Visualizing Interactions along the Escherichia coli Twin-Arginine Translocation Pathway Using Protein Fragment Complementation

The twin-arginine translocation (Tat) pathway is well known for its ability to export fully folded substrate proteins out of the cytoplasm of Gram-negative and Gram-positive bacteria. Studies of this mechanism in Escherichia coli have identified numerous transient protein-protein interactions that guide export-competent proteins through the Tat pathway. To visualize these interactions, we have adapted bimolecular fluorescence complementation (BiFC) to detect protein-protein interactions along the Tat pathway of living cells. Fragments of the yellow fluorescent protein (YFP) were fused to soluble and transmembrane factors that participate in the translocation process including Tat substrates, Tat-specific proofreading chaperones and the integral membrane proteins TatABC that form the translocase. Fluorescence analysis of these YFP chimeras revealed a wide range of interactions such as the one between the Tat substrate dimethyl sulfoxide reductase (DmsA) and its dedicated proofreading chaperone DmsD. In addition, BiFC analysis illuminated homo- and hetero-oligomeric complexes of the TatA, TatB and TatC integral membrane proteins that were consistent with the current model of translocase assembly. In the case of TatBC assemblies, we provide the first evidence that these complexes are co-localized at the cell poles. Finally, we used this BiFC approach to capture interactions between the putative Tat receptor complex formed by TatBC and the DmsA substrate or its dedicated chaperone DmsD. Our results demonstrate that BiFC is a powerful approach for studying cytoplasmic and inner membrane interactions underlying bacterial secretory pathways

DNA G-segment bending is not the sole determinant of topology simplification by type II DNA topoisomerases

DNA topoisomerases control the topology of DNA. Type II topoisomerases exhibit topology simplification, whereby products of their reactions are simplified beyond that expected based on thermodynamic equilibrium. The molecular basis for this process is unknown, although DNA bending has been implicated. To investigate the role of bending in topology simplification, the DNA bend angles of four enzymes of different types (IIA and IIB) were measured using atomic force microscopy (AFM). The enzymes tested were Escherichia coli topo IV and yeast topo II (type IIA enzymes that exhibit topology simplification), and Methanosarcina mazei topo VI and Sulfolobus shibatae topo VI (type IIB enzymes, which do not). Bend angles were measured using the manual tangent method from topographical AFM images taken with a novel amplitude-modulated imaging mode: small amplitude small set-point (SASS), which optimises resolution for a given AFM tip size and minimises tip convolution with the sample. This gave improved accuracy and reliability and revealed that all 4 topoisomerases bend DNA by a similar amount: ~120° between the DNA entering and exiting the enzyme complex. These data indicate that DNA bending alone is insufficient to explain topology simplification and that the ‘exit gate’ may be an important determinant of this process

Sulfonoquinovosyl diacylglyceride selectively targets acute lymphoblastic leukemia cells and exerts potent anti-leukemic effects in vivo

DNA topoisomerase II inhibitors e.g. doxorubicin and etoposide are currently used in the chemotherapy for acute lymphoblastic leukemia (ALL). These inhibitors have serious side effects during the chemotherapy e.g. cardiotoxicity and secondary malignancies. In this study we show that sulfonoquinovosyl diacylglyceride (SQDG) isolated from Azadirachta indica exerts potent anti-ALL activity both in vitro and in vivo in nude mice and it synergizes with doxorubicin and etoposide. SQDG selectively targets ALL MOLT-4 cells by inhibiting catalytic activity of topoisomerase I enzyme and inducing p53 dependent apoptotic pathway. SQDG treatment induces recruitment of ATR at chromatin and arrests the cells in S-phase. Down-regulation of topoisomerase I or p53 renders the cells less sensitive for SQDG, while ectopic expression of wild type p53 protein in p53 deficient K562 cells results in chemosensitization of the cells for SQDG. We also show that constant ratio combinations of SQDG and etoposide or SDQG and doxorubicin exert synergistic effects on MOLT-4 cell killing. This study suggests that doses of etoposide/ doxorubicin can be substantially reduced by combining SQDG with these agents during ALL chemotherapy and side effects caused can be minimized. Thus dual targeting of topoisomerase I and II enzymes is a promising strategy for improving ALL chemotherap