Background: Many organisms undergo closed mitosis and locate tubulin and mitotic kinases to nuclei only during mitosis. How this is regulated is unknown. Interestingly, the NIMA kinase of Aspergillus nidulans interacts with two nuclear pore complex (NPC) proteins and NIMA is required for mitotic localization of the Cdk1 kinase to nuclei. Therefore, we wished to define the mechanism by which the NPC is regulated during A. nidulans' closed mitosis.
Results: The structural makeup of the NPC is dramatically changed during A. nidulans' mitosis. At least five NPC proteins disperse throughout the cell during mitosis while at least three structural components remain at the NPC. These modifications correlate with marked changes in the function of the NPC. Notably, during mitosis, An-RanGAP is not excluded from nuclei, and five other nuclear or cytoplasmic proteins investigated fail to locate as they do during interphase. Mitotic modification of the NPC requires NIMA and Cdk1 kinase activation. NIMA appears to be particularly important. Most strikingly, ectopic induction of NIMA promotes mitotic-like changes in NPC structure and function during S phase. Furthermore, NIMA locates to the NPC during entry into mitosis, and a dominant-negative version of NIMA that causes G2 delay dwells at the NPC.
Conclusions: We conclude that partial NPC disassembly under control of NIMA and Cdk1 in A. nidulans may represent a new mechanism for regulating closed mitoses. We hypothesize that proteins locate by their relative binding affinities within the cell during A. nidulans' closed mitosis, analogous to what occurs during open mitosis.