Nuclear type I myosins actively support nucleolar and nuclear structure and function

A nucleus is a remarkably arranged organelle partitioned from the cytosol by a nuclear envelope to encapsulate a highly structured space including a well-organized genome and nucleolus. Despite the conservation, our understanding of the molecular mechanisms driving and maintaining nuclear structure are limited. Here, we investigated the influence of myosin motors in the nucleus. Classically, motor proteins are known for mediating the active transport of cargo facilitating cytoplasmic organization yet how the spacing of nucleoplasmic biomolecules is coordinated was not well defined. We found that nuclear type I myosins were essential for cell division whereas nuclear type II and V myosins were not. Depletion of type I myosin from the nucleus triggered 3D genome disorganization, gene expression changes, nucleolar disruption, and nuclear envelope morphology disfigurement. Declined nucleolar function, including rRNA synthesis and processing, occurred first followed by disorganization of the genome. Notably, nuclear depletion of type I myosin correlated with altered dynamic mobilities of the nucleolar Nop1 (fibrillarin) protein and RNA polymerase I. Overall, nuclear type I myosin is critical for the shape and activities of the nucleolus and nucleus.