The Surveillance Mechanism of the Spindle Position Checkpoint in Yeast

Abstract

The spindle position checkpoint in Saccharomyces cerevisiae delays mitotic exit until the spindle has moved into the mother–bud neck, ensuring that each daughter cell inherits a nucleus. The small G protein Tem1p is critical in promoting mitotic exit and is concentrated at the spindle pole destined for the bud. The presumed nucleotide exchange factor for Tem1p, Lte1p, is concentrated in the bud. These findings suggested the hypothesis that movement of the spindle pole through the neck allows Tem1p to interact with Lte1p, promoting GTP loading of Tem1p and mitotic exit. However, we report that deletion of LTE1 had little effect on the timing of mitotic exit. We also examined several mutants in which some cells inappropriately exit mitosis even though the spindle is within the mother. In some of these cells, the spindle pole body did not interact with the bud or the neck before mitotic exit. Thus, some alternative mechanism must exist to coordinate mitotic exit with spindle position. In both wild-type and mutant cells, mitotic exit was preceded by loss of cytoplasmic microtubules from the neck. Thus, the spindle position checkpoint may monitor such interactions.

Mad3/BubR1 Phosphorylation during Spindle Checkpoint Activation Depends on both Polo and Aurora Kinases in Budding Yeast

“…40,41 Conversely, full nocodazole-induced Mad3 phosphorylation did not require Bub2 that is involved in a different branch of the checkpoint responding to spindle misorientation. 28,[41][42][43][44][45][46][47] Two possible explanations could account for the partial dependency of Mad3 hyper-phosphorylation upon spindle checkpoint proteins in the presence of nocodazole: (1) the kinase(s) involved is specifically upregulated when the spindle checkpoint is engaged; (2) the kinase(s) involved is cell cycle-regulated and accumulates during the cell cycle arrest caused by microtubule depolymerization. In the latter case, the low levels of phosphorylated Mad3 in spindle checkpoint mutants might be the consequence of their inability to sustain cell cycle arrest upon nocodazole treatment, while the checkpoint defect of bub2∆ cells allows to escape the nocodazole-induced cell cycle block later than the other mad and bub mutants.…”

Pex3p Initiates the Formation of a Preperoxisomal Compartment from a Subdomain of the Endoplasmic Reticulum in Saccharomyces cerevisiae

“…Four-dimensional in Vivo Video Microscopy-Cells grown in YPR medium and then incubated in RIM for 16 h were prepared for fourdimensional in vivo video microscopy by placing 1-2 l of culture on a slide with a thin agarose pad containing 2% galactose, which was covered with a coverslip and sealed with petroleum jelly (28). Cells were incubated at room temperature for image capture.…”

Mitotic Exit and Separation of Mother and Daughter Cells

“…Lte1, which is localized in the daughter cell, blocks Kin4's ability to phosphorylate Bfa1 (Bertazzi et al 2011); this requires the p21-activated kinase Cla4 (Seshan et al 2002;Chiroli et al 2003). Intriguingly, cells lacking cytoplasmic SPB-anchored microtubules exhibit a partial checkpoint defect, and the interaction of these microtubules with the bud neck has been proposed to activate the spindle orientation checkpoint (Adames et al 2001;Moore et al 2009). …”