TMUB1 promotes p97-mediated segregation of hydrophobic transmembrane domains from the endoplasmic reticulum membrane

Membrane protein clients of endoplasmic reticulum (ER)-associated degradation must be retrotranslocated from the ER membrane by the AAA-ATPase p97 for proteasomal degradation. Before direct engagement with p97, client transmembrane domains (TMDs) that have partially or fully crossed the membrane must be constantly shielded to avoid non-native interactions. How client TMDs are seamlessly escorted from the membrane to p97 is unknown. Here, we identified ER-resident TMUB1 as a TMD-specific escortase. TMUB1 interacts with the TMD of clients within the membrane and holds ~10-14 residues of hydrophobic sequence that is exposed out of membrane, using its transmembrane and cytosolic regions, respectively. The ubiquitin-like domain of TMUB1 recruits p97, which can pull client TMDs from bound TMUB1 into the cytosol. Disruption of TMUB1 escortase activity dramatically stabilizes otherwise transient retrotranslocating intermediates in the ER membrane and impairs client retrotranslocation. Thus, TMUB1 promotes TMD segregation by safeguarding TMD movement from the membrane to p97.

质膜蛋白的溶酶体体相关降解客户必须由AAA-ATP酶p97从内质网（ER）膜进行逆向转运以实现蛋白酶体降解。在与p97直接结合之前，部分或全部穿过膜的客户跨膜结构域（TMDs）必须持续得到保护，以避免非天然相互作用。关于客户TMDs如何无缝地从膜 escort 到p97，尚属未知。在此研究中，我们鉴定出内质网定居的TMUB1作为一种TMD特异性escortase。TMUB1与膜内客户的TMD相互作用，并利用其跨膜和细胞质区域，分别保持约10-14个疏水性氨基酸序列的暴露，这些序列从膜外伸出。TMUB1的泛素样结构域招募p97，p97能够将客户TMDs从结合的TMUB1中拉入细胞质。破坏TMUB1 escortase活性会显著稳定内质网膜中原本短暂逆向转运的中间体，并损害客户的逆向转运。因此，TMUB1通过保护TMD从膜向p97的移动，促进TMD的分离。