If the spindle isn't aligned properly, the spindle position checkpoint (SPOC) pauses mitosis so that the cell can make adjustments. Caydasi et al. identify a master switch for the checkpoint.

When a yeast cell begins to bud, the SPOC detects whether the spindle has lined up parallel to the bud–mother cell axis. One of the main SPOC proteins is the kinase Kin4. What controls its activity is unclear, so the researchers looked for proteins that flip on Kin4.

Caydasi et al. started with yeast cells that manufacture extra Kin4, which die because they become stuck in anaphase. The team reasoned that genes whose absence allowed the cells to survive were likely to be Kin4 activators. One of the activators they identified was another kinase called Elm1, which is part of a pathway that helps cells retain their shape.

To determine how Elm1 affects the SPOC, the researchers tested a yeast strain prone to faulty spindle alignment. If the spindle was askew, control cells took at least an hour to complete anaphase because they stopped to make corrections. But Elm1-deficient cells raced through anaphase in only 24 minutes, suggesting that they couldn't activate the SPOC.

Elm1 phosphorylates Kin4 throughout the cell cycle. Where the two proteins interact remains uncertain. Elm1 normally settles at the base of the newly forming bud, but it doesn't have to remain there for Kin4 to switch on. The researchers think that Elm1 activates Kin4 so that the SPOC is primed to act if the spindle misaligns. ML

et al
J. Cell Biol.