Gating mechanism of BK (Slo1) channels: so near, yet so far.
Gating mechanism of BK (Slo1) channels: so near, yet so far.
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DOI：
10.1085/jgp.20028721
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发表时间：
2003-02
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通讯作者：
Magleby KL
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作者：
Magleby KL
大电导钙和电压激活 K+ 通道 (Slo1)，也称为“BK”或“maxi K”通道，因为它们具有高单通道电导（对称 150 mM KCl 中为 250-300 pS），广泛分布在许多不同的组织中（Kaczorowski 等，1996）。 BK 通道的一个标志性特征，除了具有高 K+ 选择性和电导性之外，还可以通过细胞内钙离子 (Ca2+ i) 和去极化以高度协同的方式激活（Marty, 1981；Pallotta 等人，1981；Barrett 等人，1982；Latorre 等人，1982）。如图 1 所示，该图绘制了通道开放概率 Po（z 轴）与 Ca2+ i（y 轴）和膜电位（x 轴）的关系。正是这种协同激活使得 BK 通道在控制许多系统的兴奋性方面发挥关键作用，包括调节平滑肌的收缩、调节耳蜗毛细胞以及调节递质释放（Robitaille 等，1993；Ramanathan 等，1999；Brenner 等，2000；Wang 等，2001）。也正是这种协同激活加上高单通道电导和不频繁的门控转变到亚电导水平，使得 BK 通道成为门控机制研究中如此有吸引力的课题。通道的双重激活允许 Po 偏置到最佳范围，以便通过适当设置其他参数来研究电压或 Ca2+ i 的激活。这篇评论将重点关注作者对一些关键实验和观察的高度偏见的观点，这些实验和观察有助于制定我们当前关于 BK 通道如何门控的概念。由于篇幅限制，β亚基对门控的调节并未包括在内，并且大多数要讨论的论文将是最近的论文，这些论文为顶峰提供了最后的华丽攻击，而不是提供关键的逐步上山的早期研究。了解 BK 通道门控的进展基于三种互补且重要的方法：动力学分析、分子生物学和 3-D 结构。动力学和分子生物学方面取得了最大的进展。目前还没有 BK 通道的结构，但有细菌相关 K+ 通道的结构。 BK 通道是四聚体（Shen 等人，1994）。四个α亚基之一组装形成功能通道的示意图如图2所示。与电压门控 K+ 通道超家族类似（Butler 等，1990；Isacoff 等，1990；Wei 等，1990），BK 通道包含跨膜片段 S1-S6，包括 S4 电压传感器和 P 区，以形成孔的选择性过滤器（Adelman 等，1992；Butler 等，1993；Diaz 等）等，1998；崔和奥尔德里奇，2000）。此外，BK 通道具有一个将 NH2 末端置于细胞外的 S0 片段（Meera 等，1997）和一个非常大的细胞内 COOH 末端，其氨基酸残基数量是 S0–S6 中所含氨基酸残基数量的两倍多（Adelman 等，1992；Butler 等，1993）。 Wei等人(1994)发现他们可以在S8和S9之间的不保守连接子处切断通道（见图2），然后从单独的核心和尾部表达功能通道。核心可进一步细分为跨膜片段 S0-S6 和细胞内片段
Large conductance calcium-and voltage-activated K+ channels (Slo1), also referred to as “BK” or “maxi K” channels because of their high single channel conductance (250–300 pS in symmetrical 150 mM KCl), are widely distributed in many different tissues (Kaczorowski et al., 1996). A signature feature of BK channels, in addition to their high K+ selectivity and conductance, is that they are activated in a highly synergistic manner by both intracellular calcium ion (Ca2+ i) and depolarization (Marty, 1981; Pallotta et al., 1981; Barrett et al., 1982; Latorre et al., 1982). This is shown in Fig. 1, which plots channel open probability Po (z axis) versus Ca2+ i (y axis) and membrane potential (x axis). It is this synergistic activation that allows BK channels to play key roles in controlling excitability in a number of systems, including regulating the contraction of smooth muscle, the tuning of hair cells in the cochlea, and regulation of transmitter release (Robitaille et al., 1993; Ramanathan et al., 1999; Brenner et al., 2000; Wang et al., 2001). It is also this synergistic activation together with the high single-channel conductance and the infrequent gating transitions to subconductance levels that has made the BK channel such an attractive subject for the study of gating mechanism. The dual activation of the channel allows the Po to be biased into optimal ranges for the study of activation by either voltage or Ca2+ i by appropriate setting of the other parameter. This review will focus on this author’s highly biased view of some of the key experiments and observations that have helped formulate our current concept of how BK channels gate. Due to space limitations, the modulation of the gating by ß subunits is not included, and most of the papers to be discussed will be recent papers that are providing the final flashy assault to the summit, rather than the earlier studies providing the crucial stepwise slog up the mountain. The progress toward understanding how BK channels gate is based on three complimentary and essential approaches: kinetic analysis, molecular biology, and 3-D structure. The greatest progress has been made on kinetics and molecular biology. There is not yet a structure of BK channels, but there are structures of related K+ channels from bacteria. BK channels are tetramers (Shen et al., 1994). A schematic diagram of one of the four α subunits that assemble to form functional channels is shown in Fig.2. Similar to the superfamily of voltage gated K+ channels (Butler et al., 1990; Isacoff et al., 1990; Wei et al., 1990), BK channels contain transmembrane segments S1–S6, including an S4 voltage sensor and a P region to form the selectivity filter of the pore (Adelman et al., 1992; Butler et al., 1993; Diaz et al., 1998; Cui and Aldrich, 2000). In addition, BK channels have an S0 segment that places the NH2 terminus extracellular (Meera et al., 1997) and a very large intracellular COOH terminus that contains more than twice the number of amino acid residues that are contained in S0–S6 (Adelman et al., 1992; Butler et al., 1993). Wei et al.(1994) found that they could cut the channel at an unconserved linker between S8 and S9 (see Fig. 2) and then express functional channels from the separate cores and tails. The core can be further subdivided into the transmembrane segments S0–S6 and an intracellu-