Modeling the Budding Yeast Cell Cycle


Cdc20 is essential for exit of mitosis. Deletion mutant (cdc20) arrests in M phase, with unseparated sister chromatids and high Clb2 activity (Alexandru et al., 1999; Lim et al., 1998). It is an activator of APC (for review, see (Prinz & Amon, 1999; Zachariae & Nasmyth, 1999; Morgan, 1999).




  • In the presence of unattached kinetochores, signaling in the MAD/BUB pathway results in the inhibition of Cdc20 (for review, Gardner & Burke, 2000, Fig. 1).
  • Since MAD and BUB genes are not essential genes (Li & Murray, 1991; Alexandru et al., 1999), it indicates that Cdc20 must be activated (by our hypothetical protein IE) before it can be functional.
  • IE may be the phosphorylated form of the APC. Rudner & Murray (Rudner & Murray, 2000b, Fig. 2B) showed that three components of the APC core, Cdc16, Cdc23 and Cdc27 are phospho-proteins, and that they are phosphorylated by Cdc28 kinase. The non-phosphorylable APC (APC-A mutants) shows reduced binding with Cdc20, and reduced mitotic Cdc20-dependent activity. However, APC-A has normal Cdh1-dependent activity in G1.