen Agrobacterium tumefaciens, we show that effectors are not just passengers but also impact on T6SS assembly. The A. tumefaciens strain C58 encodes one T6SS and two Tde DNase toxin effectors used as major weapons for interbacterial competition. Here, we demonstrate that loading of Tde effectors onto their cognate carriers, the VgrG spikes, is required for active T6SS secretion. The assembly of the TssBC contractile sheath occurs only in the presence of Tde effectors. The requirement of effector loading for efficient T6SS secretion was also validated in other A. tumefaciens strains. We propose that such a mechanism is used by bacteria as a strategy for efficacious T6SS firing and to ensure that effectors are loaded onto the T6SS prior to completing its assembly.

The potential of lumbrokinase and serratiopeptidase for the degradation of Aß 1-42 peptide - an in vitro and in silico approach.

BACKGROUND: Alzheimer's disease (AD) is diagnosed with the deposition of insoluble ß-amyloid (Aß) peptides in the neuropil of the brain leading to dementia. The extracellular deposition of the fibrillar Aß peptide on the neurons is known as senile plaques. Therefore, Aß degradation and clearance from the human body is a promising therapeutic approach in the medication of AD. METHODS: In the current study, the enzyme lumbrokinase (LK) was extracted and purified from earthworm and its activity was utilized toward Aß 1-42 amyloids degradation in vitro alongside with an additional enzyme serratiopeptidase (SP) considering nattokinase (NK) as a standard. RESULTS: The output of this study revealed that preformed Aß 1-42 amyloids was disintegrated by both LK and SP, as demonstrated from fluorescence assay using Thioflavin T dye. In addition, dynamic light scattering study revealed the lower size of the preformed fibrils Aß 1-42 at various time intervals after incubation with the enzymes LK and SP. Furthermore, in silico approach showed high affinity thermodynamically favorable interaction of LK as well as SP toward Aß 1-42 amyloid. Finally, the toxicity of degraded preformed Aß 1-42 amyloid was assessed by MTT assay which showed reduced toxicity of enzyme treated Aß 1-42 amyloid compared to only Aß 1-42 amyloid. CONCLUSION: The findings of the present study indicated that LK and SP, not only had Aß 1-42 amyloid degrading potential, but also could reduce the toxicity which can make them a suitable drug candidate for AD. Furthermore, the in vivo studies are needed to be executed in future.

VgrG C terminus confers the type VI effector transport specificity and is required for binding with PAAR and adaptor-effector complex.

Type VI secretion system (T6SS) is a macromolecular machine used by many Gram-negative bacteria to inject effectors/toxins into eukaryotic hosts or prokaryotic competitors for survival and fitness. To date, our knowledge of the molecular determinants and mechanisms underlying the transport of these effectors remains limited. Here, we report that two T6SS encoded valine-glycine repeat protein G (VgrG) paralogs in Agrobacterium tumefaciens C58 specifically control the secretion and interbacterial competition activity of the type VI DNase toxins Tde1 and Tde2. Deletion and domain-swapping analysis identified that the C-terminal extension of VgrG1 specifically confers Tde1 secretion and Tde1-dependent interbacterial competition activity in planta, and the C-terminal variable region of VgrG2 governs this specificity for Tde2. Functional studies of VgrG1 and VgrG2 variants with stepwise deletion of the C terminus revealed that the C-terminal 31 aa (C31) of VgrG1 and 8 aa (C8) of VgrG2 are the molecular determinants specifically required for delivery of each cognate Tde toxin. Further in-depth studies on Tde toxin delivery mechanisms revealed that VgrG1 interacts with the adaptor/chaperone-effector complex (Tap-1-Tde1) in the absence of proline-alanine-alanine-arginine (PAAR) and the VgrG1-PAAR complex forms independent of Tap-1 and Tde1. Importantly, we identified the regions involved in these interactions. Although the entire C31 segment is required for binding with the Tap-1-Tde1 complex, only the first 15 aa of this region are necessary for PAAR binding. These results suggest that the VgrG1 C terminus interacts sequentially or simultaneously with the Tap-1-Tde1 complex and PAAR to govern Tde1 translocation across bacterial membranes and delivery into target cells for antibacterial activity.