The TatA component of the twin-arginine protein transport system forms channel complexes of variable diameter Ulrich Gohlke*†, Lee Pullan*‡, Christopher A. McDevitt§, Ida Porcelli§, Erik de Leeuw§, Tracy Palmer¶ʈ, Helen R. Saibil*, and Ben C. Berks§ *Institute of Structural Molecular Biology, School of Crystallography, Birkbeck College, Malet Street, London WC1E 7HX, United Kingdom; §Department of Biochemistry, University of Oxford, South Parks Road, Oxford OX1 3QU, United Kingdom; ¶School of Biological Sciences, University of East Anglia, Norwich NR4 7TJ, United Kingdom; and ʈDepartment of Molecular Microbiology, John Innes Centre, Norwich NR4 7UH, United Kingdom Edited by William T. Wickner, Dartmouth Medical School, Hanover, NH, and approved June 8, 2005 (received for review April 29, 2005) The Tat system mediates Sec-independent transport of folded diameter (15, 17). TatBC has been shown to act as the receptor precursor proteins across the bacterial plasma membrane or the element of the translocation pathway (13, 16, 18), and TatC has chloroplast thylakoid membrane. Tat transport involves distinct been shown to contain the primary binding site for the signal high-molecular-weight TatA and TatBC complexes. Here we report peptide (18). The function of TatA is less clear. It is known that the 3D architecture of the TatA complex from Escherichia coli TatA is required subsequent to substrate recognition by the obtained by single-particle electron microscopy and random con- TatBC complex (16, 18, 19), and this has led to the suggestion ical tilt reconstruction. TatA forms ring-shaped structures of vari- that TatA constitutes the protein-conducting channel of the Tat able diameter in which the internal channels are large enough to system. Recent chemical crosslinking studies suggest that TatA accommodate known Tat substrate proteins. This morphology transiently associates with the TatBC–substrate complex during strongly supports the proposal that TatA forms the protein- active protein translocation (18, 19). conducting channel of the Tat system. One end of the channel is Like other protein translocation systems, the Tat pathway is closed by a lid that might gate access to the channel. On the basis presumed to form an aqueous transmembrane channel for of previous protease accessibility measurements, the lid is likely to protein transport. It is vital that there is no ion leakage through be located at the cytoplasmic side of the membrane. The observed this channel, because the bacterial cytoplasmic membrane has to variation in TatA diameter suggests a model for Tat transport in which the number of TatA protomers changes to match the size of maintain transmembrane proton and other ion electrochemical the channel to the size of the substrate being transported. Such gradients to drive essential cellular functions. To preserve the dynamic close packing would provide a mechanism to maintain the ionic permeability barrier of the membrane, opening of the Tat membrane permeability barrier during transport. channel must be gated by substrate. In addition, a mechanism is required to prevent ion leakage around the translocating sub- conical tilt reconstruction ͉ electron microscopy ͉ Tat protein transport ͉ strate protein during transport. These considerations also apply three-dimensional structure ͉ twin-arginine signal peptide to the Sec channel present in the same membrane. However, the mechanistic challenges posed for the Tat channel are consider- any proteins function in extracytoplasmic compartments ably greater than for the Sec channel. Because some folded E. Msuch as the periplasm in Gram-negative bacteria or the coli Tat substrates are close to 70 Å in diameter (20), the Tat chloroplast in plants. In bacteria, most exported proteins are system must be able to form a channel of this size as opposed to moved across the cytoplasmic membrane by the Sec pathway (1). the 12-Å channel needed to thread unfolded chains by the Sec This translocation occurs by a threading mechanism in which the translocase (1). Concerted helix tilting in the membrane may be substrate adopts an extended conformation. By contrast, the Tat sufficient to gate the pore in the Sec apparatus (1), but a radically system exports folded proteins across the cytoplasmic membrane different mechanism seems necessary to provide the huge (2). Substrates of the Tat pathway have N-terminal signal conformational change required to gate the Tat channel. An sequences containing an S-R-R-x-F-L-K consensus motif in additional challenge for the Tat system is that, in contrast to the which the arginine residues are almost invariant (3). Tat trans- Sec pathway, substrate proteins vary widely in size. If the channel port is energized by the transmembrane proton electrochemical needs a diameter of 70 Å to accommodate the largest substrates, gradient (4). The Tat system is vital for many bacterial processes, how is ion leakage between the substrate protein and the walls including energy metabolism, formation of the cell envelope, of