EVIDENCE FOR CELL CYCLE-SPECIFIC, SPINDLE POLE BODY-MEDIATED, NUCLEARPOSITIONING IN THE FISSION YEAST SCHIZOSACCHAROMYCES-POMBE

Specific changes in spatial order occur during cell cycle progression in fission yeast. Growth of the rod-shaped cells is highly regulated a nd undergoes a cell cycle and sizeregulated switch from monopolar to b ipolar tip extension, During both phases of growth, the interphase nuc leus is maintained in a central location. Following the separation of the genome to the cell tips in mitosis, the two nuclei migrate back to wards the cell equator before stopping in two new positions that will become the middle of the two new cells, Here we use simultaneous label ing of microtubules, chromatin and spindle pole bodies in wild-type an d cde mutants, to show that nuclear positioning is achieved by regulat ion of spindle pole body-mediated nuclear migration, We show that the number and location of nuclear positioning signals is regulated in a c ell cycle-specific manner and that spindle pole body-mediated forces a re likely to be responsible for maintaining correct nuclear position o nce the nuclei have reached the appropriate position in the cell. Acce ntuating the movement of the nuclei back towards the cell equator afte r mitosis by artificially increasing cell length shows that the spindl e pole body leads the nucleus during this migration, When multiple spi ndle pole bodies are associated with the same or different nuclei they all go to the same point indicating that the different spindle pole b odies are responding to the same positional cue. In a septation-defect ive mutant cell, which contains four nuclei, the spindle pole bodies o n the four different nuclei initially group as two pairs in regions th at would become the middle of the new cells, were the cell able to div ide, In the subsequent interphase, the nuclei aggregate as a group of four in the centre of the cell. The presence of two or three clusters of spindle pole bodies in larger cells with eight nuclei suggests that the mechanisms specifying the normally central location for multiple nuclei may be unable to operate properly as the cells get larger, Pert urbation of microtubules with the microtubule poison thiabendazole pre vents the spindle pole body clustering in septation mutants, demonstra ting that nuclear positioning requires a functional microtubule cytosk eleton.