Fin1-PP1 Helps Clear Spindle Assembly Checkpoint Protein Bub1 from Kinetochores in Anaphase

Abstract

The spindle assembly checkpoint (SAC) monitors chromosome attachment defects and the assembly of SAC proteins at kinetochores is essential for its activation, but the SAC disassembly process remains unknown. We found that deletion of a 14-3-3 protein, Bmh1, or hyper-activation of FEAR (Cdc14 Early Anaphase Release) allows premature SAC silencing in budding yeast, which depends on a kinetochore protein Fin1 that forms a complex with protein phosphatase PP1. Previous works suggest that FEAR-dependent Fin1 dephosphorylation promotes Bmh1-Fin1 dissociation, which enables kinetochore recruitment of Fin1-PP1. We found persistent kinetochore association of SAC protein Bub1 in fin1Δ mutants after anaphase entry. Therefore, we revealed a mechanism that clears SAC proteins from kinetochores. After anaphase entry, FEAR activation promotes kinetochore enrichment of Fin1-PP1, resulting in SAC disassembly at kinetochores. This mechanism is required for efficient SAC silencing after SAC being challenged, and untimely Fin1-kinetochore association causes premature SAC silencing and chromosome missegregation.

The Opposing Functions of Protein Kinases and Phosphatases in Chromosome Bipolar Attachment

“…FEAR-mediated Cdc14 release leads to Fin1 dephosphorylation and kinetochore recruitment of Fin1-PP1, which clears SAC protein Bub1 from the kinetochore. The results from phospho-deficient fin1 mutants (fin1-5A) support the expected consequence of Fin1 dephosphorylation [77,86]. Therefore, Cdc14 phosphatase also regulates Ipl1/PP1 balance at the kinetochore through Fin1 dephosphorylation during anaphase, although the biological significance of this regulation is not fully understood yet (Figure 3).…”

“…cdc55∆ mutant cells show premature release of Cdc14, which allows the dephosphorylating of CDK substrates, including the CPC component Sli15. We found that Cdc55 is required for efficient cell cycle delay in response to tensionless attachment [77]. It will be interesting to examine the Ipl1 kinetochore localization in cdc55 mutant cells at metaphase to clarify if the dysfunction of PP2A Cdc55 compromises CPC kinetochore localization.…”

“…Fin1 dephosphorylation by Cdc14 phosphatase during early anaphase enables kinetochore recruitment of Fin1-PP1 [76]. Although the exact target of Fin1-PP1 is unclear, we recently found that Fin1-PP1 kinetochore localization is necessary to remove SAC protein Bub1 from the kinetochore, because fin1∆ cells show persistent Bub1 kinetochore localization in anaphase [77]. Mutating the CDK phosphorylation sites of Fin1 to alanine produces a non-phosphorylatable Fin1-5A protein that is prematurely targeted to the kinetochore.…”

The phosphorylation of a kinetochore protein Dam1 by Aurora B/Ipl1 kinase promotes chromosome bipolar attachment in yeast

“…Cdc13 binds to telomeres and protects chromosome ends 30 . In cdc13-1 mutants incubated at non-permissive temperatures, unprotected telomeres activate the DNA damage checkpoint to arrest cells in pre-anaphase with established chromosome bipolar attachment 31 , 32 . To follow the process of chromosome bipolar attachment, WT and dam1-3A mutant cells with Mtw1-GFP and Tub1-mCherry were arrested in G 1 phase and then released into 34 °C medium containing nocodazole for 2 hr.…”

“…The association of PP1 with Spc105 is essential for the SAC silencing, and mutation of the PP1 binding site in Spc105 protein results in lethality due to the failure of SAC silencing 35 . Although Fin1 is also responsible kinetochore recruitment of PP1, this function is not essential for SAC silencing as fin1 ∆ deletion mutants are viable 32 , 36 , 37 . It will be our future interest to investigate the role of Spc105- or Fin1-associated PP1 in Dam1 dephosphorylation.…”