MEN (Mitotic Exit Network)
The components of this network are Lte1, Tem1, Bub2/Bfa1, Cdc15, Cdc5, Dbf2/Mob1, and Cdc14 (Hwang et al., 1998). The activation of MEN is required for exit of mitosis. Temperature sensitive mutations in these genes show arrest in telophase (review: Cid et al., 2002).
Proteins of MEN present in the model:
- Lte1: GTP-GDP exchange factor. Mutants of this gene shows a temperature sensitive phenotype, at low temperature, Lte1 is required for termination of M phase (Shirayama, et al., 1994a; Bardin et al., 2000). Lte1 is located in the daughter cell only. It activates Tem1 when Tem1 enters the bud.
- Bub2/Bfa1: checkpoint protein. Bub2 in complex with Bfa1 monitors the spindle pole orietation. It prevents the cell from exit of mitosis (Li, 1999; Pereira et al., 2000; Lee et al., 2001b; Hu et al., 2001) when the spindle is not properly oriented. In a wild-type, when BUB2 is mutated, the timing of cell cycle events is not altered. However, when treated in nocodazole (microtubule polymerization prevented), the cell shows a defect that it exits from mitotic arrest and rebud after long incubation, Alexandru et al., 1999).
- Tem1: GTP-binding protein. Mutations of this gene block the cell in telophase (Shirayama et al., 1994b). Tem1 is active when located in the bud (Bardin et al., 2000). Tem1 acts upstream of Cdc15, Cdc14 and Dbf2 since the defect of tem1-ts mutant can be overcome by over-expression of these proteins (Jaspersen et al., 1998).
- Cdc15: protein kinase. Mutations of this gene lead to a telophase arrest (Shirayama et al., 1996). Cdc15 is activated by Tem1 (Asakawa et al., 2001) and as all the components of MEN, it is required for exit from mitosis. In our model, we use Cdc15 to represent the endpoint of the MEN pathway, responsible for the release of Cdc14. We are aware that there are other elements following Cdc15 activation that lead to this release.
- Cdc14: go to details of Cdc14.
MEN controls the release of Cdc14 from the RENT complex. MEN is only activated
when the Bud2-dependent spindle assembly checkpoint is satisfied (Yoshida et al., 2002b; Visintin et al., 2003 ).
- Lte1 level does not fluctuate much throughout the cell cycle. Lte1 is located in the cell during G1 and starts moving to the bud as it is formed (Bardin et al., 2000).
- Bub2 level does not fluctuate during the cell cycle (Fraschini et al., 1999).
- Tem1 level fluctuates throughout the cell cycle. It starts accumulating in M phase, and is at its highest level in telophase (Bardin et al., 2000).
- Cdc15 level is constant throughout the cell cycle (Jaspersen et al., 1998).
- Tem1 and Bub2/Bfa1 are located on a spindle pole body (SPB) (Li, 1999; Bardin et al., 2000) that migrates to the bud, where Lte1 is located. When the SPB moves into the newly formed bud, then Lte1 is able to activate Tem1. However, Bub2/Bfa1 keeps Tem1 inactive until the checkpoint is lifted (Pereira et al., 2000; Krishnan et al., 2000), and Cdc5 is activated. Cdc5 phosphorylates Bfa1, causing the dissociation of Bub2/Bfa1 complex, resulting in Tem1 activation (Hu et al., 2001). Cdc5 is also able to phosphorylate Cdc14 (Visintin et al., 2003 ) promotes its release from the nucleolus.
- When Tem1 is active, Cdc15 is turned on. Together with Cdc5, Cdc15 leads to Dbf2/Mob1 activation (Fesquet et al., 1999; Lee et al., 2001a), and subsequently to Net1 phosphorylation (Shou et al., 2002a). Phosphorylation of both components Cdc14 and NET1 in the RENT complex (by Cdc5 directly or indirectly) leads to total release of Cdc14 from the nucleolus (Visintin et al., 2003 ). The details of this series of activation is still unclear.
- Cdc5 is not included in the present model. Its involvement in the MEN pathway in helping the release of Cdc14, by acting on Bub2/Bfa1, Lte1, and Dbf2 will be modeled in the next version.