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Amt/MEP/Rh proteins conduct ammonia

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Abstract

The structure determination of the ammonium transport protein AmtB from Escherichia coli strongly indicates that the members of the ubiquitous ammonium transporter/methylamine permease/Rhesus (Amt/MEP/Rh) protein family are ammonia-conducting channels rather than ammonium ion transporters. The most conserved part of these proteins, apart from the common overall structure with 11 transmembrane helices, is the pore lined by hydrophobic side chains except for two highly conserved histidine residues. A high-affinity ion-binding site specific for ammonium is present at the extracellular pore entry of the Amt/MEP proteins. It is proposed to play an important role in enhancing net transport at very low external ammonium concentrations and to provide discrimination against water. The site is not conserved in the animal Rhesus proteins which are implicated in ammonium homeostasis and saturate at millimolar ammonium concentrations. Many aspects of the biological function of these ammonia channels are still poorly understood and further studies in cellular systems are needed. Likewise, studies with purified, reconstituted Amt/MEP/Rh proteins will be needed to resolve open mechanistic questions and gain a more quantitative understanding of the conduction mechanism in general and for different subfamily representatives.

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Fig. 1
Scheme 1

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Notes

  1. Here, the term ammonium is used when no discrimination between NH3 and NH +4 is made. When specificity is required, chemical symbols or the terms ammonia and ammonium ion are used.

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Acknowledgments

I thank Lei Zheng for providing Fig. 1 and Xiao-Dan Li and Lei Zheng for critical comments on the manuscript. The author’s research was supported by the Swiss National Science Foundation within the framework of the National Center of Competence in Research (NCCR) in Structural Biology.

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  1. Structural Biology, Biomolecular Research, Paul Scherrer Institut, 5232, Villigen, Switzerland

    Fritz K. Winkler

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Winkler, F.K. Amt/MEP/Rh proteins conduct ammonia. Pflugers Arch - Eur J Physiol 451, 701–707 (2006). https://doi.org/10.1007/s00424-005-1511-6

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