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Title: Electrostatic lock in the transport cycle of the multidrug resistance transporter EmrE

We report that EmrE is a small, homodimeric membrane transporter that exploits the established electrochemical proton gradient across the Escherichia coli inner membrane to export toxic polyaromatic cations, prototypical of the wider small-multidrug resistance transporter family. While prior studies have established many fundamental aspects of the specificity and rate of substrate transport in EmrE, low resolution of available structures has hampered identification of the transport coupling mechanism. Here we present a complete, refined atomic structure of EmrE optimized against available cryo-electron microscopy (cryo-EM) data to delineate the critical interactions by which EmrE regulates its conformation during the transport process. With the model, we conduct molecular dynamics simulations of the transporter in explicit membranes to probe EmrE dynamics under different substrate loading and conformational states, representing different intermediates in the transport cycle. The refined model is stable under extended simulation. The water dynamics in simulation indicate that the hydrogen-bonding networks around a pair of solvent-exposed glutamate residues (E14) depend on the loading state of EmrE. One specific hydrogen bond from a tyrosine (Y60) on one monomer to a glutamate (E14) on the opposite monomer is especially critical, as it locks the protein conformation when the glutamate is deprotonated. The hydrogen bondmore » provided by Y60 lowers the pK a of one glutamate relative to the other, suggesting both glutamates should be protonated for the hydrogen bond to break and a substrate-free transition to take place. In conclusion, these findings establish the molecular mechanism for the coupling between proton transfer reactions and protein conformation in this proton-coupled secondary transporter.« less
Authors:
ORCiD logo [1] ;  [2] ; ORCiD logo [3]
  1. University of Illinois at Urbana–Champaign, Urbana, IL (United States); Sandia National Lab. (SNL-NM), Albuquerque, NM (United States)
  2. Sandia National Lab. (SNL-NM), Albuquerque, NM (United States)
  3. University of Illinois at Urbana–Champaign, Urbana, IL (United States)
Publication Date:
Report Number(s):
SAND-2018-12663J
Journal ID: ISSN 0027-8424; 670037
Grant/Contract Number:
AC04-94AL85000; NA0003525; AC52-06NA25296
Type:
Accepted Manuscript
Journal Name:
Proceedings of the National Academy of Sciences of the United States of America
Additional Journal Information:
Journal Volume: 115; Journal Issue: 32; Journal ID: ISSN 0027-8424
Publisher:
National Academy of Sciences, Washington, DC (United States)
Research Org:
Sandia National Lab. (SNL-NM), Albuquerque, NM (United States)
Sponsoring Org:
USDOE National Nuclear Security Administration (NNSA)
Country of Publication:
United States
Language:
English
Subject:
59 BASIC BIOLOGICAL SCIENCES; 37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; molecular dynamics; membrane protein; structure refinement; proton-coupled transport
OSTI Identifier:
1485839
Alternate Identifier(s):
OSTI ID: 1460924

Vermaas, Josh V., Rempe, Susan B., and Tajkhorshid, Emad. Electrostatic lock in the transport cycle of the multidrug resistance transporter EmrE. United States: N. p., Web. doi:10.1073/pnas.1722399115.
Vermaas, Josh V., Rempe, Susan B., & Tajkhorshid, Emad. Electrostatic lock in the transport cycle of the multidrug resistance transporter EmrE. United States. doi:10.1073/pnas.1722399115.
Vermaas, Josh V., Rempe, Susan B., and Tajkhorshid, Emad. 2018. "Electrostatic lock in the transport cycle of the multidrug resistance transporter EmrE". United States. doi:10.1073/pnas.1722399115.
@article{osti_1485839,
title = {Electrostatic lock in the transport cycle of the multidrug resistance transporter EmrE},
author = {Vermaas, Josh V. and Rempe, Susan B. and Tajkhorshid, Emad},
abstractNote = {We report that EmrE is a small, homodimeric membrane transporter that exploits the established electrochemical proton gradient across the Escherichia coli inner membrane to export toxic polyaromatic cations, prototypical of the wider small-multidrug resistance transporter family. While prior studies have established many fundamental aspects of the specificity and rate of substrate transport in EmrE, low resolution of available structures has hampered identification of the transport coupling mechanism. Here we present a complete, refined atomic structure of EmrE optimized against available cryo-electron microscopy (cryo-EM) data to delineate the critical interactions by which EmrE regulates its conformation during the transport process. With the model, we conduct molecular dynamics simulations of the transporter in explicit membranes to probe EmrE dynamics under different substrate loading and conformational states, representing different intermediates in the transport cycle. The refined model is stable under extended simulation. The water dynamics in simulation indicate that the hydrogen-bonding networks around a pair of solvent-exposed glutamate residues (E14) depend on the loading state of EmrE. One specific hydrogen bond from a tyrosine (Y60) on one monomer to a glutamate (E14) on the opposite monomer is especially critical, as it locks the protein conformation when the glutamate is deprotonated. The hydrogen bond provided by Y60 lowers the pKa of one glutamate relative to the other, suggesting both glutamates should be protonated for the hydrogen bond to break and a substrate-free transition to take place. In conclusion, these findings establish the molecular mechanism for the coupling between proton transfer reactions and protein conformation in this proton-coupled secondary transporter.},
doi = {10.1073/pnas.1722399115},
journal = {Proceedings of the National Academy of Sciences of the United States of America},
number = 32,
volume = 115,
place = {United States},
year = {2018},
month = {7}
}

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