The TatA component of the twin-arginine protein transport system forms channel complexes of variable diameter

doi:10.1073/pnas.0503558102

The TatA component of the twin-arginine protein transport system forms channel complexes of variable diameter

^*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 ^∥Department of Molecular Microbiology, John Innes Centre, Norwich NR4 7UH, United Kingdom

Edited by William T. Wickner, Dartmouth Medical School, Hanover, NH (received for review April 29, 2005)

Abstract

The Tat system mediates Sec-independent transport of folded precursor proteins across the bacterial plasma membrane or the chloroplast thylakoid membrane. Tat transport involves distinct high-molecular-weight TatA and TatBC complexes. Here we report the 3D architecture of the TatA complex from Escherichia coli obtained by single-particle electron microscopy and random conical tilt reconstruction. TatA forms ring-shaped structures of variable diameter in which the internal channels are large enough to accommodate known Tat substrate proteins. This morphology strongly supports the proposal that TatA forms the protein-conducting channel of the Tat system. One end of the channel is closed by a lid that might gate access to the channel. On the basis of previous protease accessibility measurements, the lid is likely to be located at the cytoplasmic side of the membrane. The observed variation in TatA diameter suggests a model for Tat transport in which the number of TatA protomers changes to match the size of the channel to the size of the substrate being transported. Such dynamic close packing would provide a mechanism to maintain the membrane permeability barrier during transport.

Footnotes

↵ † To whom correspondence should be sent at the present address: PSF Biotech AG, Heubnerweg 6, 14059 Berlin, Germany. E-mail: gohlke{at}psf-ag.com.
↵ ‡ Present address: Department of Biochemistry and Molecular Biology, University of Texas Medical School, Houston, TX 77030.
This paper was submitted directly (Track II) to the PNAS office.