所有活细胞都积累钾离子，而很高的细胞内钾浓度是很多生理过程所必需的。在细菌、酵母和植物中，钾离子吸收是由一个超级家族的运输蛋白“SKT蛋白”实现的。这些蛋白被认为是从简单的钾通道形成的，但是钾离子选择性和运输的机制却不清楚。现在，一种细菌SKT蛋白（来自“副溶血性弧菌”的TrkH钾运输蛋白）的晶体结构已被确定。它的选择性过滤器与钾离子通道中的择性过滤器相似，但较短。生物化学研究表明，K+选择性取决于一个新的门控机制，该机制将Na+和Li+等较小的离子排除在外，而让K+和Rb+等较大的离子进来。（生物谷Bioon.com）
生物谷推荐原文出处：
Nature doi:10.1038/nature09731
Crystal structure of a potassium ion transporter, TrkH
Yu Cao,1, 11 Xiangshu Jin,2, 11 Hua Huang,1, 11 Mehabaw Getahun Derebe,3 Elena J. Levin,1 Venkataraman Kabaleeswaran,1 Yaping Pan,1 Marco Punta,4, 5 James Love,4 Jun Weng,1 Matthias Quick,6, 7 Sheng Ye,3 Brian Kloss,4 Renato Bruni,4 Erik Martinez-Hackert,8 Wayne A. Hendrickson,8 Burkhard Rost,4, 5 Jonathan A. Javitch,6, 7, 9 Kanagalaghatta R. Rajashankar,10 Youxing Jiang3 & Ming Zhou1
The TrkH/TrkG/KtrB proteins mediate K+ uptake in bacteria and probably evolved from simple K+ channels by multiple gene duplications or fusions. Here we present the crystal structure of a TrkH from Vibrio parahaemolyticus. TrkH is a homodimer, and each protomer contains an ion permeation pathway. A selectivity filter, similar in architecture to those of K+ channels but significantly shorter, is lined by backbone and side-chain oxygen atoms. Functional studies showed that TrkH is selective for permeation of K+ and Rb+ over smaller ions such as Na+ or Li+. Immediately intracellular to the selectivity filter are an intramembrane loop and an arginine residue, both highly conserved, which constrict the permeation pathway. Substituting the arginine with an alanine significantly increases the rate of K+ flux. These results reveal the molecular basis of K+ selectivity and suggest a novel gating mechanism for this large and important family of membrane transport proteins.
