Cdc20 and Cdh1
Cdc20-homologue 1 (Cdh1) plays a complimentary role to Cdc20 in cell cycle progression. During the time of APCCdc20 activity, Cdh1 is phosphorylated and cannot bind to the APC. After metaphase, however, S/M-Cdks are inactivated by APCCdc20, and Cdh1 can exist in a non-phosphorylated state and bind the APC. This enables the APC to continue to degrade S/M cyclins (and thus S/M Cdks) until they are needed again in the next S-phase. How can S/M cyclins reappear to shepherd the cell into mitosis? The APCCdc20 does not recognize G1/S cyclins. Their concentration rises during G1, activativing G1/S Cdks, which in turn phosphorylate Cdh1 and gradually relieve the inhibition on S/M cyclins.
Read more about this topic: CDC20