Editors' ChoiceIon Channels

Keeping Sodium Out

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Science's STKE  22 Jul 2003:
Vol. 2003, Issue 192, pp. tw281-TW281
DOI: 10.1126/stke.2003.192.tw281

Hua et al. analyzed the ion selectivity profiles of two cloned Arabidopsis ion channels that are similar to members of the animal cyclic nucleotide-gated nonselective cation channel family (CNGC) and discovered that one of them, AtCNGC2, used a novel mechanism to achieve selectivity for K+. The selective permeability of ion channels in the cell membrane allows cells to exclude unwanted ions and helps them maintain steep transmembrane ion gradients that are exploited in signaling. Research on bacterial and animal channels selectively permeable to potassium (K+ channels) has implicated a characteristic GYG (glycine-tyrosine-glycine) motif as providing the K+ selectivity filter; this motif has been considered essential for K+ selectivity. Hua et al. used structural modeling to identify a unique alanine-asparagine-aspartate (AND) motif as providing a putative selectivity filter for AtCNCG2, which displays a cyclic adenosine monophosphate (cAMP)-dependent inwardly rectifying K+ current. The authors used mutational analysis in combination with electrophysiological measurements of cAMP-dependent ion currents through channels expressed in human embryonic kidney cells or Xenopus oocytes to confirm that the ND portion of the motif was required for AtCNGC's selective permeability to K+ over Na+ (alanine was not mutated). The second channel, AtCNCG1, did not contain the AND motif and was equally permeable to Na+ and K+. AtCNCG1 represents the first cloned plant channel shown to be permeable to Na+ and may represent a physiologically significant pathway for Na+ uptake. Advances in understanding the basis of plant ion channel selectivity may have important implications regarding the ability to grow crops in regions of high soil salinity.

B.-G. Hua, R. W. Mercier, Q. Liang, G. A. Berkowitz, Plants do it differently: A new basis for potassium/sodium selectivity in the pore of an ion channel. Plant Physiol. 132, 1353-1361 (2003). [Abstract] [Full Text]

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