White P J
Department of Cell Physiology, Horticulture Research International, Wellesbourne, Warwick, CV35 9EF.
J Membr Biol. 1996 Jul;152(1):89-99. doi: 10.1007/s002329900088.
Nitrogen is available to the plant in the form of NH+4 in the soil solution. Here it is shown that a voltage-independent K+ channel in the plasma membrane of rye (Secale cereale L.) roots is permeable to NH+4. The channel was studied following its incorporation into planar 1-palmitoyl-2-oleoyl phosphatidyl ethanolamine bilayers. The unitary conductance of the channel was greater when assayed in the presence of 100 mM NH4Cl than 100 mM KCl. However, the probability of finding the channel open (Po) was lower in the presence of 100 mm NH4Cl (Po = 0.63) than in 100 mM KCl (Po = 0.8), suggesting that Po can be regulated by the (permeant) ions present in solution. When assayed in equimolar concentrations of NH4Cl (cis) and KCl (trans), the zero-current (reversal) potential for the channel (Erev) exhibited a complex concentration dependence. At low cation concentrations, the apparent permeability of NH+4 relative to K+ (PNH4/PK) was greater than 1.0. However, as the cation concentration was increased, PNH4/PK initially decreased to a minimum of 0.95 at 3 mM before increasing again to a maximum of 1.89 at 300 mM. At cation concentrations above 300 mM, PNH4/PK decreased slightly. This implies that the pore of the channel can be occupied by more than one cation simultaneously. Ammonium permeation through the pore was simulated using a model which is composed of three energy barriers and two energy wells (the ion-binding sites). The model (3B2S) allowed for single-file permeation, double cation occupancy, ion-ion repulsion within the pore and surface potential effects. Results indicated that energy peaks and energy wells were situated asymmetrically within the electrical distance of the pore, that cations repel each other within the pore and that the vestibules to the pore contain negligible surface charge. The energy profile obtained for NH+4 is compared with ones obtained for K+ and Na+. This information allows the fluxes through the K+ channel of the three major monovalent cations present in the soil solution to be predicted.
土壤溶液中的铵根离子（NH₄⁺）可为植物提供氮元素。本文表明，黑麦（Secale cereale L.）根细胞质膜上一种不依赖电压的钾离子通道对NH₄⁺具有通透性。该通道在被整合到平面的1 - 棕榈酰 - 2 - 油酰磷脂酰乙醇胺双层膜后进行了研究。当在100 mM氯化铵存在的情况下进行检测时，该通道的单通道电导大于在100 mM氯化钾存在时的情况。然而，在100 mM氯化铵存在时通道开放的概率（Po）低于在100 mM氯化钾存在时（Po = 0.63对Po = 0.8），这表明Po可由溶液中存在的（可通透的）离子调节。当在等摩尔浓度的氯化铵（顺式）和氯化钾（反式）中进行检测时，该通道的零电流（反转）电位（Erev）呈现出复杂的浓度依赖性。在低阳离子浓度下（相对于K⁺），NH₄⁺的表观通透性（PNH₄/PK）大于1.0。然而，随着阳离子浓度增加，PNH₄/PK最初下降至3 mM时的最小值0.95，然后再次上升至300 mM时的最大值1.89。在阳离子浓度高于300 mM时，PNH₄/PK略有下降。这意味着通道孔可同时被多个阳离子占据。使用一个由三个能量势垒和两个能量阱（离子结合位点）组成的模型对铵根离子通过该孔的渗透进行了模拟。该模型（3B2S）允许单排渗透、双阳离子占据、孔内离子 - 离子排斥以及表面电位效应。结果表明，能量峰和能量阱在孔的电距离内不对称分布，阳离子在孔内相互排斥，并且孔的前庭表面电荷可忽略不计。将NH₄⁺获得的能量分布图与K⁺和Na⁺获得的能量分布图进行了比较。这些信息使得能够预测土壤溶液中存在的三种主要单价阳离子通过钾离子通道的通量。
