Print ISSN 1933-6950; Online ISSN 1933-6969

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Research Papers

Role of Positively Charged Amino Acids in the M2_D Transmembrane Helix of Ktr/Trk/HKT Type Cation Transporters

Naoki Kato, Masaro Akai, Lalu Zulkifli, Nobuyuki Matsuda, Yasuhiro Kato, Shinobu Goshima, Akihiro Hazama, Mutsumi Yamagami, Robert H. Guy and Nobuyuki Uozumi

volume 1 | issue 3

May/June 2007
Pages: 161 - 171

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Studies suggest that Ktr/Trk/HKT type transporters have evolved from multiple gene fusions of simple K⁺ channels of the KcsA type into proteins that span the membrane at least eight times. Several positively charged residues are present in the eighth transmembrane segment, M2_D, in the transporters but not K⁺ channels. Some models of ion transporters require a barrier to prevent free diffusion of ions down their electrochemical gradient, and it is possible that the positively charged residues within the transporter pore may prevent transporters from being channels. Here we studied the functional role of these positive residues in three Ktr/Trk/HKT type transporters (Synechocystis KtrB mediated K⁺ uniporter, Arabidopsis AtHKT1 mediated Na uniporter and wheat TaHKT1 mediated K⁺/Na symporter) by examining K⁺ uptake rates in E. coli, electrophysiological measurements in oocytes and growth rates of E. coli and yeast. The conserved Arg near the middle of the M2_D segment was essential for the K⁺ transport activity of KtrB and plant HKTs. Combined replacement of several positive residues in TaHKT1 showed that the positive residue at the beginning of the M2_D, which is conserved in many K⁺ channels, also contributed to cation transport activity. This positive residue and the conserved Arg both face towards the ion conducting pore side. We introduced an atomic scale homology model for predicting amino acid interactions. Based on the experimental results and the model, we propose that a salt bridge(s) exists between positive residues in the M2_D and conserved negative residues in the pore region to reduce electrostatic repulsion against cation permeation caused by the positive residue(s). This salt bridge may help stabilize the transporter configuration, and may also prevent the conformational change that occurs in channels.

Authors

Naoki Kato

Bioscience and Biotechnology Center; Nagoya University; Nagoya, Japan

Masaro Akai

Bioscience and Biotechnology Center; Nagoya University; Nagoya, Japan

Lalu Zulkifli

Bioscience and Biotechnology Center; Nagoya University; Nagoya, Japan

Nobuyuki Matsuda

Bioscience and Biotechnology Center; Nagoya University; Nagoya, Japan

Yasuhiro Kato

Bioscience and Biotechnology Center; Nagoya University; Nagoya, Japan

Shinobu Goshima

Bioscience and Biotechnology Center; Nagoya University; Nagoya, Japan

Akihiro Hazama

Department of Physiology; School of Medicine; Fukushima Medical University; Fukushima, Japan

Mutsumi Yamagami

Institute for Environmental Sciences; Aomori, Japan

Robert H. Guy

National Cancer Institute/National Institutes of Health

Nobuyuki Uozumi

Bioscience and Biotechnology Center; Nagoya University; Nagoya, Japan

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We now provide open access to journal articles published online for one year or more. This article may be downloaded at the following link:
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