Misfolded proteins in the endoplasmic reticulum (ER) are degraded by ER-associated degradation (ERAD). Although ERAD components involved in degradation of luminal substrates are well characterized, much less is known about quality control of membrane proteins. Here, we analyzed the degradation pathways of two short-lived ER membrane model proteins in mammalian cells. Using a CRISPR-Cas9 genome-wide library screen, we identified an ERAD branch required for quality control of a subset of membrane proteins. Using biochemical and mass spectrometry approaches, we showed that this ERAD branch is defined by an ER membrane complex consisting of the ubiquitin ligase RNF185, the ubiquitin-like domain containing proteins TMUB1/2 and TMEM259/Membralin, a poorly characterized protein. This complex cooperates with cytosolic ubiquitin ligase UBE3C and p97 ATPase in degrading their membrane substrates. Our data reveal that ERAD branches have remarkable specificity for their membrane substrates, suggesting that multiple, perhaps combinatorial, determinants are involved in substrate selection.

内质网（ER）中错误折叠的蛋白质会被ER相关降解（ERAD）降解。尽管与内腔基质降解有关的ERAD成分已得到很好的表征，但对膜蛋白的质量控制知之甚少。在这里，我们分析了哺乳动物细胞中两种短寿命的ER膜模型蛋白的降解途径。使用CRISPR-Cas9全基因组文库筛选，我们鉴定了对膜蛋白子集进行质量控制所需的ERAD分支。使用生化和质谱方法，我们显示该ERAD分支由ER膜复合物定义，该膜复合物由泛素连接酶RNF185（泛素样结构域，包含蛋白质TMUB1 / 2和TMEM259 / Membralin）组成，特征不明确。该复合物与胞质泛素连接酶UBE3C和p97 ATPase协同作用，降解了其膜底物。我们的数据表明，ERAD分支对它们的膜底物具有显着的特异性，表明底物选择涉及多个决定因素。