Probing the mechanism of protein export by the bacterial Tat transport system

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All bacteria produce proteins that operate on the outside of the bacterial cell. A good example of this is 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. The Tat system is found in many different bacteria, including those which cause human diseases such as Mycobacterium tuberculosis (which causes tuberculosis) and Salmonella (which causes food poisoning). Scientists have shown for many different pathogenic bacteria that the Tat system is essential for them to cause disease, and they are becoming increasingly interested in developing drugs that prevent the Tat system from working.In the bacterium E. coli, the Tat transporter is a large protein complex made up of three types of proteins, named TatA, TatB and TatC. These proteins are organised into two different sub-complexes. There is a sub-complex made up of equal amounts of TatB and TatC. This sub-complex plays a very important role in recognising the proteins that are going to be transported across the membrane. The other sub-complex is made up of many copies of the TatA protein. This forms the channel in the membrane through which the proteins are transported.In order to understand how the Tat machinery works, and ultimately to design new drugs to stop it working, we need to know how the different proteins (or subunits) in the sub-complexes are arranged. This application is aimed at understanding how the TatBC sub-complex is organised. Some of the questions we want to answer in this project are (i) where are the interfaces between the TatB and TatC proteins located? (ii) How do the interfaces change to allow dynamic movement of TatB and TatC during the transport of proteins through the Tat pathway? Finally we know that during the transport of proteins through the Tat system the TatA and TatBC subcomplexes come together to transport proteins. We believe that when this happens TatA might bind to TatC in the same way that TatB does, and we would like to test this by carrying out similar experiments.