Instituto de Biología Molecular y Celular, Universidad Miguel Hernández, Elche, 03202 Alicante, Spain.
Instituto de Biología Molecular y Celular, Universidad Miguel Hernández, Elche, 03202 Alicante, Spain.
Biochim Biophys Acta Biomembr. 2017 May;1859(5):779-788. doi: 10.1016/j.bbamem.2017.01.014. Epub 2017 Jan 12.
This work explores whether the ion selectivity and permeation properties of a model potassium channel, KcsA, could be explained based on ion binding features. Non-permeant Na or Li bind with low affinity (millimolar K's) to a single set of sites contributed by the S1 and S4 sites seen at the selectivity filter in the KcsA crystal structure. Conversely, permeant K, Rb, Tl and even Cs bind to two different sets of sites as their concentration increases, consistent with crystallographic evidence on the ability of permeant species to induce concentration-dependent transitions between conformational states (non-conductive and conductive) of the channel's selectivity filter. The first set of such sites, assigned also to the crystallographic S1 and S4 sites, shows similarly high affinities for all permeant species (micromolar K's), thus, securing displacement of potentially competing non-permeant cations. The second set of sites, available only to permeant cations upon the transition to the conductive filter conformation, shows low affinity (millimolar K's), thus, favoring cation dissociation and permeation and results from the contribution of all S1 through S4 crystallographic sites. The differences in affinities between permeant and non-permeant cations and the similarities in binding behavior within each of these two groups, correlate fully with their permeabilities relative to K, suggesting that binding is an important determinant of the channel's ion selectivity. Conversely, the complexity observed in permeation features cannot be explained just in terms of binding and likely relates to reported differences in the occupancy of the S2 and S3 sites by the permeant cations.
这项工作探讨了模型钾通道 KcsA 的离子选择性和渗透特性是否可以基于离子结合特征来解释。非渗透离子 Na 或 Li 以低亲和力（毫摩尔 K）与单个由 S1 和 S4 位贡献的一组结合位点结合，这些结合位点在 KcsA 晶体结构的选择性过滤器中可见。相反，随着浓度的增加，可渗透离子 K、Rb、Tl 甚至 Cs 结合到两个不同的结合位点，这与晶体学证据一致，表明可渗透离子具有诱导通道选择性过滤器构象状态（非传导和传导）之间浓度依赖性转变的能力。第一个这样的结合位点也被分配给晶体学 S1 和 S4 位，对所有可渗透离子（微摩尔 K）显示出相似的高亲和力，从而确保置换潜在竞争的非渗透阳离子。第二个结合位点仅在过渡到传导过滤器构象时对可渗透阳离子可用，对其亲和力低（毫摩尔 K），从而有利于阳离子解离和渗透，这是由于所有 S1 到 S4 晶体学位的贡献。可渗透和非渗透阳离子之间的亲和力差异以及这两个组内结合行为的相似性，与它们相对于 K 的渗透率完全相关，这表明结合是通道离子选择性的重要决定因素。相反，渗透特征的复杂性不能仅仅用结合来解释，可能与可渗透阳离子对 S2 和 S3 位的占有率的差异有关。
