Sodium Blocking Induced by a Point Mutation at the C-Terminal End of the Pore Helix of the KAT1 Channel |
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Authors: | N Uozumi K Yamada S Goshima T Ona S Oiki |
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Institution: | (1) Department of Physiology, Fukui Medical University, Yoshida-gun, Fukui 910-1193, Japan, JP;(2) Nagoya University Bioscience Center, Nagoya University, Chikusa-ku, Nagoya 464-8601, Japan, JP;(3) Forestry Research Institute, Oji Paper Co., Ltd., 24-9 Nobono-cho, Kameyama 519-0212, Japan, JP |
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Abstract: | A plant hyperpolarization-activating K+ channel, KAT1, is highly selective for K+ over Na+ and is little affected by external Na+, which is crucial to take up K+ effectively in a Na+-containing environment. It has been shown that a mutation at the location (Thr256) preceding the selectivity signature sequence
dramatically enhanced the sensitivity of the KAT1 channel to external Na+. We report here electrophysiological experiments for the mechanism of action of external Na+ on KAT1 channels. The Thr256 residue was substituted with either glutamine (Q) or glutamate (E). The wild-type channel was
insensitive to external Na+. However, the activity of both mutant channels was significantly depressed by Na+ with apparent dissociation constants of 6.7 mm and 11.3 mm for T256Q and T256E, respectively. The instantaneous current-voltage relationships revealed distinct blocking mechanisms
for these mutants. For T256Q a typical voltage-dependent fast blocking was shown. On the other hand, the blocking for the
T256E mutant was voltage-independent at low Na+ concentrations and became voltage-dependent at higher concentrations. At extreme hyperpolarization the blocking was relieved
significantly. These data strongly suggest that the mutation at the end of the pore helix rearranged the selectivity filter
and allows Na+ to penetrate into the pore.
Received: 16 October 2000/Revised: 20 February 2001 |
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Keywords: | : Open channel blocking — Electrophysiology — Selectivity — Channel structure — Hyperpolarization activating channel — Instantaneous current-voltage curves |
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