Clb2 phosphorylation and de-phosphorylation dynamics set the conditions for anaphase progression
Mitotic events are orchestrated by oscillations in the activity of cyclin-dependent kinase 1 (Cdk1), which is modulated by the mitotic cyclins (Clb2 in yeast) associating with it. To progress through anaphase and next exit from mitosis, cells must inactivate Cdk1. The latter is achieved mainly via proteolysis of Clb2 by the proteasome. Clb2 degradation is mediated by the ubiquitin ligase APC/C (anaphase-promoting complex) in association with co-factor Cdc20 or Cdh1. Clb2 degradation is bi-phasic; about half of Clb2 molecules is turned over at the metaphase-anaphase transition by the APC/C-Cdc20, while the rest becomes degraded at mitotic exit by the APC/C-Cdh1.
One question that has remained unanswered is why and how APC/C-Cdc20 degrades only half of the Clb2 pool. We propose that Cdc5-mediated phosphorylation of Clb2 represents a signal for APC/C-Cdc20 recognition. Concomitantly with Clb2 turn over, the phosphatase Cdc14 becomes activated. This initial activation of Cdc14 requires the function of Clb2. We show that a non-phosphorylatable Clb2 protein besides being protected from degradation is also impaired in Cdc14 release. Taken together, Clb2 phos-phorylation by Cdc5 is required for APC/C-Cdc20-mediated Clb2 degradation and for Clb2 function within the FEAR network to promote Cdc14 activation. Active Cdc14 dephosphorylates Clb2 therewith restricting its own activity at early anaphase and protecting the remaining Clb2 molecules from degrading.