Brüggemann L, Dietrich P, Dreyer I, Hedrich R
Julius-von-Sachs-Institut, Universität Würzburg, Germany.
Planta. 1999 Jan;207(3):370-6. doi: 10.1007/s004250050494.
Stomatal opening is the result of K(+)-salt accumulation in guard cells. Potassium uptake in these motor cells is mediated by voltage-dependent, K(+)-selective ion channels. Here we compare the invitro properties of two guard-cell K(+)-channel alpha-subunits from Arabidopsis thaliana (L.) Heynh. (KAT1) and Solanum tuberosum L. (KST1) after heterologous expression with the respective K(+)-transport characteristics in their mother cell. The KAT1 and KST1 subunits when expressed in Xenopus oocytes shared the basic features of the K(+)-uptake channels in the corresponding guard cells, including voltage dependence and single-channel conductance. Besides these similarities, the electrophysiological comparison of K+ channels in the homologous and the heterologous expression systems revealed pronounced differences with respect to modulation and block by extracellular cations. In the presence of 1 mM Cs+, 50% of the guard-cell K(+)-uptake channels (GCKClin) in A. thaliana and S. tuberosum, were inhibited upon hyperpolarization to -90 mV. For a similar effect on KAT1 and KST1 in oocytes, voltages as negative as -155 mV were required. In contrast, compared to the K+ channels in vivo the functional alpha-subunit homomers almost lacked a voltage-dependent block by extracellular Ca2+. Similar to the block by Cs+ and Ca2+, the acid activation of the alpha-homomers was less pronounced in oocytes. Upon acidification the voltage-dependence shifted by 82 and 90 mV for GCKCLin in A. thaliana and S. tuberosum, respectively, but only by 25 mV for KAT1 and KST1. From the differences in K(+)-channel modulation in vivo and after heterologous expression we conclude that the properties of functional guard-cell K(+)-uptake channels result either from the heterometric assembly of different alpha-subunits or evolve from cell-type specific posttranslational modification.
气孔开放是保卫细胞中钾盐积累的结果。这些运动细胞对钾的吸收由电压依赖性钾选择性离子通道介导。在此，我们比较了拟南芥（L.）Heynh.（KAT1）和马铃薯（Solanum tuberosum L.）（KST1）的两个保卫细胞钾通道α亚基在异源表达后的体外特性及其在母细胞中的相应钾转运特性。KAT1和KST1亚基在非洲爪蟾卵母细胞中表达时，具有相应保卫细胞中钾吸收通道的基本特征，包括电压依赖性和单通道电导。除了这些相似之处，同源和异源表达系统中钾通道的电生理比较显示，在细胞外阳离子的调节和阻断方面存在明显差异。在存在1 mM Cs + 的情况下，拟南芥和马铃薯中50%的保卫细胞钾吸收通道（GCKClin）在超极化至-90 mV时受到抑制。对于卵母细胞中的KAT1和KST1要产生类似的效果，则需要低至-155 mV的电压。相比之下，与体内的钾通道相比，功能性α亚基同聚体几乎没有细胞外Ca2 + 引起的电压依赖性阻断。与Cs + 和Ca2 + 的阻断类似，α同聚体的酸激活在卵母细胞中也不太明显。酸化后，拟南芥和马铃薯中GCKClin的电压依赖性分别偏移82和90 mV，但KAT1和KST1仅偏移25 mV。从体内和异源表达后钾通道调节的差异中，我们得出结论，功能性保卫细胞钾吸收通道的特性要么源于不同α亚基的异源组装，要么源于细胞类型特异性的翻译后修饰。
