The integration of tail anchored membrane proteins by the twin-arginine translocase

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All bacteria, whether they are 'friendly' or otherwise, have one thing in common. In order to colonize their niches they need to communicate with the outside world. They achieve this by secreting protein molecules that allow them firstly to detect and then manipulate their environment. Bacteria are surrounded by one or more membranes and a rigid wall, which together form a protective barrier. Secreting proteins into the environment requires that these molecules are able to pass through the membrane barrier. In order to achieve this, bacteria have transporters located in the membrane that allow the passage of proteins to the outside. Understanding how these protein transporters work is critical if we wish to control this process to prevent disease, or engineer microbes to decontaminate toxic environments.We study a protein transporter called the Tat system, that is conserved in almost all bacteria and in plant chloroplasts, and we use E. coli as a convenient model system in which to study these processes. The Tat system plays a very important role in the physiology of many different bacteria and it is essential for photosynthesis in plants. Proteins are made up of long, linear chains of amino acids which fold up after they are made. Proteins are only functional once they have folded into their final 3-dimensional structure. Proteins that are secreted are functional outside the bacterial cell. The Tat system is unusual because unlike most other protein transporters it only transports proteins after they have already folded. Proteins that are destined to be secreted by the Tat system have a special signature sequence of amino acids, termed a 'twin arginine signal' at their start. This signal targets the protein to the Tat machinery that is embedded in the membrane, and facilitates its secretion. The Tat machinery itself is made up of three components - TatA, TatB and TatC. The TatA, TatB and TatC components initially form a 1:1:1 complex with each other and this complex is responsible for recognizing each of the different proteins that are targeted for secretion by Tat system, by interacting with the twin arginine signal. After the signal has been bound by TatABC this triggers additional copies of TatA component to assemble into a ring-like structure, which can then allow transport of the protein. After the protein has been transported the TatA ring disassembles ready for another round of secretion.Some of the Tat substrate proteins are unusual because they contain a membrane interaction domain at their C-terminus that anchors them into the membrane. In E. coli these membrane-anchored Tat substrates are essential for the bacterium to adapt to different growth conditions, and to survive during infection of mammals. We are interested in understanding how the Tat pathway is able to integrate these proteins into the membrane. We have isolated mutants in the Tat system that cannot integrate these proteins, instead secreting them completely across the membrane. We want to understand why these mutants are defective in integrating the Tat substrate proteins. This work will help us to understand how the Tat machinery recognises membrane proteins, and may also help us to understand the mechanism by which the Tat machinery disassembles once transport is complete.