Pervasive Roles of Ubiquitination in Orchestrating the Deubiquitinase Activity and Function of the BAP1/ASXL2 Tumor Suppressor Complex

The tumor suppressor and deubiquitinase (DUB) BAP1 regulates chromatin-associated processes and is frequently mutated in various malignancies. BAP1 and its drosophila orthologue Calypso assemble DUB complexes with ASXL-1, -2, -3 paralogues and ASX respectively, and these cofactors are required for stimulating their DUB activity. However how the DUB activity of BAP1 is regulated remains largely unknown. Here we show that BAP1 promotes monoubiquitination of ASXLs on the ASXM/DEUBAD domain. ASXL2 monoubiquitination promotes its stability or proteasomal degradation, stimulates BAP1 DUB activity and is required for mammalian cell proliferation. Monoubiquitination of ASXL2 is directly catalyzed by UBE2E family of ubiquitin conjugating enzymes and is regulated by deubiquitination. Monoubiquitination of ASX is regulated by Calypso and is required for drosophila development. We further revealed a switch mechanism that tightly regulate BAP1 function, as a monoubiquitination of BAP1 UCH domain is mutually exclusive with ASXL2 monoubiquitination, thus ensuring highly coordinated DUB-mediated signaling.

肿瘤抑制因子与去泛素化酶（deubiquitinase, DUB）BAP1可调控染色质相关生物学过程，且在多种恶性肿瘤中频发突变。BAP1及其果蝇同源蛋白Calypso分别与ASXL-1、-2、-3旁系同源物以及ASX组装形成DUB复合物，此类辅助因子是激活其DUB活性的必需条件。然而，BAP1的DUB活性具体调控机制目前仍不明确。本研究发现，BAP1可促进ASXL家族蛋白在ASXM/DEUBAD结构域发生单泛素化修饰。ASXL2的单泛素化可调控其蛋白稳定性或介导其蛋白酶体降解，增强BAP1的DUB活性，且对于哺乳动物细胞增殖不可或缺。ASXL2的单泛素化修饰由UBE2E家族泛素结合酶直接催化，并受去泛素化过程调控。果蝇ASX的单泛素化修饰则受Calypso调控，且对果蝇发育至关重要。本研究进一步揭示了一种可精准调控BAP1功能的切换机制：BAP1的UCH结构域单泛素化与ASXL2的单泛素化修饰呈互斥关系，从而确保DUB介导的信号通路实现高度协同调控。