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Structural model for the protein-translocating element of the twin-arginine transport system

Significance The twin-arginine transport system (Tat) has the remarkable ability of transporting folded proteins across membranes while avoiding uncontrolled ion leakage. Tat is essential for plant photosynthesis and is required for bacterial pathogenesis. The mechanism by which folded proteins are translocated is poorly understood. We have determined the structure of the TatA oligomer, which is responsible for the translocation step, and evaluated its impact on lipid bilayers. The results suggest a mechanism of protein translocation involving thinning and perturbing the membrane bilayer. The approach used here will be useful for structural analysis of other oligomeric proteins that weakly assemble in the membrane. The twin-arginine translocase (Tat) carries out the remarkable process of translocating fully folded proteins across the cytoplasmic membrane of prokaryotes and the thylakoid membrane of plant chloroplasts. Tat is required for bacterial pathogenesis and for photosynthesis in plants. TatA, the protein-translocating element of the Tat system, is a small transmembrane protein that assembles into ring-like oligomers of variable size. We have determined a structural model of the Escherichia coli TatA complex in detergent solution by NMR. TatA assembly is mediated entirely by the transmembrane helix. The amphipathic helix extends outwards from the ring of transmembrane helices, permitting assembly of complexes with variable subunit numbers. Transmembrane residue Gln8 points inward, resulting in a short hydrophobic pore in the center of the complex. Simulations of the TatA complex in lipid bilayers indicate that the short transmembrane domain distorts the membrane. This finding suggests that TatA facilitates protein transport by sensitizing the membrane to transient rupture.