Divalent cation gating of an ammonium permeable channel in the symbiotic membrane from soybean nodules

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The symbiotic membrane between N2-fixing bacteroids and plant cytoplasm in nodules of soybean contains a sub-picoSiemen cation channel permeable to NH4+. With millimolar concentrations of Ca2+ or Mg2+ on the cytoplasmic side, the channel rectifies current in the direction of cation influx to the cytoplasm. When Ca2+ 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 Kd 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 NH4+, shifting negative at lower concentrations. The time-course of activation upon hyperpolarisation can be described by the Hodgkin-Huxley equation with Ca2+ present on the cytoplasmic side. With Mg2+ 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. Mg2+ on the cytoplasmic side may function in vivo as the gating particle of the channel.

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