Zheng J, Sigworth F J
Department of Cellular and Molecular Physiology, Yale University School of Medicine, New Haven, Connecticut 06520, USA.
J Gen Physiol. 1998 Oct;112(4):457-74. doi: 10.1085/jgp.112.4.457.
A previous study of the T442S mutant Shaker channel revealed activation-coupled subconductance levels that apparently represent kinetic intermediates in channel activation (Zheng, J., and F.J. Sigworth. 1997. J. Gen. Physiol. 110:101-117). We have now extended the study to heteromultimeric channels consisting of various numbers of mutant subunits as well as channels without mutant subunits, all in the background of a chimeric Shaker channel having increased conductance. It has been found that activation-coupled sublevels exist in all these channel types, and are traversed in at least 80% of all deactivation time courses. In symmetric K+ solutions, the currents in the two sublevels have a linear voltage dependence, being 23-44% and 54-70% of the fully open conductance. Sublevels in different channel types share similar voltage dependence of the mean lifetime and similar ion selectivity properties. However, the mean lifetime of each current level depends approximately geometrically on the number of mutant subunits in the channel, becoming shorter in channels having fewer mutant subunits. Each mutant subunit appears to stabilize all of the conducting states by approximately 0.5 kcal/mol. Consistent with previous results in the mutant channel, sublevels in channels with two or no mutant subunits also showed ion selectivities that differ from that of the fully open level, having relatively higher K+ than Rb+ conductances. A model is presented in which Shaker channels have two coupled activation gates, one associated with the selectivity filter and a second associated with the S6 helix bundle.
先前对T442S突变型Shaker通道的一项研究揭示了激活偶联的亚电导水平，这些水平显然代表了通道激活过程中的动力学中间体（郑，J.，和F.J.西格沃思。1997年。《普通生理学杂志》110:101 - 117）。我们现在已将该研究扩展到由不同数量突变亚基组成的异源多聚体通道以及没有突变亚基的通道，所有这些都处于电导增加的嵌合Shaker通道背景中。已发现所有这些通道类型中都存在激活偶联的亚水平，并且在至少80%的所有失活时间进程中会经历这些亚水平。在对称的K⁺溶液中，两个亚水平的电流具有线性电压依赖性，分别为完全开放电导的23 - 44%和54 - 70%。不同通道类型中的亚水平具有相似的平均寿命电压依赖性和相似的离子选择性特性。然而，每个电流水平的平均寿命大致按几何比例取决于通道中突变亚基的数量，在具有较少突变亚基的通道中会变得更短。每个突变亚基似乎能使所有导电状态稳定约0.5千卡/摩尔。与先前在突变通道中的结果一致，具有两个或没有突变亚基的通道中的亚水平也显示出与完全开放水平不同的离子选择性，其K⁺电导相对于Rb⁺电导相对较高。本文提出了一个模型，其中Shaker通道有两个偶联的激活门，一个与选择性过滤器相关，另一个与S6螺旋束相关。
