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Title: Regulation of type III secretion hierarchy in Shigella flexneri
Author: Röhrich-Dönitz, Anelia Dorothea
Awarding Body: University of Bristol
Current Institution: University of Bristol
Date of Award: 2013
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Type III secretion systems (T3SS) are protein injection devices used by Gram-negative bacteria to manipulate eukaryotic cells. In Shigella, the T3SS is assembled when the environmental conditions are appropriate for invasion. However, secretion is only activated when physical contact of the injection needle with the host cell generates an activation signal. The signal is transmitted to the cytoplasm where it triggers secretion. First, translocators are secreted which form a pore in the host cell membrane. Second, effector proteins are translocated into the host cell. The activation process is controlled by conserved T3SS components: the needle tip proteins IpaD and IpaB, the needle itself and the intracellular gate-keeper protein MxiC. The major tip protein IpaD provides a scaffold for pore-forming translocators. In its absence no needle tip is formed, the T3SS secretes constitutively and is unable to sense host cell contact. Using random mutagenesis combined with a genetic screen we have mapped the region of IpaD required for activation signal generation/transmission and identified an additional intracellular role for IpaD in secretion control. Thus, IpaD has a dual role in secretion regulation. The gate-keeper protein MxiC is a cytoplasmic protein that plays a key role in mediating secretion hierarchy. In its absence, the secretion of translocator proteins is decreased and effector proteins are leaked. We have used site-directed mutagenesis, genetics and analysis of native protein complexes to further characterise its function. While MxiC seems to be a predominantly cytoplasmic and monomeric protein, we show that it acts in the same intracellular pathway as IpaD to control translocator secretion. We have identified the areas of MxiC required for activation signal reception, promoting translocator secretion, blocking premature effector secretion and for regulating its own secretion. We also provide evidence that a conformational change in MxiC might be involved in its function. Taken together, our work suggests how cytoplasmic mechanisms block premature secretion of translocators and effectors and in which steps secretion activation might occur.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available