Wurzenberger et al. identify two phosphatase-targeting proteins that help chromosomes to segregate properly during anaphase.
In early mitosis, the kinase Aurora B localizes to centromeres, where it eliminates incorrect attachments to the mitotic spindle by phosphorylating microtubule-binding kinetochore proteins like Dsn1. When sister chromatids are attached to opposite spindle poles, tension pulls the kinetochore proteins away from Aurora B, stabilizing their interactions with spindle microtubules. This tension is lost when sister chromatids separate at the beginning of anaphase, however, so cells must somehow prevent Aurora B from weakening kinetochore–microtubule attachments during chromosome segregation. This is partly achieved by relocating Aurora B to the central spindle, but Wurzenberger et al. suspected that phosphatases might also counteract Aurora B on anaphase chromosomes.
In an RNAi screen of phosphatase subunits, the researchers identified two proteins whose depletion enhanced the phosphorylation of Aurora B substrates on anaphase chromatin. Sds22 and Repo-Man both target the phosphatase PP1 to chromosomes. Though knocking down either protein had no effect on Aurora B's activity or relocation to the central spindle, Dsn1 was more phosphorylated in anaphase cells lacking Sds22 or Repo-Man. Segregating chromosomes often paused in these cells, suggesting that their attachments to the spindle were unstable. This frequently resulted in chromosome missegregation.
Senior author Daniel Gerlich now wants to investigate which Aurora B substrates, besides Dsn1, are hyperphosphorylated in the absence of Sds22 or Repo-Man and to determine which of these are responsible for the defects in kinetochore–microtubule attachment during anaphase.
