Ion permeation in a K+ channel inChara australis: Direct evidence for diffusion limitation of ion flow in a maxi-K channel |
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Authors: | D R Laver K A Fairley N A Walker |
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Institution: | (1) Biophysics Laboratory, School of Biological Sciences, The University of Sydney A12, 2006, New South Wales, Australia |
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Abstract: | Summary We report a study of a potassium-selective channel in the membrane delineating cytoplasmic drops fromChara australis. The relatively large conductance (170 pS in 150 mol/m3 (mm) KCl), high ion selectivity (P
Cl/P
K=0.015±0.01) and voltagedependent kinetics of this channel indicate that it is a type of maxi-K channel commonly found in animal cells but not previously detected in any plant cell.The current-voltage (I/V) characteristic of these channels was examined in drop-attached and in excised outside-out patches using the patch-clamp technique, over the unusually large voltage range of –250 to 200 mV. TheI/V characteristic is nonlinear and shows saturation at extreme voltages; the current also saturates at high K+]. In solutions with symmetrical KCl concentrations the saturation behavior of the current is asymmetrical. The permeability of the channel depends on whether it is observed in excised or in drop-attached membrane patches.Here we investigate the main factors affecting the permeation of K+ ions through this maxi-K channel. We present the first direct evidence for the importance of diffusion external to the pore in limiting ion flow through maxi-K channels. The data are consistent with an ion translocation mechanism whose current is limited (i) at high voltages by ion diffusion external to the pore and (ii) at high K+] by the maximum transport rate of the channel. We fit the data to a diffusion-limited pore model in which the pore exhibits saturation described by Michaelis-Menten kinetics with aK
m=50±25 mol/m3 andG
max=300±20 pS. |
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Keywords: | K+ channel permeation kinetics patch clamp Chara australis cytoplasmic drop diffusion limited |
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