Mutations in fission yeast Cut15, an importin α homolog, lead to mitotic progression without chromosome condensation

Chromosome condensation is a major mitotic event. Fission yeast mutations in topoisomerase II and condensin subunits produce the characteristic 'cut' phenotypes, in which the septum bisects the nuclear material in the absence of normal condensation and sister chromatid separation. We show here that the same condensation defect is produced in cut15 temperature-sensitive mutants at the restrictive temperature (36 degrees C). The gene product of cut15+ is, surprisingly, very similar to importin alpha, which binds proteins containing a nuclear localization signal (NLS) and forms the heterodimer with importin beta that mediates translocation through the nuclear pore complex. We show that in a nuclear import assay, purified Cut15 protein behaved identically to mammalian importin alpha but mutant Cut15 did not. Mutant Cut15 failed to bind an NLS-containing protein in vitro but could still bind importin beta. Unexpectedly, however, NLS proteins were imported into the nucleus in cut15 mutants. Cut15 is thus essential for mitotic chromosome condensation, but its role in nuclear import might be dispensable. Green fluorescent protein (GFP)-tagged Cut15 was enriched within the nucleus specifically during prometaphase-metaphase, so the interaction of Cut15 with nuclear NLS proteins during mitosis might be important for condensation.