Eukaryotic cells have developed mechanisms for regulating the nuclear transport of macromolecules that control various cellular events including movement through defined stages of the cell cycle. In yeast cells, where the nuclear envelope remains intact throughout the cell cycle, these transport regulatory mechanisms must also function during mitosis. We have uncovered a mechanism for regulating transport that is controlled by M phase specific molecular rearrangements in the nuclear pore complex (NPC). These changes allow a transport inhibitory nucleoporin, Nup53p, to bind the karyopherin Kap121p specifically during mitosis, slowing its movement through the NPC and inducing cargo release. Yeast strains that possess defects in the function of Kap121p or the fidelity of the inhibitory pathway are delayed in mitosis. We propose that fluctuations in Kap121p transport mediated by the NPC contribute to controlling the subcellular distribution of molecules that direct progression through mitosis.

† These authors contributed equally to this work.Several components of the nuclear transport machinery play a role in mitotic spindle assembly in higher eukaryotes. To further investigate the role of this family of proteins in microtubule function, we screened for mutations in Saccharomyces cerevisiae that confer sensitivity to microtubule-destabilizing drugs. One mutant exhibiting this phenotype lacked the gene encoding the karyopherin Kap123p. Analysis of kap123 cells revealed that the drug sensitivity was caused by a defect in microtubule stability and/or assembly. In support of this idea, we demonstrated genetic interactions between the kap123 mutation and mutated alleles of genes encoding α-tubulins and factors controlling microtubule dynamics. Moreover, kap123 cells exhibit defects in spindle structure and dynamics as well as nuclear positioning defects during mitosis. Cultures of kap123 strains are enriched for mononucleated large-budded cells often containing short spindles and nuclei positioned away from the budneck, phenotypes indicative of defects in both cytoplasmic and nuclear microtubules. Finally, we identified a gene, CAJ1, which when deleted in combination with KAP123 exacerbated the microtubule-related defects of the kap123 mutants. We propose that Kap123p and Caj1p, a member of the Hsp40 family of proteins, together play an essential role in normal microtubule function.