Poster: Membrane Transport

Abs # 1205: Identification of modifications in cation selectivity of the Arabidopsis Na⁺ translocating AtHKT1 protein

Presenter:	Kato, Yasuhiro , kato@yasu.net
Authors	Kato, Yasuhiro  (A)   Hazama, Akihiro  (B)   Yamagami, Mutsumi  (C)   Guy, Robert H. (D)   Uozumi, Nobuyuki  (E) (A)
Affiliations:	(A): Graduate School of Bioagricultural Sciences, Nagoya University, Nagoya (B): Department of Cell Biology, National Institute for Basic Biology, Okazaki (C): Institute of Environmental Sciences, Aomori (D): Laboratory of Experimental and Computational Biology, National Cancer Institute, NIH, Bethesda (E): Bioscience Center, Nagoya University, Nagoya
Web Site:	http://www.agr.nagoya-u.ac.jp/~donew/index-e.html

The Arabidopsis thaliana AtHKT1 protein mediates inward Na⁺ currents in Xenopus oocytes¹⁾²⁾. HKT1 proteins are members of a superfamily of K⁺ transporters^1-4). These proteins contain eight transmembrane segments and four pore forming regions arranged in a mode similar to that of a K⁺ channel tetramer^1-4). Mutation analysis on AtHKT1 and wheat HKT1 revealed that the Gly conserved in the four pore forming region are of particular importance for K⁺ selectivity²⁾⁴⁾. The selectivity filter of four Gly arrangement corresponds to the first Gly of the Gly-Tyr-Gly sequence in K⁺ channel, which suggests that HKT transporter has a common ancestor with K⁺ channels⁴⁾. HKT however possesses charged residues within transmembrane regions which are not conserved in K⁺ channels. These positive charged residues may involve in the electrostatic barrier to the passage of cations through HKT⁴⁾. In this study, we did site-directed mutations on the charged residues to replace the residues with neutral amino acids, Ala (A) or Gln (Q), or negatively charged residue, Glu (E). Their ion selectivities was measured by the reversal potential shift in the two electrode voltage clamp using X. oocytes. R487A and R487Q did not show any significant difference in ion selectivity compared with that of the wild type. R487E transported K⁺ without loss of Na⁺ uptake activity. This suggested that R487 may be one of the crucial residues for cation selectivity of HKT.

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