Institute of Biomedical Sciences, Academia Sinica, Taipei 11529, Taiwan.
J Gen Physiol. 2012 Mar;139(3):245-59. doi: 10.1085/jgp.201110736.
Outward currents through Kir2.1 channels play crucial roles in controlling the electrical properties of excitable cells, and such currents are subjected to voltage-dependent block by intracellular Mg(2+) and polyamines that bind to both high- and low-affinity sites on the channels. Under physiological conditions, high-affinity block is saturated and yet outward Kir2.1 currents can still occur, implying that high-affinity polyamine block cannot completely eliminate outward Kir2.1 currents. However, the underlying molecular mechanism remains unknown. Here, we show that high-affinity spermidine block, rather than completely occluding the single-channel pore, induces a subconducting state in which conductance is 20% that of the fully open channel. In a D172N mutant lacking the high-affinity polyamine-binding site, spermidine does not induce such a substate. However, the kinetics for the transitions between the substate and zero-current state in wild-type channels is the same as that of low-affinity block in the D172N mutant, supporting the notion that these are identical molecular events. Thus, the residual outward current after high-affinity spermidine block is susceptible to low-affinity block, which determines the final amplitude of the outward current. This study provides a detailed insight into the mechanism underlying the emergence of outward Kir2.1 currents regulated by inward rectification attributed to high- and low-affinity polyamine blocks.
外向电流通过 Kir2.1 通道在控制可兴奋细胞的电特性方面起着至关重要的作用，而这些电流会受到细胞内 Mg(2+)和多胺的电压依赖性阻断，多胺结合在通道的高亲和和低亲和位点上。在生理条件下，高亲和性阻断是饱和的，但外向 Kir2.1 电流仍然可以发生，这意味着高亲和性多胺阻断不能完全消除外向 Kir2.1 电流。然而，其潜在的分子机制仍然未知。在这里，我们表明高亲和性亚精胺阻断，而不是完全阻塞单通道孔，会诱导亚导通状态，其电导为完全开放通道的 20%。在缺乏高亲和性多胺结合位点的 D172N 突变体中，亚精胺不会诱导这种亚状态。然而，在野生型通道中，亚状态和零电流状态之间的转变动力学与 D172N 突变体中的低亲和性阻断相同，支持这些是相同的分子事件的观点。因此，高亲和性亚精胺阻断后残留的外向电流易受低亲和性阻断的影响，这决定了外向电流的最终幅度。本研究深入了解了由高亲和性和低亲和性多胺阻断引起的内向整流调节的外向 Kir2.1 电流出现的机制。
