Department of Physiology and Biophysics, University of Miami, Miller School of Medicine , Miami, Florida.
Neuroscience Program, University of Miami, Miller School of Medicine , Miami, Florida.
Am J Physiol Cell Physiol. 2018 Aug 1;315(2):C155-C163. doi: 10.1152/ajpcell.00297.2017. Epub 2018 Apr 25.
UNC-8 and MEC-4 are two members of the degenerin/epithelial Na channel (DEG/ENaC) family of voltage-independent Na channels that share a high degree of sequence homology and functional similarity. For example, both can be hyperactivated by genetic mutations [UNC-8(d) and MEC-4(d)] that induce neuronal death by necrosis. Both depend in vivo on chaperone protein MEC-6 for function, as demonstrated by the finding that neuronal death induced by hyperactive UNC-8 and MEC-4 channels is prevented by null mutations in mec-6. UNC-8 and MEC-4 differ functionally in three major ways: 1) MEC-4 is calcium permeable, whereas UNC-8 is not; 2) UNC-8, but not MEC-4, is blocked by extracellular calcium and magnesium in the micromolar range; and 3) MEC-6 increases the number of MEC-4 channels at the cell surface in oocytes but does not have this effect on UNC-8. We previously reported that Capermeability of MEC-4 is conferred by the second transmembrane domain. We show here that the extracellular "finger" domain of UNC-8 is sufficient to mediate inhibition by divalent cations and that regulation by MEC-6 also depends on this region. Thus, our work confirms that the finger domain houses residues involved in gating of this channel class and shows for the first time that the finger domain also mediates regulation by chaperone protein MEC-6. Given that the finger domain is the most divergent region across the DEG/ENaC family, we speculate that it influences channel trafficking and function in a unique manner depending on the channel subunit.
UNC-8 和 MEC-4 是电压非依赖性钠通道（DEG/ENaC）家族的两个成员，它们具有高度的序列同源性和功能相似性。例如，它们都可以被导致神经元坏死的遗传突变[UNC-8(d) 和 MEC-4(d)]超激活。体内，两者都依赖于伴侣蛋白 MEC-6 发挥功能，这一点可以通过以下事实证明：超激活的 UNC-8 和 MEC-4 通道诱导的神经元死亡可以被 mec-6 的无效突变所预防。UNC-8 和 MEC-4 在三个主要方面存在功能差异：1）MEC-4 是钙离子可渗透的，而 UNC-8 不是；2）UNC-8 而不是 MEC-4 被细胞外钙镁离子在微摩尔范围内阻断；3）MEC-6 在卵母细胞中增加 MEC-4 通道在细胞表面的数量，但对 UNC-8 没有这种作用。我们之前报道过 MEC-4 的通透性由第二跨膜域赋予。我们在这里表明，UNC-8 的细胞外“指”域足以介导二价阳离子的抑制，而 MEC-6 的调节也依赖于该区域。因此，我们的工作证实了指域包含参与该通道类门控的残基，并首次表明指域还介导伴侣蛋白 MEC-6 的调节。鉴于指域是整个 DEG/ENaC 家族中最具差异的区域，我们推测它根据通道亚基以独特的方式影响通道运输和功能。
