Allosteric Signaling Is Bidirectional in an Outer-Membrane Transport Protein.
In BtuB, the Escherichia coli TonB-dependent transporter for vitamin B12, substrate binding to the extracellular surface unfolds a conserved energy coupling motif termed the Ton box into the periplasm. This transmembrane signaling event facilitates an interaction between BtuB and the inner-membrane protein TonB. In this study, continuous-wave and pulse electron paramagnetic resonance in a native outer-membrane preparation demonstrate that signaling also occurs from the periplasmic to the extracellular surface in BtuB. The binding of a TonB fragment to the periplasmic interface alters the configuration of the second extracellular loop and partially dissociates a spin-labeled substrate analog. Moreover, mutants in the periplasmic Ton box that are transport-defective alter the binding site for vitamin B12in BtuB. This work demonstrates that the Ton box and the extracellular substrate binding site are allosterically coupled in BtuB, and that TonB binding may initiate a partial round of transport.
Department of Chemistry and Center for Membrane Biology, University of Virginia, Charlottesville, Virginia.
Arthur
A
Sikora
Institute of Physical and Theoretical Chemistry and Center for Biomolecular Magnetic Resonance, University of Frankfurt, Frankfurt am Main, Germany.
Benesh
B
Joseph
Department of Chemistry and Center for Membrane Biology, University of Virginia, Charlottesville, Virginia.
Morgan
M
Matson
Department of Chemistry and Center for Membrane Biology, University of Virginia, Charlottesville, Virginia.
Jacob R
JR
Staley
Department of Chemistry and Center for Membrane Biology, University of Virginia, Charlottesville, Virginia. Electronic address: cafiso@virginia.edu.
David S
DS
Cafiso
D000494:Allosteric Regulation; D001425:Bacterial Outer Membrane Proteins / Q000737:chemistry* / Q000378:metabolism*; D001426:Bacterial Proteins / Q000737:chemistry / Q000378:metabolism; D001692:Biological Transport; D002118:Calcium / Q000378:metabolism; D002462:Cell Membrane / Q000378:metabolism; D004926:Escherichia coli / Q000166:cytology / Q000378:metabolism; D029968:Escherichia coli Proteins / Q000737:chemistry* / Q000378:metabolism*; D005110:Extracellular Space / Q000378:metabolism; D008565:Membrane Proteins / Q000737:chemistry / Q000378:metabolism; D026901:Membrane Transport Proteins / Q000737:chemistry* / Q000378:metabolism*; D008958:Models, Molecular; D011487:Protein Conformation; D015398:Signal Transduction; D013113:Spin Labels; D014805:Vitamin B 12 / Q000378:metabolism
D001425:Bacterial Outer Membrane Proteins; D001426:Bacterial Proteins; C044592:BtuB protein, E coli; D029968:Escherichia coli Proteins; D008565:Membrane Proteins; D026901:Membrane Transport Proteins; D013113:Spin Labels; C057616:tonB protein, Bacteria; D014805:Vitamin B 12; D002118:Calcium