A novel mechanism of nuclear envelope break-down in a fungus: nuclear migration strips off the envelope.

In animals, the nuclear envelope disassembles in mitosis, while budding and fission yeast form an intranuclear spindle. Ultrastructural data indicate that basidiomycetes, such as the pathogen Ustilago maydis, undergo an 'open mitosis'. Here we describe the mechanism of nuclear envelope break-down in U. maydis. In interphase, the nucleus resides in the mother cell and the spindle pole body is inactive. Prior to mitosis, it becomes activated and nucleates microtubules that reach into the daughter cell. Dynein appears at microtubule tips and exerts force on the spindle pole body, which leads to the formation of a long nuclear extension that reaches into the bud. Chromosomes migrate through this extension and together with the spindle pole bodies leave the old envelope, which remains in the mother cell until late telophase. Inhibition of nuclear migration or deletion of a Tem1p-like GTPase leads to a 'closed' mitosis, indicating that spindle pole bodies have to reach into the bud where MEN signalling participates in envelope removal. Our data indicate that dynein-mediated premitotic nuclear migration is essential for envelope removal in U. maydis.