Abstract
Plasma-membrane vesicles were purified by aqueous-polymer two-phase partitioning of a microsomal membrane fraction from rye (Secale cereale L.) roots and incorporated into planar 1-palmitoyl-2-oleoyl phosphatidylethanolamine bilayers. A high-conductance cation channel (a maxi cation channel) was characterized from single-channel electrical recordings. The channel was incorporated into the bilayer with its cytoplasmic surface facing the trans compartment and voltages were referenced cis with respect to trans. The channel was permeable to both monovalent and divalent cations. The unitary conductance was 451 pS in symmetrical 100 mM KCl and 213 pS in symmetrical 100 mM BaCl2. The permeability ratio PK∶PBa was 1.00∶2.56. Unitary conductances declined in the order K+≥Rb+>Cs+>Na+> Li+ (monovalent cations) and Ba2+>Sr2+>Ca2+> Mg2+>Co2+>Mn2+ (divalent cations). The relative permeabilities of monovalent cations mirrored their conductivity sequence, whereas the permeabilities of all divalent cations were similar. The maxi cation channel showed complex kinetics, exhibiting both voltage- and time-dependent inactivation and voltage-dependent gating. The voltage dependence of the kinetics shifted in parallel with changes in the reversal potential of the channel. In symmetrical 100 mM KCl, following a voltage step from zero to the test voltage, the channel inactivated and the active-channel lifetime (τ i) shortened exponentially as the test voltage was increased. The channel always opened immediately upon depolarization to zero volts, indicating that inactivation of the channel did not result from the loss of any intrinsic factor. The probability of finding an active channel in the open state (P0) exhibited a bell-shaped relationship with membrane potential. At voltages between -40 and 80 mV, P0 exceeded 0.99, but p0 declined abruptly at more extreme voltages. Under ionic conditions which approximated physiological conditions, in the presence of 100 mM KCl on the trans (cytoplasmic) side and 1 mM KCl plus 2 mM CaCl2 on the cis (extracellular) side, the reversal potential was 15.6 mV and the kinetics approximated those observed in symmetrical 100 mM KCl. Thus, the channel would open upon depolarization of the plasma membrane in vivo. If the channel functioned physiologically as a Ca2+ channel it might be involved in intracellular signalling: the channel could open in response to a variety of environmental, developmental and pathological stimuli which depolarize the plasma membrane, allowing Ca2+ into the cytoplasm and thereby initiating a physiological response.
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Abbreviations
- EK :
-
Nernst (equilibrium) potential for potassium
- Erev :
-
zero-current (reversal) potential
- I/V:
-
current/voltage
- τ c :
-
apparent mean lifetime of the activated-channel closed state
- τ i :
-
apparent mean lifetime of the activated channel following a voltage step from zero volts
- τ 0 :
-
apparent mean lifetime of the activated-channel open state
- PE:
-
1-palmitoyl-2-oleoyl phosphatidylethonlamine
- P0 :
-
probability of finding the activated channel in an open state
- TEA+ :
-
tetraethylammonium
References
Berestovskii, G.N., Zherelova, O.M., Katayev, A.A. (1987) Ionic channels in characean algal cells. Biophysics 32, 1101–1120
Blatt, M.R. (1991) Ion channel gating in plants: Physiological implications and integration for stomatal function. J. Membr. Biol. 124, 95–112
Colquhoun, D. (1987) Practical analysis of single channel records. In: Microelectrode techniques (The Plymouth Workshop Handbook), pp. 83–104. Standen, N.B., Gray, P.T.A., Whitaker, M.J., eds. Company of Biologists, Cambridge
Davies, E. (1987) Action potentials as multifunctional signals in plants: a unifying hypothesis to explain apparently disparate wound responses. Plant Cell Environ. 10, 623–631
Hedrich, R., Schroeder, J.I. (1989) The physiology of ion channels and electrogenic pumps in higher plants. Annu. Rev. Plant Physiol. Plant Mol. Biol. 40, 539–569
Hepler, P.K., Wayne R.O. (1985) Calcium and plant development. Annu. Rev. Plant Physiol. 36, 397–439
Hille, B. (1992) Ionic channels of excitable membranes. Sinauer Associates Inc., Massachusetts
Johannes, E., Brosnan, J.M., Sanders, D. (1991) Calcium channels and signal transduction in plant cells. BioEssays 13, 331–336
Kauss, H. (1987) Some aspects of calcium-dependent regulation in plant metabolism. Annu. Rev. Plant Physiol. 38, 47–72
Lewis, C.A. (1979) Ion-concentration dependence of the reversal potential and the single channel conductance of ion channels at the frog neuromuscular junction. J. Physiol. 286, 417–445
Lunevsky, V.Z., Zherelova, O.M., Vostrikov, I.Y., Berestovsky, G.N. (1983) Excitation of Characeae cell membranes as a result of activation of calcium and chloride channels. J. Membr. Biol. 72, 43–58
Marschner, H. (1986) Mineral nutrition of higher plants. Academic Press, London
Minorsky, P.V. (1989) Temperature sensing by plants: a review and hypothesis. Plant Cell Environ. 12, 119–135
Okazaki, Y., Tazawa, M. (1990) Calcium ion and turgor regulation in plant cells. J. Membr. Biol. 114, 189–194
Robinson, R.A., Stokes, R.H. (1959) Electrolyte solutions. Butterworths Scientific Publications, London
Sanders, D. (1990) Kinetic modelling of plant and fungal membrane transport systems. Annu. Rev. Plant Physiol. Plant Mol. Biol. 41, 77–107
Sibaoka, T. (1991) Rapid plant movements triggered by action potentials. Bot. Mag. Tokyo 104, 73–95
Tamamushi, R., Goto, S. (1970) Determination of ion activity coefficients from the measurements of membrane concentration potentials: Activity behaviour of alkali-metal cations in aqueous solutions at 25° C. Bull. Chem. Soc. Jap. 43, 3420–3424
Tester, M. (1990) Plant ion channels: whole-cell and single-channel studies. New Phytol. 114, 305–340
White, P.J., Tester, M.A. (1992a) Potassium channels from the plasma membrane of rye roots characterized following incorporation into planar lipid bilayers. Planta 186, 188–202
White, P.J., Tester, M. (1992b) The application of planar lipid bilayers to the study of plant ion channels. In: Transport and receptor proteins of plant membranes: Molecular structure and function, pp. 119–133, Clarkson, D.T., Cooke D.T., eds. Plenum Press, New York
White, P.J., Earnshaw, M.J., Clarkson, D.T. (1991) Effects of growth and assay temperatures on unidirectional K+ fluxes in roots of rye (Secale cereale). J. Exp. Bot. 42, 1031–1041
White, P.J., Smahel, M., Thiel, G. (1993) Characterization of ion channels from Acetabularia plasma membrane in planar lipid bilayers. J. Membr. Biol. 133, 145–160
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This work was supported by the Agriculture and Food Research Council and by a grant from the Science and Engineering Research Council Membrane Initiative (GR/F 33971) to Prof. E.A.C. MacRobbie (University of Cambridge, UK).
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White, P.J. Characterization of a high-conductance, voltage-dependent cation channel from the plasma membrane of rye roots in planar lipid bilayers. Planta 191, 541–551 (1993). https://doi.org/10.1007/BF00195756
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DOI: https://doi.org/10.1007/BF00195756