On the molecular basis of ion permeation in the epithelial Na+ channel.
On the molecular basis of ion permeation in the epithelial Na+ channel.
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DOI：
10.1085/jgp.114.1.13
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发表时间：
1999-07
期刊：
影响因子：
--
通讯作者：
Schild L
中科院分区：
文献类型：
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作者：
Kellenberger S;Hoffmann-Pochon N;Gautschi I;Schneeberger E;Schild L
上皮 Na+ 通道 (ENaC) 对 Na+ 和 Li+ 的选择性高于对 K+ 的选择性，并被利尿剂阿米洛利阻断。 ENaC是由两个α、一个β和一个γ同源亚基组成的异四聚体，每个亚基包含两个跨膜片段。参与孔阻断剂阿米洛利结合的氨基酸残基位于 β 和 γ 亚基的前 M2 片段中，位于第二个假定的跨膜 α 螺旋 (M2) 之前。 M2 NH2 末端的 α 亚基 (αS589) 中的残基对于 ENaC 的分子筛分特性至关重要。 ENaC 比 Na+ 离子对 Li+ 的渗透性更强。 Na+ 的半最大单一电导浓度为 38 mM，Li+ 的半最大单位电导浓度为 118 mM，这是一种可以解释 Li+ 和 Na+ 渗透性差异的动力学特性。我们在此表明，α、β 和 γ 亚基（αG587、βG529、γS541）的前 M2 片段中同源位置的氨基酸残基突变通过改变对 Li+ 和 Na+ 的表观通道亲和力来降低 Li+/Na+ 选择性。将 Li+ 渗透的单通道数据拟合到包括三个屏障和两个结合位点的离散状态模型表明，这些突变增加了 Li+ 从外部离子结合位点通过选择性过滤器易位所需的能量。 βG529 突变为 Ser、Cys 或 Asp 使 ENaC 对 K+ 和更大的离子具有部分渗透性，类似于之前报道的 αS589 突变。我们得出结论，残基 αG587 至 αS589 以及 β 和 γ 亚基中的同源残基形成选择性过滤器，它紧密容纳 Na+ 和 Li+ 离子，并排除较大的离子（如 K+）。
The epithelial Na+ channel (ENaC) is highly selective for Na+ and Li+ over K+ and is blocked by the diuretic amiloride. ENaC is a heterotetramer made of two α, one β, and one γ homologous subunits, each subunit comprising two transmembrane segments. Amino acid residues involved in binding of the pore blocker amiloride are located in the pre-M2 segment of β and γ subunits, which precedes the second putative transmembrane α helix (M2). A residue in the α subunit (αS589) at the NH2 terminus of M2 is critical for the molecular sieving properties of ENaC. ENaC is more permeable to Li+ than Na+ ions. The concentration of half-maximal unitary conductance is 38 mM for Na+ and 118 mM for Li+, a kinetic property that can account for the differences in Li+ and Na+ permeability. We show here that mutation of amino acid residues at homologous positions in the pre-M2 segment of α, β, and γ subunits (αG587, βG529, γS541) decreases the Li+/Na+ selectivity by changing the apparent channel affinity for Li+ and Na+. Fitting single-channel data of the Li+ permeation to a discrete-state model including three barriers and two binding sites revealed that these mutations increased the energy needed for the translocation of Li+ from an outer ion binding site through the selectivity filter. Mutation of βG529 to Ser, Cys, or Asp made ENaC partially permeable to K+ and larger ions, similar to the previously reported αS589 mutations. We conclude that the residues αG587 to αS589 and homologous residues in the β and γ subunits form the selectivity filter, which tightly accommodates Na+ and Li+ ions and excludes larger ions like K+.