Starace Dorine M, Bezanilla Francisco
Department of Physiology, David Geffen School of Medicine at UCLA, Los Angeles, California 90095, USA.
Nature. 2004 Feb 5;427(6974):548-53. doi: 10.1038/nature02270.
Voltage-dependent potassium channels are essential for the generation of nerve impulses. Voltage sensitivity is conferred by charged residues located mainly in the fourth transmembrane segment (S4) of each of the four identical subunits that make up the channel. These charged segments relocate when the potential difference across the membrane changes, controlling the ability of the pore to conduct ions. In the crystal structure of the Aeropyrum pernix potassium channel KvAP, the S4 and part of the third (S3B) transmembrane alpha-helices are connected by a hairpin turn in an arrangement termed the 'voltage-sensor paddle'. This structure was proposed to move through the lipid bilayer during channel activation, transporting positive charges across a large fraction of the membrane. Here we show that replacing the first S4 arginine by histidine in the Shaker potassium channel creates a proton pore when the cell is hyperpolarized. Formation of this pore does not support the paddle model, as protons would not have access to a lipid-buried histidine. We conclude that, at hyperpolarized potentials, water and protons from the internal and external solutions must be separated by a narrow barrier in the channel protein that focuses the electric field to a small voltage-sensitive region.
电压依赖性钾通道对于神经冲动的产生至关重要。电压敏感性由主要位于构成通道的四个相同亚基中每个亚基的第四个跨膜片段（S4）中的带电残基赋予。当跨膜电位差发生变化时，这些带电片段会重新定位，从而控制孔道传导离子的能力。在嗜热栖热菌钾通道KvAP的晶体结构中，S4和第三个（S3B）跨膜α螺旋的一部分通过一个发夹结构相连，这种排列被称为“电压传感器桨叶”。该结构被认为在通道激活过程中穿过脂质双层，将正电荷运输穿过大部分膜。在此我们表明，在Shaker钾通道中用组氨酸取代第一个S4精氨酸，当细胞超极化时会形成一个质子孔道。这个孔道的形成并不支持桨叶模型，因为质子无法接触到埋在脂质中的组氨酸。我们得出结论，在超极化电位下，来自内部和外部溶液的水和质子必须被通道蛋白中的一个狭窄屏障隔开，该屏障将电场聚焦到一个小的电压敏感区域。
