Structural studies of the inner-membrane platform of the bacterial type II secretion system
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The type II secretion system (T2SS) is widespread in Gram-negative bacteria that cause disease in animals and plants. In human and animal pathogens toxins are secreted (e.g. cholera toxin) and in plant pathogens lytic enzymes that breakdown the plant cell wall are exported in to the extracellular milieu (e.g. pectate lyase). Structurally the T2SS comprises at least 11 core proteins that form three major subassemblies spanning the inner-membrane, periplasmic space and outer-membrane: (i) the inner-membrane platform and associated cytoplasmic ATPase (E); (ii) the pseudopilus, which consists of five pseudopilins, G to K; and (iii) a large, pore-forming outer-membrane complex secretin D. The inner-membrane platform comprises three single transmembrane helix proteins, and one three transmembrane helix protein, OutF. The evidence from cryo-electron microscopy on the related type IVa pilus machine (T4PS) places the protein corresponding to OutF at the centre of this platform. This platform is responsible for assembling the pilus and for communicating between the periplasm and the cytoplasmic ATPase. To date, no high-resolution structure of a full-length OutF/PilC family protein is available. A low-resolution electron microscopy reconstruction of isolated PilG (PilC ortholog from Neisseria meningitides T4PS) showed a tetrameric two lobed structure. Here I report the results of studying the structure of the inner-membrane protein OutF from Dickeya dadantii and the complete inner-membrane platform comprising 9 proteins: OutEFGHIJKLM. This work involved cloning the corresponding operon, purifying the proteins, and using crystallography and electron microscopy. Key results reported here are the crystal structure of the first cytoplasmic domain of Dickeya dadantii, OutF65-172 and a preliminary three-dimensional model of the Dickeya dadantii inner-membrane platform. This model, and higher-resolution models to come, will provide valuable information about the oligomeric state, and arrangement of the inner-membrane proteins. These studies will help us to understand how the type II secretion system works.
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