The cortical protein Lte 1 promotes mitotic exit by inhibiting the spindle position checkpoint kinase Kin 4

The unequal distribution of cell fate determinants during asymmetric cell division is a fundamental process that underlies the generation of cell diversity in a variety of multicellular organisms (Yamashita et al., 2007). The positioning of the mitotic spindle relative to the cell polarity axis is critical to mediate asymmetric cell divisions (Siller and Doe, 2009). Several mechanisms ensure correct spindle alignment in the asymmetrically dividing unicellular organism, budding yeast Saccharomyces cerevisiae. First, spindle positioning relative to the intrinsic polarity axis of the cell is ensured by two partially redundant pathways; one reliant upon the function of the conserved protein Kar9, the other upon the microtubule motor protein dynein (Segal and Bloom, 2001). Second, if spindle orientation fails, a surveillance mechanism called the spindle positioning checkpoint (SPOC) pauses cell cycle progression until proper spindle alignment is achieved (Fraschini et al., 2008; Caydasi et al., 2010a). Under normal cell cycle progression, the mitotic exit network (MEN) initiates mitotic exit and cytokinesis in late anaphase. The MEN is a GTPase-driven signaling pathway whose components associate with the yeast microtubule organizing center, the spindle pole body (SPB). MEN signaling starts with the activation of the GTPase Tem1 that triggers the sequential activation of the protein kinase Cdc15 and the Dbf2–Mob1 kinase complex. Dbf2–Mob1 promotes the full activation of the protein phosphatase Cdc14, which is needed to promote Cdk inactivation, leading to mitotic exit and cytokinesis (Bardin and Amon, 2001). Feedback mechanisms involving Cdc14 and Cdk contribute to MEN activation at the level of Cdc15 and Dbf2– Mob1 (Jaspersen and Morgan, 2000; König et al., 2010). However, the mechanisms by which the activation of Tem1 is controlled remain debatable. Tem1 preferentially associates with the SPB that enters the daughter cell (dSPB; Bardin et al., 2000; Pereira et al., 2000). Based on genetic studies and the presence of the guanine nucleotide exchange factor (GEF) homology domain, the putative GEF Lte1 has been implicated in Tem1 activation (Keng et al., 1994; Shirayama et al., 1994b). Lte1 is confined at the daughter cell cortex and the entrance of the dSPB bound to Tem1 brings Tem1 in close proximity to Lte1, thus promoting Tem1 activation (Bardin et al., 2000; Pereira et al., 2000). However, Lte1 is not essential for mitotic exit at temperatures above 30°C and no GEF activity toward Tem1 has been The spindle position checkpoint (SPOC) is an essen­ tial surveillance mechanism that allows mitotic exit only when the spindle is correctly oriented along the cell axis. Key SPOC components are the kinase Kin4 and the Bub2–Bfa1 GAP complex that inhibit the mitotic exit– promoting GTPase Tem1. During an unperturbed cell cycle, Kin4 associates with the mother spindle pole body (mSPB), whereas Bub2–Bfa1 is at the daughter SPB (dSPB). When the spindle is mispositioned, Bub2–Bfa1 and Kin4 bind to both SPBs, which enables Kin4 to phosphorylate Bfa1 and thereby block mitotic exit. Here, we show that the daughter cell protein Lte1 physically interacts with Kin4 and inhibits Kin4 kinase activity. Specifically, Lte1 binds to cata­ lytically active Kin4 and promotes Kin4 hyperphosphory­ lation, which restricts Kin4 binding to the mSPB. This Lte1­mediated exclusion of Kin4 from the dSPB is essential for proper mitotic exit of cells with a correctly aligned spindle. Therefore, Lte1 promotes mitotic exit by inhibiting Kin4 activity at the dSPB. The cortical protein Lte1 promotes mitotic exit by inhibiting the spindle position checkpoint kinase Kin4