title: Roles of different pools of the mitotic checkpoint complex and the mechanisms of their disassembly.
abstract: The mitotic (or spindle assembly) checkpoint system prevents premature separation of sister chromatids in mitosis. When the checkpoint is turned on, the mitotic checkpoint complex (MCC) inhibits the ubiquitin ligase anaphase-promoting complex/cyclosome (APC/C). MCC is composed of the checkpoint proteins BubR1, Bub3, and Mad2 associated with the APC/C activator Cdc20. The mechanisms of the assembly of MCC when the checkpoint is turned on, and of its disassembly when the checkpoint is inactivated, are not sufficiently understood. Previous reports indicated that APC/C-mediated polyubiquitylation of Cdc20 in MCC is required for the dissociation of APC/C-associated MCC, but not of free MCC. The pool of free MCC is disassembled by an ATP-dependent process stimulated by the Mad2-binding protein p31(comet). It remained unknown whether free MCC is the precursor or the dissociation product of APC/C-bound MCC. By characterizing the mechanisms of the disassembly of APC/C-bound MCC in a purified system, we find that it cannot be the source of free MCC, because it is bound at high affinity and is released only in ubiquitylated or partially disassembled forms. By the use of a cell-free system from Xenopus eggs that reproduces the mitotic checkpoint, we show that MCC can be assembled in the absence of APC/C in a checkpoint-dependent manner. We propose that when the checkpoint is turned on, free MCC is the precursor of APC/C-bound MCC. When the mitotic checkpoint is extinguished, both APC/C-bound and free MCC pools have to be disassembled to release APC/C from inhibition.
authors.affiliation: Unit of Biochemistry, The Rappaport Faculty of Medicine, Technion-Israel Institute of Technology, Haifa 31096, Israel.
authors.forename: Esther
authors.initials: E
authors.lastname: Eytan
authors.forename: Danielle
authors.initials: D
authors.lastname: Sitry-Shevah
authors.forename: Adar
authors.initials: A
authors.lastname: Teichner
authors.forename: Avram
authors.initials: A
authors.lastname: Hershko
mesh_terms: D048868:Adaptor Proteins, Signal Transducing / Q000378:metabolism; D064173:Anaphase-Promoting Complex-Cyclosome; D000818:Animals; D002135:Calcium-Binding Proteins / Q000737:chemistry / Q000378:metabolism; D064199:Cdc20 Proteins; D018797:Cell Cycle Proteins / Q000737:chemistry* / Q000378:metabolism*; D002474:Cell-Free System; D005260:Female; D006367:HeLa Cells; D006801:Humans; D059566:M Phase Cell Cycle Checkpoints / Q000502:physiology*; D064246:Mad2 Proteins; D046912:Multiprotein Complexes / Q000737:chemistry / Q000378:metabolism; D009687:Nuclear Proteins / Q000378:metabolism; D009865:Oocytes / Q000378:metabolism; D000075223:Poly-ADP-Ribose Binding Proteins; D017346:Protein Serine-Threonine Kinases / Q000737:chemistry / Q000378:metabolism; D012097:Repressor Proteins / Q000737:chemistry / Q000378:metabolism; D043743:Ubiquitin-Protein Ligase Complexes / Q000737:chemistry* / Q000378:metabolism*; D014981:Xenopus; D029867:Xenopus Proteins / Q000737:chemistry / Q000378:metabolism
chemical_list: D048868:Adaptor Proteins, Signal Transducing; C489444:BUB3 protein, human; D002135:Calcium-Binding Proteins; D064199:Cdc20 Proteins; D018797:Cell Cycle Proteins; C577697:MAD2L1 protein, human; C470309:MAD2L1BP protein, human; D064246:Mad2 Proteins; D046912:Multiprotein Complexes; D009687:Nuclear Proteins; D000075223:Poly-ADP-Ribose Binding Proteins; D012097:Repressor Proteins; D029867:Xenopus Proteins; C577488:CDC20 protein, human; D043743:Ubiquitin-Protein Ligase Complexes; D064173:Anaphase-Promoting Complex-Cyclosome; C584277:BUB1 protein, human; C090464:Bub1 spindle checkpoint protein; D017346:Protein Serine-Threonine Kinases