I. Hagan et M. Yanagida, EVIDENCE FOR CELL CYCLE-SPECIFIC, SPINDLE POLE BODY-MEDIATED, NUCLEARPOSITIONING IN THE FISSION YEAST SCHIZOSACCHAROMYCES-POMBE, Journal of Cell Science, 110, 1997, pp. 1851-1866
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.