Contractile actomyosin network on nuclear envelope remnants positions chromosomes for mitosis

To ensure proper segregation during mitosis, chromosomes must be efficiently captured by kinetochore microtubules and subsequently aligned on the mitotic spindle. The efficacy of chromosome capture by the mitotic spindle can be influenced by how widely chromosomes are scattered in space. Here, we quantify chromosome-scattering volume (CSV) and find that it is reduced immediately after nuclear envelope breakdown (NEBD) in human cells. The reduction of CSV occurs independently of microtubules and is therefore not an outcome of interactions between chromosomes and the spindle. We find that prior to NEBD, an actomyosin network is assembled in a LINC complex-dependent manner on the cytoplasmic surface of the nuclear envelope. Myosin-II-mediated contraction of this nuclear envelope-associated actomyosin network around chromosomes reduces CSV and facilitates their interaction with spindle microtubules.