Novel twin-arginine translocase (Tat) complexes from Gram-positive bacteria

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Bacteria export many essential proteins across the plasma membrane that surrounds the cell, and the export mechanisms have attracted a great deal of interest. One reason for this interest is the complexity of the processes involved: the plasma membrane is designed to be tightly sealed and the transport of a large protein molecule is an inherently difficult process. Another factor is that these transport mechanisms represent excellent potential targets for novel anti-microbial compounds, since they are surface-exposed and there are no homologues in mammals. One protein export system of particular interest is the twin-arginine translocation (Tat) system, because this exports fully-folded proteins across this membrane by an unknown mechanism. Previous studies have focused on Gram-negative bacteria (which have 2 cell membranes) but we have recently found that the Tat system of the Gram-positive Bacillus subtilis has features that are very different in fundamental respects (yet the system still functions when expressed in a Gram-negative cell, indicating a common translocation mechanism). The aim of the work is to characterise the two Tat complexes of this organism, and determine how they come together to transport proteins. We will study the composition of the substrate-binding TatAC complex, study the structure and function of the uniquely simple TatA complex, and generate high-resolution structural data through a combination of electron microscopy and X-ray crystallography.