The mitotic exit network

Abstract

What is it? The mitotic exit network – commonly known by the acronym MEN – is a signal transduction cascade which triggers the exit of mitosis. The MEN has been characterized in the budding yeast Saccharomyces cerevisiae, and the protein names below refer to components from this species, though many are conserved in vertebrates. The main switch of this cascade is the small G protein Tem1 and its regulators: a two-component GAP made of Bub2 and Bfa1, which keeps Tem1 inactive, and the putative exchange factor Lte1, which contributes to the activation of Tem1. Upon activation, Tem1 activates the Cdc15 protein kinase, which phosphorylates and activates another kinase made up of the Mob1 and Dbf2 proteins. Another protein, the Cdc5 polo-like kinase, is involved at several steps in the cascade.What is its function? Upon activation in late anaphase, and through a still unknown mechanism, the MEN ensures release into the cytoplasm of the Cdc14 phosphatase from the nucleolus, where it was kept inactive by binding to Net1. The resulting dephosphorylation wave leads through different mechanisms to inactivation of cyclin B–cyclin-dependent kinase (Cdk), allowing exit from mitosis and preparation for a new G1 phase. The MEN is thus a crucial checkpoint in the cell cycle that tightly coordinates completion of chromosome segregation and exit from mitosis, ensuring the proper segregation of genetic information. The MEN may also control cytokinesis (see below).How is the MEN regulated? One important aspect of this in S. cerevisiae is spatial regulation. Whereas Tem1, Bub2 and Bfa1 localise preferentially to the spindle pole body (SPB) – the yeast centrosome – destined for the daughter cell, Lte1 is restricted to the bud cortex (Figure 1Figure 1). Tem1 is thus held inactive by Bub2–Bfa1 until the SPB enters the bud and encounters Lte1. Upon activation of Tem1, Cdc15, Dbf2 and Mob1 are recruited to the SPBs as well, where they are activated.Figure 1Towards the end of mitosis, the MEN components asymmetrically localise to the SPB that enters the bud and so encounter Lte1 and are activated. This triggers the release of Cdc14 from the nucleolus which drives inactivation of mitotic kinase, relocalisation of the MEN complex to the mother/bud neck and cytokinesis (see text for details).View Large Image | View Hi-Res Image | Download PowerPoint SlideIs spatial regulation the whole story? Some studies have suggested the MEN is also regulated by phosphorylation. Bfa1 is phosphorylated by Cdc5, which inhibits the GAP activity and thus activates the MEN. Phosphorylation of Lte1 by the Cdk and by the PAK kinase Cla4 also contribute to MEN regulation, at least by controlling Lte1 localisation. Bub2, Cdc15, Dbf2 and Mob1 are also phosphoproteins. The recent identification of the FEAR pathway (see below) raises the possibility that Cdc14 itself may activate the MEN.Isn't the idea that Cdc14 activates the MEN confusing? Although the MEN is responsible for full release of Cdc14 into the cytoplasm in late anaphase, recent work has shown that another pathway – the ‘FEAR’ network – is involved in a transient and partial release of Cdc14 from the nucleolus in early anaphase. This allows Cdc14 to load onto the SPBs, probably to activate the MEN. Later in anaphase, after MEN activation, Cdc14 dephosphorylates Bfa1, Cdc15 and Lte1, switching Tem1 off to release Cdc15, Dbf2 and Mob1 from the SPBs and allow them to migrate to the neck of the budding yeast cell (Figure 1Figure 1).Why do MEN components go to the neck? There is considerable evidence indicating that some MEN components go to the neck to contribute to cytokinesis, probably by controlling the contraction of actomyosin and septin protein ‘rings’, although the molecular targets are still not known. In fact, the homologues of MEN components in the fission yeast Schizosaccharomyces pombe are predominantly involved in septum initiation, and less if at all in mitotic kinase inactivation.