Cyclin B, degradation

APC is active from mid-M phase (anaphase) to the end of G1 phase and required for disconnecting sister chromatids and exit from M-Phase to Gl. The complex mediates the ubiquitination of Securin and Cyclin B. Degradation of these proteins, which block mitotic progression, promotes anaphase onset and exit from mitosis. 

The biochemical mechanism of Mos action is not yet established. Mos has been found to phosphorylate cyclin B in vitro, and it is possible that this phosphorylation directly inhibits cyclin B proteolysis (Roy et al., 1990). However, such a direct effect of phosphorylation on cyclin B stability remains to be demonstrated, and it is alternatively possible that Mos inhibits (directly or indirectly) the proteolytic pathway responsible for cyclin B degradation. Mos has recently been found to stimulate mitogen-activated protein kinase (MAP kinase) in Xenopus oocytes,... 

Hunt There is another complication here it turns out that the cyclin A in CSF-arrested frog eggs is extremely unstable. The APC is certainly active in that situation, and the checkpoint that prevents the degradation of B-type cyclins can somehow recognize and spare the cyclin B, but has no effect on cyclin A. 

Mailer Of course, cyclin B is being degraded at a certain rate it is not zero. 

In higher eukaryotes, at the onset of S phase cyclin A accumulates which stimulates DNA synthesis. The amount of cyclin A continues to be high after the S phase because of its role in chromosome condensation. Cyclin A is degraded when cells enter prometaphase. The level of another cyclin called cyclin B rises during G2 phase, which helps to complete the chromosome condensation and spindle assembly, which allow transition to metaphase. Cyclin B is degraded by APC during metaphase. ... 

Important APC substrates are the mitotic cyclins and the anaphase inhibitors. Proteolysis of mitotic cyclins is activated at the metaphase/anaphase transition and is only switched off at the start of S phase. The mitotic cyclin A is degraded before cyclin B. Lasting activity of the Hctl-APC during Gx phase is thought to be responsible for the lack of detection of mitotic cyclins in Gx phase. Renewed accumulation of mitotic cyclins is only possible again when APC activity is switched off at the start of S phase. 

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