Prior to their delivery to and degradation by the 26S proteasome, misfolded transmembrane proteins of the ER and inner-nuclear membrane must be extracted from lipid bilayers. This extraction process, known as retrotranslocation, requires both quality-control E3 ubiquitin ligases and dislocation factors that diminish the energetic cost of dislodging the transmembrane segments of a protein. Recently, we showed that retrotranslocation of all ER transmembrane proteins requires the Dfm1 rhomboid pseudoprotease. However, we did not investigate whether Dfm1 also mediated retrotranslocation of transmembrane substrates in the inner-nuclear membrane (INM), which is contiguous with the ER but functionally separated from it by nucleoporins. Here, we show that canonical retrotranslocation occurs during INM-associated degradation (INMAD) but proceeds independently of Dfm1. Despite this independence, ERAD-M and INMAD cooperate to mitigate proteotoxicity. We show a novel misfolded-transmembrane-protein toxicity that elicits genetic suppression, demonstrating the cell's ability to tolerate a toxic burden of misfolded transmembrane proteins without functional INMAD or ERAD-M. This strikingly contrasted the suppression of the dfm1Δ null, which leads to the resumption of ERAD-M through HRD-complex remodeling. Thus, we conclude that INM retrotranslocation proceeds through a novel, private channel, which can be studied by virtue of its role in alleviating membrane-associated proteotoxicity.

在将错误折叠的内质网和核内膜跨膜蛋白递送至 26S 蛋白酶体并被 26S 蛋白酶体降解之前，必须将其从脂质双层中提取出来。这种提取过程称为逆转录易位，需要质量控制 E3 泛素连接酶和位错因子，以减少移出蛋白质跨膜片段的能量成本。最近，我们发现所有 ER 跨膜蛋白的逆转位都需要 Dfm1 菱形假蛋白酶。然而，我们没有研究 Dfm1 是否也介导内核膜 (INM) 中跨膜底物的逆转位，该内核膜与 ER 相邻，但通过核孔蛋白在功能上与其分开。在这里，我们表明典型的逆转位发生在 INM 相关降解 (INMAD) 期间，但独立于 Dfm1 进行。尽管存在这种独立性，ERAD-M 和 INMAD 仍合作减轻蛋白毒性。我们展示了一种新的错误折叠跨膜蛋白毒性，可引起基因抑制，证明细胞能够在没有功能性 INMAD 或 ERAD-M 的情况下耐受错误折叠跨膜蛋白的毒性负担。这与dfm1Δ null 的抑制形成鲜明对比，dfm1Δ null 的抑制导致 ERAD-M 通过 HRD 复合体重塑恢复。因此，我们得出结论，INM 逆转位通过一种新颖的、私人的通道进行，可以利用其在减轻膜相关蛋白毒性方面的作用来研究该通道。