Mechanism of M18BP1-mediated activation of condensin II for mitotic DNA compaction.

Phosphorylated M18BP1 activates human condensin II by inducing conformational changes that relieve autoinhibition, enabling stable DNA binding and timely mitotic DNA compaction.

• Why it matters: Condensin II must be activated during mitosis to organize chromosomes correctly, but how M18BP1 turns on condensin II at the molecular level was unknown.
• What they did: Using cryo-electron microscopy and single-molecule imaging, the study tested how phosphorylated M18BP1 binding alters condensin II structure and its behavior on DNA.
• The result: The phosphorylated interaction promotes stable DNA binding and compaction, and it also yields multimeric condensin II species that resist optical-tweezers forces, implying mechanical reinforcement of mitotic chromosomes.

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