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PLoS Biology:细胞膜上离子通道通过体积识别离子

来源:生物谷 2007-07-02 09:12

    生物谷报道:离子通道是细胞膜上控制带电粒子进出的微小通道。但是它们是非常严格的“门卫”:每种通道只允许特定的一种离子通过。最近,来自Rockefeller大学的科学家揭示了至少其中一种离子通道——钾离子通道——背后的热动力学机制。

    为了使神经将电信号快速传递到全身各处,它们必须保持在细胞内和周围存在电荷分布。尽管科学家一直知道离子通道是保持这种分布的原因,但他们不清楚通道如何区分不同的离子,例如带有同样电荷的钾和钠。

    在发表于PLoS  Biology上的文章中,Rockefeller分子神经及生物物理学实验室负责人Rod  MacKinnon教授证明,钾离子通道通过离子大小来进行区分。

    钾离子比钠离子稍大——半径大约大1/3——较大的体积意味着更少的表面电荷分布。小组的博士后Steve  Lockless是文章第一作者,他说:“钾离子需要更多空间,但同时其电荷分布区域也更大,因此钾离子的电荷密度较低。”

    为了证明钾离子通道依赖于其体积而非电荷密度进行离子区分,Lockless、MacKinnon和博士后Ming  Zhou设计了一个实验。他们将细胞膜上的钾离子通道分离开,并将它们放入钡溶液中。钡的体积和钾类似,但是带有两个负电荷,因此其电荷密度和钠离子差不多。

    Lockless说:“如果通道通过电荷密度识别,那么钡就不会结合到上面。但是结果正好相反。”钡最终结合到了离子通道上,这证明了离子通道通过离子大小进行识别。(引自教育部科技发展中心)

原文链接:http://www.physorg.com/news102176422.html

刘乐译自:physorg.com

原始出处:

PLoS  Biology

Structural and Thermodynamic Properties of Selective Ion Binding in a K+ Channel

Steve W. Lockless1,2, Ming Zhou1,2¤, Roderick MacKinnon1,2*

1 Laboratory of Molecular Neurobiology and Biophysics, The Rockefeller University, New York, New York, United States of America, 2 Howard Hughes Medical Institute, The Rockefeller University, New York, New York, United States of America

Thermodynamic measurements of ion binding to the Streptomyces lividans K+ channel were carried out using isothermal titration calorimetry, whereas atomic structures of ion-bound and ion-free conformations of the channel were characterized by x-ray crystallography. Here we use these assays to show that the ion radius dependence of selectivity stems from the channel's recognition of ion size (i.e., volume) rather than charge density. Ion size recognition is a function of the channel's ability to adopt a very specific conductive structure with larger ions (K+, Rb+, Cs+, and Ba2+) bound and not with smaller ions (Na+, Mg2+, and Ca2+). The formation of the conductive structure involves selectivity filter atoms that are in direct contact with bound ions as well as protein atoms surrounding the selectivity filter up to a distance of 15 Å from the ions. We conclude that ion selectivity in a K+ channel is a property of size-matched ion binding sites created by the protein structure.

Funding. This work was supported in part by NIH grant number GM43949 to RM.

Competing interests. The authors have declared that no competing interests exist.

Academic Editor: Richard W. Aldrich, University of Texas Austin, United States of America

Citation: Lockless SW, Zhou M, MacKinnon R (2007) Structural and Thermodynamic Properties of Selective Ion Binding in a K+ Channel. PLoS Biol 5(5): e121 doi:10.1371/journal.pbio.0050121

Received: December 13, 2006; Accepted: March 1, 2007; Published: May 1, 2007

Figure 1. K+ Ion Binding Sites within the KcsA K+ Channel

(A) A ribbon representation of KcsA with two of the four subunits is shown; the subunits closest to and furthest from the viewer are removed for clarity. K+ ions (in green) are located within the selectivity filter (orange) and in the water-filled cavity. The gray lines indicate the presumed interior and exterior membrane boundaries.

(B) A stick representation of the selectivity filter containing four K+ ion-binding sites (S1–S4). Each K+ ion-binding site is composed of eight oxygen atoms made from the K+ channel TVGYG signature sequence. The figure was made using PyMOL [33].

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