Department of Biological Sciences, University of Pittsburgh, Pittsburgh, PA 15260, USA.
Mol Biol Cell. 2010 Mar 15;21(6):1047-58. doi: 10.1091/mbc.e09-11-0944. Epub 2010 Jan 28.
The epithelial sodium channel (ENaC) is composed of a single copy of an alpha-, beta-, and gamma-subunit and plays an essential role in water and salt balance. Because ENaC assembles inefficiently after its insertion into the ER, a substantial percentage of each subunit is targeted for ER-associated degradation (ERAD). To define how the ENaC subunits are selected for degradation, we developed novel yeast expression systems for each ENaC subunit. Data from this analysis suggested that ENaC subunits display folding defects in more than one compartment and that subunit turnover might require a unique group of factors. Consistent with this hypothesis, yeast lacking the lumenal Hsp40s, Jem1 and Scj1, exhibited defects in ENaC degradation, whereas BiP function was dispensable. We also discovered that Jem1 and Scj1 assist in ENaC ubiquitination, and overexpression of ERdj3 and ERdj4, two lumenal mammalian Hsp40s, increased the proteasome-mediated degradation of ENaC in vertebrate cells. Our data indicate that Hsp40s can act independently of Hsp70 to select substrates for ERAD.
上皮钠通道（ENaC）由一个α、β和γ亚基组成，在水盐平衡中发挥着重要作用。由于 ENaC 在插入内质网后组装效率低下，因此大量的每个亚基都被靶向内质网相关降解（ERAD）。为了确定 ENaC 亚基如何被选择进行降解，我们为每个 ENaC 亚基开发了新型酵母表达系统。该分析的数据表明，ENaC 亚基在一个以上的隔间中显示出折叠缺陷，并且亚基周转可能需要一组独特的因子。与该假设一致，缺乏腔 Hsp40s Jem1 和 Scj1 的酵母表现出 ENaC 降解缺陷，而 BiP 功能是可有可无的。我们还发现 Jem1 和 Scj1 有助于 ENaC 的泛素化，并且内质网腔哺乳动物 Hsp40s 中的 ERdj3 和 ERdj4 的过表达增加了脊椎动物细胞中蛋白酶体介导的 ENaC 降解。我们的数据表明，Hsp40s 可以独立于 Hsp70 发挥作用，选择 ERAD 的底物。