the channel prevented when the smallest substrates (20–30 Å biofilm formation, heavy metal resistance, nitrogen-fixing sym- in diameter) (21) are being transported? biosis, and bacterial pathogenesis (5). A Tat system is also found In this report we present the 3D density maps of the TatA in the thylakoid membrane of plant chloroplasts, where it plays complex from E. coli obtained by random conical tilt electron an essential role in the biogenesis of the photosynthetic electron microscopy of negatively stained specimens. This is the first 3D transport chain (6). structure determination for any component of the Tat translo- In Escherichia coli, the minimal components of the Tat cation pathway. It strongly supports the hypothesis that TatA translocation system are the integral membrane proteins TatA, forms the protein-conducting channel of the Tat system and TatB, and TatC (7–11). Attempts to purify the Tat components suggests possible mechanisms by which the channel can transport from E. coli membranes have led to the identification of two distinct high-molecular-mass complexes, one corresponding to TatA and the other containing predominantly TatB and TatC This paper was submitted directly (Track II) to the PNAS office. (12–15). Both types contain multiple subunits per complex. †To whom correspondence should be sent at the present address: PSF Biotech AG, Analogous complexes have been described in de-energized Heubnerweg 6, 14059 Berlin, Germany. E-mail: [email protected]. thylakoids (16). Projection maps of Tat complexes obtained by ‡Present address: Department of Biochemistry and Molecular Biology, University of Texas negative stain electron microscopy show that each type of Medical School, Houston, TX 77030. complex forms particles of Ϸ90–160 Å (1 Å ϭ 0.1 nm) in © 2005 by The National Academy of Sciences of the USA 10482–10486 ͉ PNAS ͉ July 26, 2005 ͉ vol. 102 ͉ no. 30 www.pnas.org͞cgi͞doi͞10.1073͞pnas.0503558102 Downloaded by guest on September 26, 2021 Fig. 2. Classification of TatA complexes. (A) The untilted data were sorted into 10 classes according to size (28). The averages of all images in each class are shown. The number of images per class ranges from 86 (class 9) to 249 (class 3). (B) Averages of the untilted data in A after reference-free align- ment. Four representative classes used in the interpretation of the 3D analysis are outlined. Pairs of low-dose exposures were taken of each selected area, the Fig. 1. Characterization of TatA complexes. (A and B) Micrographs of TatA, first at a nominal tilt angle of 45° and the second at 0°. The stained with uranyl acetate. (A) Untilted specimen. (B) Forty-five-degree tilted ␮ view of the same area. The tilt axis runs horizontally. Corresponding particles average defocus was 1.5 m. Negatives were developed in Kodak are marked in each micrograph (white circles). A few large, collapsed vesicles, full-strength developer for 12 min, and their quality was assessed probably consisting of lipid and͞or detergent are also present. (C) SDS͞PAGE by optical diffraction. Micrographs were digitized on a SCAI analysis and Coomassie brilliant blue staining of the sample used for image microdensitometer (Zeiss) with a pixel size of 14 ␮m. analysis. The molecular masses (kDa) of marker proteins are given on the left. The band corresponding to TatA as well as the expected positions of TatB and Image Processing. Image analysis was performed by using TatC are indicated on the right. (D) Immunodetection of Tat proteins in the IMAGIC-5 (24) and SPIDER/WEB software (25). The random con- TatA sample used for image analysis. The sample was subjected to SDS͞PAGE ical tilt analysis (26) was based on SPIDER scripts written by N. followed by immunoblotting with the indicated subunit-specific antisera. Identical exposures were used in all three cases. (E) Blue native PAGE analysis Boisset (Universite´Pierre et Marie Curie, Paris). of the sample used for image analysis, stained by Coomassie brilliant blue. The Particles were picked interactively using the ‘‘tilted particles’’ molecular masses (kDa) of marker proteins are given on the left. Arrows on the option in WEB, which provided the tilt geometry for each right indicate bands corresponding to the variable-size TatA complexes negative. CTFTILT3 (27) was used to calculate the defocus in present in the sample. each tilted micrograph and to verify the tilt geometry found during particle picking. The particles were corrected for the effect of the contrast transfer function by phase-flipping in folded proteins while maintaining the ionic permeability barrier SPIDER according to their positions on the micrograph. of the membrane. The untilted particles were centered by multi-reference Experimental Methods alignment. Seven soft-edged discs with radii of 14–20 pixels were used as initial references for translational alignment. Protein Chemistry. E. coli TatA with a C-terminal hexahistidine Next, the centered images were sorted by size by using the tag was overproduced from plasmid pFAT75AH (13) in E.