Abstract
The twin-arginine translocation (Tat) system of bacteria and plant plastids serves to translocate folded proteins across energized biological membranes. In Escherichia coli, the three components TatA, TatB, and TatC mediate this membrane passage. Here we demonstrate that TatA can assemble to form clusters of tube-like structures in vivo. While the presence of TatC is essential for their formation, TatB is not required. The TatA tubes have uniform outer and inner diameters of about 11.5 nm and 6.7 nm, respectively. They align to form a crystalline-like structure in which each tube is surrounded by six TatA tubes. The tube structures become easily detectable even at only a 15-fold overexpression of the tatABC genes. The TatA tubes could also be visualized by fluorescence when untagged TatA was mixed with low amounts of TatA-GFP. The structures were often found in contact with the cell poles. Because TatC is most likely polar in E. coli, as demonstrated by a RR-dependent targeting of translocation-incompatible Tat substrates to the cell poles, and because TatC is required for the formation of aligned TatA tubes, it is proposed that the TatA tubes are initiated at polarly localized TatC.
Highlights
Folded proteins can be transported across membranes by the twin-arginine translocation (Tat)3 system [1]
The association of TatA with the TatBC complex occurs strictly after substrate binding [6], whereas TatA of Gram-positive bacteria has been detected inside the cytoplasm, where it has a high affinity for Tat substrates that might be targeted by TatA to the membranes (8 –10)
Complexes might result from disassembly or reorganization of much larger complexes that could possibly be detectable by electron microscopy of cells after high pressure freeze-fixation (HPF), a method that can preserve sensitive structures within the cells
Summary
Folded proteins can be transported across membranes by the twin-arginine translocation (Tat) system [1]. The association of TatA with the TatBC complex occurs strictly after substrate binding [6], whereas TatA of Gram-positive bacteria has been detected inside the cytoplasm, where it has a high affinity for Tat substrates that might be targeted by TatA to the membranes (8 –10). TatA of the E. coli Tat system is known to form homooligomeric complexes [11]. It has been purified from detergent-solubilized membrane fractions of strains overexpressing the tatABC genes [12]. E. coli TatA has a previously unrecognized capacity to extend in three dimensions Because these associations can sediment with membranes due to their size, the proposed strict membrane integral localization of E. coli TatA has to be questioned. The Tat systems from Gram-positive and Gram-negative bacteria appear to be more similar than previously thought
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