The Journal of General Physiology
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Published 29 September 2003. doi:10.1085/jgp.200308818
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© Rockefeller University Press, 0022-1295/2003/10/459/ $5.00
Journal of General Physiology, Volume 122, Number 4, October 2003 459-469

Molecular Coupling between Voltage Sensor and Pore Opening in the Arabidopsis Inward Rectifier K+ Channel KAT1

Ramon Latorre1,2, Riccardo Olcese3, Claudia Basso4, Carlos Gonzalez1,2, Fabian Muñoz1, Diego Cosmelli1 and Osvaldo Alvarez1,2

1 Laboratory of Biophysics and Molecular Physiology, Centro de Estudios Científicos, Valdivia, Chile
2 Department of Biology Faculty of Sciences, University of Chile Santiago, Chile
3 Department of Anesthesiology, Division of Molecular Medicine, and Brain Research Institute, David Geffen School of Medicine, University of California at Los Angeles, Los Angeles, CA 90095
4 Biomedical Sciences Institute Faculty of Medicine, University of Chile, Casilla 70058, Santiago-7, Chile

Address correspondence to Dr. Ramon Latorre Laboratory of Biophysics and Molecular Physiology Centro de Estudios Científicos Arturo Prat 514 Valdivia, Chile. Fax: (56) 63-234515; email: ramon{at}cecs.cl

Animal and plant voltage-gated ion channels share a common architecture. They are made up of four subunits and the positive charges on helical S4 segments of the protein in animal K+ channels are the main voltage-sensing elements. The KAT1 channel cloned from Arabidopsis thaliana, despite its structural similarity to animal outward rectifier K+ channels is, however, an inward rectifier. Here we detected KAT1-gating currents due to the existence of an intrinsic voltage sensor in this channel. The measured gating currents evoked in response to hyperpolarizing voltage steps consist of a very fast ({tau} = 318 ± 34 µs at -180 mV) and a slower component (4.5 ± 0.5 ms at -180 mV) representing charge moved when most channels are closed. The observed gating currents precede in time the ionic currents and they are measurable at voltages (less than or equal to -60) at which the channel open probability is negligible ({approx}10-4). These two observations, together with the fact that there is a delay in the onset of the ionic currents, indicate that gating charge transits between several closed states before the KAT1 channel opens. To gain insight into the molecular mechanisms that give rise to the gating currents and lead to channel opening, we probed external accessibility of S4 domain residues to methanethiosulfonate-ethyltrimethylammonium (MTSET) in both closed and open cysteine-substituted KAT1 channels. The results demonstrate that the putative voltage–sensing charges of S4 move inward when the KAT1 channels open.

Key Words: KAT1 channels • inward rectifier • gating currents • cysteine accessibility • voltage sensor


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