title: Nucleolar separation from chromosomes during Aspergillus nidulans mitosis can occur without spindle forces.
abstract: How the nucleolus is segregated during mitosis is poorly understood and occurs by very different mechanisms during closed and open mitosis. Here we report a new mechanism of nucleolar segregation involving removal of the nucleolar-organizing regions (NORs) from nucleoli during Aspergillus nidulans mitosis. This involves a double nuclear envelope (NE) restriction which generates three NE-associated structures, two daughter nuclei (containing the NORs), and the nucleolus. Therefore, a remnant nucleolar structure can exist in the cytoplasm without NORs. In G1, this parental cytoplasmic nucleolus undergoes sequential disassembly releasing nucleolar proteins to the cytoplasm as nucleoli concomitantly reform in daughter nuclei. By depolymerizing microtubules and mutating spindle assembly checkpoint function, we demonstrate that a cycle of nucleolar "segregation" can occur without a spindle in a process termed spindle-independent mitosis (SIM). During SIM physical separation of the NOR from the nucleolus occurs, and NE modifications promote expulsion of the nucleolus to the cytoplasm. Subsequently, the cytoplasmic nucleolus is disassembled and rebuilt at a new site around the nuclear NOR. The data demonstrate the existence of a mitotic machinery for nucleolar segregation that is normally integrated with mitotic spindle formation but that can function without it.
authors.affiliation: Department of Molecular Genetics, Ohio State University, Columbus, 43210, USA.
authors.forename: Leena
authors.initials: L
authors.lastname: Ukil
authors.forename: Colin P
authors.initials: CP
authors.lastname: De Souza
authors.forename: Hui-Lin
authors.initials: HL
authors.lastname: Liu
authors.forename: Stephen A
authors.initials: SA
authors.lastname: Osmani
mesh_terms: D001233:Aspergillus nidulans / Q000166:cytology* / Q000378:metabolism*; D002466:Cell Nucleolus / Q000378:metabolism*; D020090:Chromosome Segregation; D015825:Chromosomes, Fungal / Q000378:metabolism*; D003593:Cytoplasm / Q000378:metabolism; D004271:DNA, Fungal / Q000378:metabolism; D005656:Fungal Proteins / Q000378:metabolism; D007734:Kymography; D008938:Mitosis; D008954:Models, Biological; D009685:Nuclear Envelope / Q000378:metabolism; D009687:Nuclear Proteins / Q000378:metabolism; D009697:Nucleolus Organizer Region / Q000378:metabolism; D008941:Spindle Apparatus / Q000378:metabolism*; D013997:Time Factors
chemical_list: D004271:DNA, Fungal; D005656:Fungal Proteins; D009687:Nuclear Proteins