Mps1 sends condensin II off to work

The kinase Mps1 has a long to-do list during mitosis, and Kagami et al. identify yet another responsibility: promoting chromosome condensation.

The condensin I and condensin II complexes prompt diffuse chromatin to scrunch up into mitotic chromosomes. Condensin II gets a head start on its partner, but researchers still aren’t sure which enzymes switch on condensin II at the start of mitosis.

One of them, Kagami et al. found, is Mps1. This protein’s multiple obligations during mitosis include spurring spindle microtubules to connect to kinetochores, helping chromosomes align, and fostering accurate chromosome separation. The researchers searched for proteins that partner with Mps1 and found that the kinase associated with SMC2, a component of both condensin complexes. Condensin I remains in the cytoplasm until the nuclear envelope breaks down, whereas condensin II resides in the nucleus. The team used this difference to show that Mps1 interacts specifically with condensin II.

The researchers then asked how Mps1 influences condensin II’s function. They found that Mps1 phosphorylates the CAP-H2 subunit of condensin II. Less of this subunit appeared on the chromosomes in cells lacking Mps1, suggesting that the protein directs the complex to the chromosomes. Reducing Mps1 levels also cut the amount of chromosome condensation during prophase, indicating that the kinase spurs the early stages of chromosome compaction by stimulating condensin II.