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A Unique Voltage Sensor Sensitizes the Potassium Channel AKT2 to Phosphoregulation

A Unique Voltage Sensor Sensitizes the Potassium Channel AKT2 to Phosphoregulation Among all voltage-gated K + channels from the model plant Arabidopsis thaliana , the weakly rectifying K + channel (K weak channel) AKT2 displays unique gating properties. AKT2 is exceptionally regulated by phosphorylation: when nonphosphorylated AKT2 behaves as an inward-rectifying potassium channel; phosphorylation of AKT2 abolishes inward rectification by shifting its activation threshold far positive (>200 mV) so that it closes only at voltages positive of +100 mV. In its phosphorylated form, AKT2 is thus locked in the open state in the entire physiological voltage range. To understand the molecular grounds of this unique gating behavior, we generated chimeras between AKT2 and the conventional inward-rectifying channel KAT1. The transfer of the pore from KAT1 to AKT2 altered the permeation properties of the channel. However, the gating properties were unaffected, suggesting that the pore region of AKT2 is not responsible for the unique K weak gating. Instead, a lysine residue in S4, highly conserved among all K weak channels but absent from other plant K + channels, was pinpointed in a site-directed mutagenesis approach. Substitution of the lysine by serine or aspartate abolished the “open-lock” characteristic and converted AKT2 into an inward-rectifying channel. Interestingly, phosphoregulation of the mutant AKT2-K197S appeared to be similar to that of the K in channel KAT1: as suggested by mimicking the phosphorylated and dephosphorylated states, phosphorylation induced a shift of the activation threshold of AKT2-K197S by about +50 mV. We conclude that the lysine residue K197 sensitizes AKT2 to phosphoregulation. The phosphorylation-induced reduction of the activation energy in AKT2 is ∼6 kT larger than in the K197S mutant. It is discussed that this hypersensitive response of AKT2 to phosphorylation equips a cell with the versatility to establish a potassium gradient and to make efficient use of it. Footnotes E. Michard's present address is Instituto Gulbenkian de Ciência, R. Quinta Grande 6, PT-2780-156 Oeiras, Portugal. Submitted: 21 September 2005 Accepted: 10 November 2005 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png The Journal of General Physiology Rockefeller University Press

A Unique Voltage Sensor Sensitizes the Potassium Channel AKT2 to Phosphoregulation

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References (54)

Publisher
Rockefeller University Press
Copyright
Copyright © 2005, by The Rockefeller University Press
ISSN
0022-1295
eISSN
1540-7748
DOI
10.1085/jgp.200509413
pmid
16316977
Publisher site
See Article on Publisher Site

Abstract

Among all voltage-gated K + channels from the model plant Arabidopsis thaliana , the weakly rectifying K + channel (K weak channel) AKT2 displays unique gating properties. AKT2 is exceptionally regulated by phosphorylation: when nonphosphorylated AKT2 behaves as an inward-rectifying potassium channel; phosphorylation of AKT2 abolishes inward rectification by shifting its activation threshold far positive (>200 mV) so that it closes only at voltages positive of +100 mV. In its phosphorylated form, AKT2 is thus locked in the open state in the entire physiological voltage range. To understand the molecular grounds of this unique gating behavior, we generated chimeras between AKT2 and the conventional inward-rectifying channel KAT1. The transfer of the pore from KAT1 to AKT2 altered the permeation properties of the channel. However, the gating properties were unaffected, suggesting that the pore region of AKT2 is not responsible for the unique K weak gating. Instead, a lysine residue in S4, highly conserved among all K weak channels but absent from other plant K + channels, was pinpointed in a site-directed mutagenesis approach. Substitution of the lysine by serine or aspartate abolished the “open-lock” characteristic and converted AKT2 into an inward-rectifying channel. Interestingly, phosphoregulation of the mutant AKT2-K197S appeared to be similar to that of the K in channel KAT1: as suggested by mimicking the phosphorylated and dephosphorylated states, phosphorylation induced a shift of the activation threshold of AKT2-K197S by about +50 mV. We conclude that the lysine residue K197 sensitizes AKT2 to phosphoregulation. The phosphorylation-induced reduction of the activation energy in AKT2 is ∼6 kT larger than in the K197S mutant. It is discussed that this hypersensitive response of AKT2 to phosphorylation equips a cell with the versatility to establish a potassium gradient and to make efficient use of it. Footnotes E. Michard's present address is Instituto Gulbenkian de Ciência, R. Quinta Grande 6, PT-2780-156 Oeiras, Portugal. Submitted: 21 September 2005 Accepted: 10 November 2005

Journal

The Journal of General Physiology – Rockefeller University Press

Published: Dec 1, 2005

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