As the nuclear envelope is not broken down during a closed mitosis, the cell must regulate the nuclear entry of mitotic kinases and tubulin. This was assumed to occur through cell cycle regulated alterations of specific transport pathways. “I, and probably most everyone,” says Osmani, “almost took for granted that subtle changes in the transport properties of the pore alter import pathways slightly. Probably in [budding yeast] this is the case. But no one thought it could be completely open to allow diffusion.”
But diffusion is just what his group noted during Aspergillus mitosis. The nuclear pores of this fungus were opened by the dispersal of FG-repeat containing nucleoporins (FG-Nups), which normally form the diffusive barrier to the nucleus. Core structural proteins, however, were retained at the pore.
The dispersal of FG-Nups was accompanied by their phosphorylation and required the mitotic kinases NIMA and Cdk1. NIMA moved to the nuclear periphery at mitosis and may alter FG–Nup interactions via direct phosphorylation.
Many proteins that are restricted to the nucleus or cytoplasm during interphase were found in both compartments during mitosis, including RanGAP. The likely resulting loss of the RanGTP gradient is expected to impair regulated transport further. Since the gradient must be rebuilt to reestablish active transport after mitosis, Osmani speculates that the nuclear envelope may break down transiently and reform around the daughter masses of DNA, where the RanGEF is concentrated. No one has seen such an event as yet, as the envelope is difficult to visualize by this stage of division.