FISSION YEAST CUT3 AND CUT14, MEMBERS OF A UBIQUITOUS PROTEIN FAMILY,ARE REQUIRED FOR CHROMOSOME CONDENSATION AND SEGREGATION IN MITOSIS

Fission yeast temperature-sensitive mutants cut3-477 and cut14-208 fai l to condense chromosomes but small portions of the chromosomes can se parate along the spindle during mitosis, producing phi-shaped chromoso mes. Septation and cell division occur in the absence of normal nuclea r division, causing the cut phenotype. Fluorescence in situ hybridizat ion demonstrated that the contraction of the chromosome arm during mit osis was defective. Mutant chromosomes are apparently not rigid enough to be transported poleward by the spindle. Loss of the cut3 protein b y gene disruption fails to maintain the nuclear chromatin architecture even in interphase. Both cut3 and cutl4 proteins contain a putative n ucleoside triphosphate (NTP)-binding domain and belong to the same ubi quitous protein family which includes the budding yeast Smc1 protein. The cut3 mutant was suppressed by an increase in the cut14(+) gene dos age. The cut3 protein, having the highest similarity to the mouse prot ein, is localized in the nucleus throughout the cell cycle. Plasmids c arrying the DNA topoisomerase I gene partly suppressed the temperature sensitive phenotype of cut3-477, suggesting that the cut3 protein mig ht be involved in chromosome DNA topology.