Protein secretion via the Twin-arginine translocation pathway of Bacillus subtilis

The term ‘protein secretion’ summarises a complex mechanism by which (bacterial) cells transport proteins across membranes. Protein secretion by bacteria is an extensively studied subject, because the understanding of the mechanism behind it enables us to apply it for biotechnological and pharmaceutical purposes. The Gram-positive, soil-dwelling bacterium Bacillus subtilis for instance, is already used for the production and secretion of many useful proteins, such as enzymes, for the development of food-related and non food-related products. For quick and efficient secretion of proteins to the extracellular environment, bacteria have developed several mechanisms that enable the translocation of the proteins across the membrane of the cell. The Twin-arginine translocation (Tat) pathway is one of these mechanisms, which is able to secrete folded proteins. Bacillus subtilis contains two of these pathways, which are very alike but operate independently and display a different substrate specificity. The underlying mechanism of the substrate specificity of the two Tat pathways of Bacillus subtilis is the main topic of this thesis. Detailed studies concerning the Tat components, the complexes they form and the proteins that are translocated by those complexes are described in the experimental chapters of the thesis. The main conclusion is that the substrate specificity is most likely determined by a combination of the growth conditions, the organism and specific properties of the Tat components and the Tat substrate. This provided novel leads for optimisation of the identification of general elements that can guarantee efficient secretion of a wide variety of proteins. The application of the Bacillus subtilis Tat system in the biotechnolocial industry is discussed.