A molecular mechanism of direction switching in the flagellar motor of Escherichia coli
Koushik Paul, Duncan Brunstetter, Sienna Titen, and David F. Blair1
Author Affiliations
Department of Biology, University of Utah, Salt Lake City, UT 84112
Edited by Howard C. Berg, Harvard University, Cambridge, MA, and approved September 2, 2011 (received for review June 23, 2011)
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National Science Foundation
Abstract
The direction of flagellar rotation is regulated by a rotor-mounted protein assembly, termed the “switch complex,” formed from multiple copies of the proteins FliG, FliM, and FliN. The structures of major parts of these proteins are known, and the overall organization of proteins in the complex has been elucidated previously using a combination of protein-binding, mutational, and cross-linking approaches. In Escherichia coli, the switch from counterclockwise to clockwise rotation is triggered by the signaling protein phospho-CheY, which binds to the lower part of the switch complex and induces small movements of FliM and FliN subunits relative to each other. Direction switching also must produce movements in the upper part of the complex, particularly in the C-terminal domain of FliG (FliGC), which interacts with the stator to generate the torque for flagellar rotation. In the present study, protein movements in the middle and upper parts of the switch complex have been probed by means of targeted cross-linking and mutational analysis. Switching induces a tilting movement of the FliM domains that form the middle part of the switch and a consequent rotation of the affixed FliGC domains that reorients the stator interaction sites by about 90°. In a recently proposed hypothesis for the motor mechanism, such a reorientation of FliGC would reverse the direction of motor rotation.
molecular machines, motility, signal transduction
Footnotes
1To whom correspondence should be addressed. E-mail:blair@bioscience.utah.edu.
Author contributions: K.P. and D.F.B. designed research; K.P., D.B., and S.T. performed research; K.P. and D.F.B. contributed new reagents/analytic tools; K.P., D.B., S.T., and D.F.B. analyzed data; and K.P. and D.F.B. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1110111108/-/DCSupplemental.
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