Ect2/Pbl Acts via Rho and Polarity Proteins to Direct the Assembly of an Isotropic Actomyosin Cortex upon Mitotic Entry

Entry into mitosis is accompanied by profound changes in cortical actomyosin organization. Here, we delineate a pathway downstream of the RhoGEF Pbl/Ect2 that directs this process in a model epithelium. Our data suggest that the release of Pbl/Ect2 from the nucleus at mitotic entry drives Rho-dependent activation of Myosin-II and, in parallel, induces a switch from Arp2/3 to Diaphanous-mediated cortical actin nucleation that depends on Cdc42, aPKC, and Par6. At the same time, the mitotic relocalization of these apical protein complexes to more lateral cell surfaces enables Cdc42/aPKC/Par6 to take on a mitosis-specific function—aiding the assembly of a relatively isotropic metaphase cortex. Together, these data reveal how the repolarization and remodeling of the actomyosin cortex are coordinated upon entry into mitosis to provide cells with the isotropic and rigid form they need to undergo faithful chromosome segregation and division in a crowded tissue environment. •Pbl/Ect2 drives a shift in epithelial polarity upon entry into mitosis•Lateral spreading of Cdc42/aPKC/Par6 aids assembly of an isotropic metaphase cortex•Mitosis triggers a switch from Arp2/3 to Dia-mediated cortical actin nucleation Rosa et al. identify a pathway activated upon entry into mitosis that drives lateral spreading of “apical” polarity proteins to coordinate the remodeling of cell shape and cytoskeleton. This mitotic repurposing of polarity proteins generates the rigid rounded cortex required for successful division in a crowded tissue.