X-ray crystallographic studies of the TatBC complex - linchpin of the twin-arginine protein transport system

Some bacterial proteins operate on the outside of the 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 protein transport system is not only found in bacteria it 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. We aim to determine a detailed structure of the complex containing TatB and TatC. We will also try to determine the structure of TatC alone since this may be easier and TatC appears to form the stable functional core of the Tat system. To obtain these structures we first need to grow crystals of the purified proteins. This is likely to be quite challenging because it is not easy to form crystals of proteins found in membranes. For this reason we have proposed a number or different crystallization approaches to maximize our chances of success. We will also make full use of the latest high-throughput technologies to speed the work. Determination of a structure for TatBC and/or TatC is essential if we are to understand the unusual mechanism of transport by 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.

The membrane proteins TatA, TatB and TatC are the essential components of the Tat protein transport pathway. The complex formed by the TatB and TatC proteins is the linchpin of the Tat pathway. It is the membrane receptor for substrate proteins and it recruits TatA to form the active translocation machinery. We aim to determine a high resolution structure for the key TatBC complex, and/or the potentially more crystallographically amenable TatC core complex, using X-ray crystallographic methods. Building on our success in the overexpression and purification of the TatBC complex of Escherichia coli our initial efforts will be concentrated on crystallizing TatBC and TatC from this source. We will utilise not only the native complexes but also engineered variants and complexes putatively stabilized by site-specific disulfide crosslinks or by the presence of a substrate signal peptide. In addition to the crystallization trials with the E. coli proteins we will attempt to increase the chances of successful structure determination by undertaking high throughput heterologous expression and crystallization trials of TatC and TatBC from a range of other prokaryotes.