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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References
Andersen, O.S. 1983. Ion movement through gramicidin A channels: Studies on the diffusion-controlled association step.Biophys. J. 41:147–165
Andersen, O.S., Procopio, J. 1980. Ion movement through gramicidin A channels. On the importance of the aqueous diffusion resistance and ion-water interactions.Acta Physiol. Scand. Suppl. 481:27–35
Apell, H.J., Bamberg, E., Läuger, P. 1979. Effects of surface charge on the conductance of the gramicidin channel.Biochim. Biophys. Acta 552:369–378
Barrett, J.N., Magleby, K.L., Pallotta, B.S. 1982. Properties of single calcium-activated potassium channels in cultured rat muscle.J. Physiol. (London) 331:211–230
Barry, P.H., Diamond, J.M. 1970. Junction potentials, electrode standard potentials, and other problems in interpreting electrical properties of membranes.J. Membrane Biol. 3:93–122
Barry, P.H., Gage, P.W. 1984. Ionic selectivity of channels at the end-plate.In: Ion Channels: Molecular and Physiological Aspects. W.D. Stein, editor. Ch 1, pp. 1–51. Academic, New York
Barry, P.H., Gage, P.W., Van Helden, D.F. 1979. Cation permeation at the amphibian motor end-plate.J. Membrane Biol. 45:245–276
Bell, J., Miller, C. 1984. Effects of phospholipid surface charge on ion conduction in the K+ channel of sarcoplasmic reticulum.Biophys. J. 45:279–287
Blatz, A.L., Magleby, K.L. 1987. Calcium-activated potassium channels.Trends Neurosci. 10:463–467
Cecchi, X., Wolff, D., Alvarez, O., Latorre, R. 1987. Mechanisms of Cs+ blockade in a Ca2+-activated K+ channel from smooth muscle.Biophys. J. 52:707–716
Eisenman, G., Latorre, R., Miller, C. 1986. Multi-ion conduction and selectivity in the high-conductance Ca++-activated K+ channel from skeletal muscle.Biophys. J. 50:1025–1034
Hamill, O.P., Marty, A., Neher, E., Sakmann, B., Sigworth, F.J. 1981. Improved patch-clamp techniques for high-resolution current recording from cells and cell-free membrane patches.Pfluegers Arch. 391:85–100
Harned, H.S., Shropshire, J.A. 1958. The diffusion coefficient at 25° of potassium chloride at low concentrations in 0.25 molar sucrose solutions.J. Am. Chem. Soc. 80:5652–5653
Jordan, P.C. 1987. How pore mouth charge distributions alter the permeability of transmembrane ionic channels.Biophys. J. 51:297–311
Kamiya, N., Kuroda, K. 1957. Cell operation inNitella: I. Cell amputation and effusion of the endoplasm.Proc. Jpn. Acad. 33:149–152
Kundu, K.K., Das, A.K. 1979. Transfer free energies of some ions from water to dimethylsulfoxide-water and urea-water mixtures.J. Sol. Chem. 8:259–263
Latorre, R., Miller, C. 1983. Conduction and selectivity in potassium channels.J. Membrane Biol. 71:11–30
Läuger, P. 1973. Ion transport through pores: A rate-theory analysis.Biochim. Biophys. Acta 311:423–441
Läuger, P. 1976. Diffusion-limited ion flow through pores.Biochim. Biophys. Acta 455:493–509
Laver, D.R., Walker, N.A. 1987. Steady-state voltage-dependent gating and conduction kinetics of single K+ channels in the membrane of cytoplasmic drops ofChara australis.J. Membrane Biol. 100:31–42
Lühring, H. 1986. Recording of single K+ channels in the membrane of cytoplasmic drop ofChara australis.Protoplasma 133:19–27
Marty, A. 1983. Ca2+-dependent K+ channels with large unitary conductance.Trends Neurosci. 6:262–265
Moczydlowski, E., Alvarez, O., Vergara, C., Latorre, R. 1985. Effect of phospholipid surface charge on the conductance and gating of a Ca2+-activated channel in planar lipid bilayers.J. Membrane Biol. 83:273–282
Quartararo, N., Barry, P.H. 1987. A simple technique for transferring excised patches of membrane to different solutions for single-channel measurements.Pfluegers Arch. 410:677–678
Reeves, M., Shimmen, T., Tazawa, M. 1985. Ionic activity gradients across the surface membrane of cytoplasmic drops fromChara australis.Plant Cell Physiol. 26:1185–1193
Robinson, R.A., Stokes, R.H., Marsh, K.N. 1970. Activity coefficients in the ternary system: Water+sucrose+sodium chloride.J. Chem. Thermodynam. 2:745–750
Sakano, K., Tazawa, M. 1986. Tonoplast origin of the membrane of cytoplasmic droplets prepared fromChara internodal cells.Protoplasma 131:247–249
Takeda, K., Gage, P.W., Barry, P.H. 1982. Effects of divalent cations on toad end-plate channels.J. Membrane Biol. 64:55–66
Yellen, G. 1984. Ionic permeation and blockade in Ca2+-activated K+ channels of bovine chromaffin cells.J. Gen. Physiol. 84:157–186
Zimmerberg, J., Parsegian, V.A. 1986. Polymer inaccessible volume changes during opening and closing of a voltage-dependent ionic channel.Nature (London) 323:36–39
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Laver, D.R., Fairley, K.A. & Walker, N.A. Ion permeation in a K+ channel inChara australis: Direct evidence for diffusion limitation of ion flow in a maxi-K channel. J. Membrain Biol. 108, 153–164 (1989). https://doi.org/10.1007/BF01871026
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DOI: https://doi.org/10.1007/BF01871026