Substrate-transporter complexes in the twin-arginine protein transport system

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Some proteins operate on the outside of the bacterial cell, for example the toxins produced by bacterial pathogens. Since all proteins are made inside the bacterium the extracellular proteins must be moved out of the cell across the normally impermeable cell membrane. This task is carried out by machines termed protein transporters that are located in the cell membrane. One type of transporter moves unfolded proteins, threading them across the membrane like string through the eye of a needle. By contrast, a second type of transporter, which we term the Tat system, moves folded proteins across the membrane. This is much more challenging than threading and so it is thought that the Tat system operates by an unusual mechanism. The Tat system is required for many bacterial processes including energy generation, cell division, pathogenesis, and the nitrogen-fixing symbiosis of soil bacteria with plants. The Tat protein transport system is not only found in bacteria but is also present in the chloroplasts of plants where it is essential to form and maintain the proteins required to carry out photosynthesis. The heart of the Tat system is a complex formed by two proteins called TatB and TatC. This complex sits in the cell membrane and recognises and binds the proteins that are to be transported. It then binds another protein called TatA to form the active transporter. This project aims to characterise the complexes formed between the Tat components and the transported protein in more detail with the aim of increasing our understanding of the unusual mechanism of the Tat system. The Tat system is a possible drug target because it is required for bacterial pathogenesis but is not found in humans. It is also of biotechnological interest because it could be utilised to secrete useful protein products.