The human UBR5 is a single polypeptide chain homology to E6AP C terminus (HECT)-type E3 ubiquitin ligase essential for embryonic development in mammals. Dysregulated UBR5 functions like an oncoprotein to promote cancer growth and metastasis. Here, we report that UBR5 assembles into a dimer and a tetramer. Our cryoelectron microscopy (cryo-EM) structures reveal that two crescent-shaped UBR5 monomers assemble head to tail to form the dimer, and two dimers bind face to face to form the cage-like tetramer with all four catalytic HECT domains facing the central cavity. Importantly, the N-terminal region of one subunit and the HECT of the other form an “intermolecular jaw” in the dimer. We show the jaw-lining residues are important for function, suggesting that the intermolecular jaw functions to recruit ubiquitin-loaded E2 to UBR5. Further work is needed to understand how oligomerization regulates UBR5 ligase activity. This work provides a framework for structure-based anticancer drug development and contributes to a growing appreciation of E3 ligase diversity.

人 UBR5 是与哺乳动物胚胎发育必需的E6AP C 末端 (HECT) 型 E3 泛素连接酶同源的单多肽链。失调的 UBR5 功能类似于癌蛋白，可促进癌症生长和转移。在这里，我们报告 UBR5 组装成二聚体和四聚体。我们的冷冻电子显微镜 (cryo-EM) 结构显示，两个新月形 UBR5 单体头尾相连形成二聚体，两个二聚体面对面结合形成笼状四聚体，所有四个催化 HECT 域都面向中央空腔。重要的是，一个亚基的 N 端区域和另一个亚基的 HECT 在二聚体中形成“分子间颚”。我们证明下颌衬里残基对于功能很重要，这表明分子间下颌的功能是将负载泛素的 E2 募集到 UBR5。需要进一步的工作来了解寡聚化如何调节 UBR5 连接酶活性。这项工作为基于结构的抗癌药物开发提供了框架，并有助于增强对 E3 连接酶多样性的认识。