Carrillo Elisa, Pacheco Lucero, Balleza Daniel, Gomez-Lagunas Froylan
Departamento de Fisiología, Universidad Nacional Autónoma de México, DF, México.
PLoS One. 2015 Mar 23;10(3):e0120431. doi: 10.1371/journal.pone.0120431. eCollection 2015.
Potassium channels allow the selective flux of K⁺ excluding the smaller, and more abundant in the extracellular solution, Na⁺ ions. Here we show that Shab is a typical K⁺ channel that excludes Na⁺ under bi-ionic, Na(o)/K(i) or Na(o)/Rb(i), conditions. However, when internal K⁺ is replaced by Cs⁺ (Na(o)/Cs(i)), stable inward Na⁺ and outward Cs⁺ currents are observed. These currents show that Shab selectivity is not accounted for by protein structural elements alone, as implicit in the snug-fit model of selectivity. Additionally, here we report the block of Shab channels by external Ca²⁺ ions, and compare the effect that internal K⁺ replacement exerts on both Ca²⁺ and TEA block. Our observations indicate that Ca²⁺ blocks the channels at a site located near the external TEA binding site, and that this pore region changes conformation under conditions that allow Na⁺ permeation. In contrast, the latter ion conditions do not significantly affect the binding of quinidine to the pore central cavity. Based on our observations and the structural information derived from the NaK bacterial channel, we hypothesize that Ca²⁺ is probably coordinated by main chain carbonyls of the pore's first K⁺-binding site.
钾通道允许K⁺选择性通过，而排除细胞外溶液中更小且更丰富的Na⁺离子。我们在此表明，Shab是一种典型的钾通道，在双离子条件下，即Na(o)/K(i)或Na(o)/Rb(i)条件下，能够排除Na⁺。然而，当内部的K⁺被Cs⁺取代时（Na(o)/Cs(i)），会观察到稳定的内向Na⁺电流和外向Cs⁺电流。这些电流表明，Shab的选择性并非仅由蛋白质结构元件决定，这与选择性的紧密契合模型所暗示的情况不同。此外，我们在此报告了外部Ca²⁺离子对Shab通道的阻断作用，并比较了内部K⁺替代对Ca²⁺和TEA阻断的影响。我们的观察结果表明，Ca²⁺在靠近外部TEA结合位点的位置阻断通道，并且该孔区域在允许Na⁺通透的条件下会发生构象变化。相比之下，后一种离子条件对奎尼丁与孔中央腔的结合没有显著影响。基于我们的观察结果以及从NaK细菌通道获得的结构信息，我们推测Ca²⁺可能与孔的第一个K⁺结合位点的主链羰基配位。
