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首页> 外文期刊>Proceedings of the National Academy of Sciences of the United States of America >Direct observation of electrogenic NH_4~+ transport in ammonium transport (Amt) proteins
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Direct observation of electrogenic NH_4~+ transport in ammonium transport (Amt) proteins

机译：Direct observation of electrogenic NH_4~+ transport in ammonium transport (Amt) proteins

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摘要

Ammonium transport (Amt) proteins form a ubiquitous family of integral membrane proteins that specifically shuttle ammonium across membranes. In prokaryotes, archaea, and plants, Amts are used as environmental NH_4~+ scavengers for uptake and assimilation of nitrogen. In the eukaryotic homologs, the Rhesus proteins, NH_4~+/NH_3 transport is used instead in acid–base and pH homeostasis in kidney or NH_4~+/NH_3 (and eventually CO_2) detoxification in erythrocytes. Crystal structures and variant proteins are available, but the inherent challenges associated with the unambiguous identification of substrate and monitoring of transport events severely inhibit further progress in the field. Here we report a reliable in vitro assay that allows us to quantify the electrogenic capacity of Amt proteins. Using solid-supported membrane (SSM)-based electrophysiology, we have investigated the three Amt orthologs from the euryarchaeon Archaeoglobus fulgidus. Af-Amt1 and Af-Amt3 are electrogenic and transport the ammonium and methylammonium cation with high specificity. Transport is pH-dependent, with a steep decline at pH values of ～5.0. Despite significant sequence homologies, functional differences between the three proteins became apparent. SSM electrophysiology provides a long-sought-after functional assay for the ubiquitous ammonium transporters.

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