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The budding-yeast Polo-like kinase, with multiple mitotic functions including regulation of the Bub2-Bfa1 complex.

A budding-yeast kinase contributing to the spindle position checkpoint through regulation of the Bub2-Bfa1 complex. Its activation and location are both relevant.

Both checkpoints couple cell-cycle timing to spatial organization, but they act at different transitions. The budding-yeast spindle position checkpoint prevents premature mitotic exit when the anaphase spindle has not been appropriately placed across the mother-bud axis. The fly centrosome orientation checkpoint delays entry into mitosis until interphase centrosomes have the orientation needed for a properly oriented division.

In Saccharomyces cerevisiae, one product of division must enter the bud. If both spindle poles and chromosome masses remain in the mother, cytokinesis would generate the wrong nuclear distribution. The mitotic exit network promotes sustained activation of Cdc14 and the decline of mitotic CDK activity. Its upstream GTPase Tem1 is restrained by the Bub2-Bfa1 complex. When the spindle is mispositioned, Kin4 helps preserve that inhibition and opposes the checkpoint-inactivating action of Cdc5 on Bfa1. Thus spindle mispositioning is connected to continued mitotic arrest rather than simply to an attempt to pull harder on chromosomes.^[ref]

Spatial regulation makes this pathway sensitive to which compartment a spindle pole occupies. Kin4 activity is associated with the mother-cell side, whereas the bud environment favours release of the inhibitory state when a pole enters appropriately. Elm1 contributes to activation of Kin4. This is a regulated network involving localization and phosphorylation, rather than an infallible single switch at the bud neck. It is also distinct from the spindle checkpoint, which monitors kinetochore attachment and restrains anaphase; a spindle can have attached chromosomes yet be in the wrong position.^[ref]

In male Drosophila melanogaster germline stem cells, contact with the hub defines the stem-cell niche. Normally one centrosome is retained toward the hub and the other moves away, preparing a division in which one daughter retains niche contact. Live imaging showed that misoriented centrosomes can delay division until orientation is restored, including in aged or dedifferentiated cells. Counting misoriented interphase centrosomes alone therefore overestimates the frequency of misoriented mitoses.^[ref]

The fly checkpoint involves Centrosomin, Par-1, and regulation of cyclin A localization. Par-1 and the germline spectrosome help restrain the mitotic programme; perturbing this organization can uncouple orientation from mitotic entry. Bazooka provides a cortical platform associated with centrosome anchoring. These observations support spatial control of mitotic competence, not identity of the molecular pathway with yeast Tem1 signalling.^[ref][ref]

A decisive experimental comparison measures both position and transition timing in live cells. A pure orientation defect raises misorientation and prolongs the relevant pause; loss of the checkpoint permits the transition despite misorientation. Rescue with an appropriate wild-type component helps establish specificity. In yeast, score mitotic exit and nuclear inheritance; in fly stem cells, score entry into mitosis and daughter contact with the niche. This separates the mechanism that constructs the correct geometry from the mechanism that waits for it.