The Tol/Pal system of Escherichia coli is composed of the YbgC, TolQ, TolA, TolR, TolB, Pal and YbgF proteins. It is involved in maintaining the integrity of the outer membrane, and is required for the uptake of group A colicins and DNA of filamentous bacteriophages. To identify new interactions between the components of the Tol/Pal system and gain insight into the mechanism of colicin import, we performed a yeast two-hybrid screen using the different components of the Tol/Pal system and colicin A. Using this system, we confirmed the already known interactions and identified several new interactions. TolB dimerizes and the periplasmic domain of TolA interacts with YbgF and TolB. Our results indicate that the central domain of TolA (TolAII) is sufficient to interact with YbgF, that the C-terminal domain of TolA (TolAIII) is sufficient to interact with TolB, and that the amino terminal domain of TolB (D1) is sufficient to bind TolAIII. The TolA/TolB interaction was confirmed by cross-linking experiments on purified proteins. Moreover, we show that the interaction between TolA and TolB is required for the uptake of colicin A and for the membrane integrity. These results demonstrate that the TolA/TolB interaction allows the formation of a trans-envelope complex that brings the inner and outer membranes in close proximity.
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The Tol Pal system integrates maintenance of the three layered cell envelope.
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Department of Biochemistry, South Parks Road, University of Oxford, Oxford, OX1 3QU, UK.
The rapid emergence of antibiotic-resistant superbugs poses a significant global health threat. Gram-negative bacteria are the primary culprits due to their robust, tripartite cell envelope. This review explores the emerging role of the trans-envelope Tol-Pal system in maintaining envelope integrity, by connecting envelope layers and serving as a protein interaction hub.
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