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.