Cdc15 was reported to bind either first to the mSPB, the dSPB, or with equal timing to both SPBs ( Cenamor et al., 1999 Xu et al., 2000 Menssen et al., 2001 Molk et al., 2004).
The mSPB recruits only a small fraction of Tem1 ( Molk et al., 2004). Throughout the majority of anaphase Tem1 at the dSPB resides in a complex with the Bfa1–Bub2 GAP and therefore is probably in its inactive GDP-bound form ( Pereira et al., 2000). During an unperturbed cell cycle the inhibitory Bfa1–Bub2 GAP complex localizes preferentially at the dSPB, where it inhibits the MEN, until Cdc5 polo-like kinase inactivates the Bfa1–Bub2 complex in late anaphase ( Bardin et al., 2000 Pereira et al., 2000 Hu et al., 2001 Caydasi and Pereira, 2009). In addition, MEN proteins bind differently to the mSPB and dSPB. The preexisting, older SPB is inherited by the daughter cell, the bud ( Pereira et al., 2001). This polarity is reflected in the two SPBs as they are functionally and biochemically distinct. Growth by budding generates an inherent polarity in the yeast cell. One function of the Dbf2–Mob1 complex is to phosphorylate Cdc14 at sites adjacent to its nuclear localization sequence, thereby retaining Cdc14 in the cytoplasm ( Mohl et al., 2009). Tem1 interacts with the Pak-like kinase Cdc15 ( Asakawa et al., 2001), which in turn activates the Dbf2–Mob1 kinase complex via phosphorylation of the kinase subunit Dbf2 ( Mah et al., 2001).
One of the most upstream MEN components is the Ras-like GTPase Tem1 that is controlled by the putative guanine nucleotide exchange factor Lte1 and the GTPase-activating protein (GAP) complex Bfa1–Bub2 ( Shirayama et al., 1994 Bardin et al., 2000 Pereira et al., 2000 Geymonat et al., 2002).
SPB DAUGHTER FULL
Mitotic exit and cytokinesis require full activation of Cdc14 by the mitotic exit network (MEN), a GTPase-driven signaling cascade that is associated with the SPB ( Shirayama et al., 1994 Luca and Winey, 1998 Cenamor et al., 1999 Gruneberg et al., 2000 Xu et al., 2000 Menssen et al., 2001 Pereira and Schiebel, 2001 Stegmeier and Amon, 2004). Consistent with this model, only triple mutants that lack BUB2 and the Cdk1 phosphorylation sites in Mob1 and Cdc15 show mitotic exit defects. Although MEN activity in the daughter cells is controlled by Bfa1–Bub2, Cdk1 inhibits MEN activity at the mSPB. Our data revise the understanding of the spatial regulation of the MEN. In addition, Cdk1 phosphorylates the Mob1 protein to inhibit the activity of Dbf2–Mob1 kinase that regulates Cdc14 phosphatase. Conversely, Cdk1 negatively regulates binding of Cdc15 to the mSPB. In early anaphase Cdk1 becomes recruited to the mSPB depending on the activity of the MEN kinase Cdc15.
Here, we show mutual regulation of cyclin-dependent kinase 1 (Cdk1) and the MEN. The inhibitory Bfa1–Bub2 GTPase-activating protein (GAP) only associates with the daughter SPB (dSPB), raising the question as to how the MEN is regulated on the mother SPB (mSPB). The mitotic exit network (MEN) is a spindle pole body (SPB)–associated, GTPase-driven signaling cascade that controls mitotic exit.