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Francisella bacteria rely on a phylogenetically distinct type VI secretion system (T6SS) to escape host phagosomes and cause the fatal disease tularemia, but the structural and molecular mechanisms involved are unknown. Here we report the atomic structure of the Francisella T6SS central spike complex, obtained by cryo-electron microscopy. Our structural and functional studies demonstrate that, unlike the single-protein spike composition of other T6SS subtypes, Francisella T6SS's central spike is formed by two proteins, PdpA and VgrG, akin to T4-bacteriophage gp27 and gp5, respectively, and that PdpA has unique characteristics, including a putative cargo within its cavity and an N-terminal helical lid. Structure-guided mutagenesis demonstrates that the PdpA N-terminal lid and C-terminal spike are essential to Francisella T6SS function. PdpA is thus both an adaptor, connecting VgrG to the tube, and a likely carrier of secreted cargo. These findings are important to understanding Francisella pathogenicity and designing therapeutics to combat tularemia.
Pubmed ID: 31677891
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The MRC Laboratory of Molecular Biology (LMB) has long been, and remains, a world-class research laboratory. Our primary goal is to understand biological processes at the molecular level, through the application of methods drawn from physics, chemistry and genetics. This quest extends from structural studies of individual macromolecules, through their interactions and beyond to the functioning of subcellular systems, cells and multicellular systems in whole organisms, with the ultimate aim of using this knowledge to tackle specific problems in human health and disease. The LMB is one of the birthplaces of modern molecular biology. Many techniques were pioneered at the laboratory, most notably methods for determining the three-dimensional structure of proteins and DNA sequencing. Whole genome sequencing was initiated at the LMB. Another landmark discovery was the invention of monoclonal antibodies. Over the years, the work of LMB scientists has attracted 9 Nobel Prizes, shared between 13 LMB scientists, as well as numerous other prizes and scientific awards.
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