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Divalent cation gating of an ammonium permeable channel in the symbiotic membrane from soybean nodules
Author(s) -
Whitehead Lynne F.,
Day David A.,
Tyerman Stephen D.
Publication year - 1998
Publication title -
the plant journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 3.058
H-Index - 269
eISSN - 1365-313X
pISSN - 0960-7412
DOI - 10.1046/j.1365-313x.1998.00298.x
Subject(s) - chemistry , divalent , cytoplasm , membrane , conductance , gating , membrane potential , biophysics , kinetics , time constant , analytical chemistry (journal) , crystallography , biochemistry , chromatography , physics , biology , organic chemistry , engineering , quantum mechanics , electrical engineering , condensed matter physics
Summary The symbiotic membrane between N 2 ‐fixing bacteroids and plant cytoplasm in nodules of soybean contains a sub‐picoSiemen cation channel permeable to NH 4 + . With millimolar concentrations of Ca 2+ or Mg 2+ on the cytoplasmic side, the channel rectifies current in the direction of cation influx to the cytoplasm. When Ca 2+ is present on the bacteroid side of the membrane the current is rectified in the opposite direction. With submicromollar concentrations of divalent on both sides, the channel no longer rectifies. The channel is inhibited by verapamil on the bacteroid side of the membrane with a K d of 2.6 μM. In the presence of millimolar concentrations of divalents on the cytoplasmic side, the conductance as a function of voltage is fitted by a simple Boltzmann equation with an effective gating charge equal to one. The voltage at which the conductance reaches 50% of maximum is dependent on external NH 4 + , shifting negative at lower concentrations. The time‐course of activation upon hyperpolarisation can be described by the Hodgkin‐Huxley equation with Ca 2+ present on the cytoplasmic side. With Mg 2+ the channel activates with single exponential kinetics. The time constant for activation is weakly voltage dependent. Upon depolarisation of the membrane the channel deactivates with double exponential kinetics, the time constants being slightly voltage dependent. We propose a model of the channel in which divalent block is relieved when the blocking ion is dislodged by univalent cation flux into the pore. Mg 2+ on the cytoplasmic side may functionin vivoas the gating particle of the channel.