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J. Cell Biol.,
Volume 143, Number 6, December 14, 1998 1603-1616
* Department of Biology, University of North Carolina, Chapel Hill, North Carolina 27599-3280; The fission yeast Schizosaccharomyces
pombe divides symmetrically using a medial F-actin-
based contractile ring to produce equal-sized daughter
cells. Mutants defective in two previously described
genes, mid1 and pom1, frequently divide asymmetrically. Here we present the identification of three new
temperature-sensitive mutants defective in localization
of the division plane. All three mutants have mutations
in the polo kinase gene, plo1, and show defects very
similar to those of mid1 mutants in both the placement and organization of the medial ring. In both cases, ring
formation is frequently initiated near the cell poles, indicating that Mid1p and Plo1p function in recruiting
medial ring components to the cell center. It has been
reported previously that during mitosis Mid1p becomes
hyperphosphorylated and relocates from the nucleus to
a medial ring. Here we show that Mid1p first forms a
diffuse cortical band during spindle formation and then
coalesces into a ring before anaphase. Plo1p is required
for Mid1p to exit the nucleus and form a ring, and
Pom1p is required for proper placement of the Mid1p
ring. Upon overexpression of Plo1p, Mid1p exits the nucleus prematurely and displays a reduced mobility on
gels similar to that of the hyperphosphorylated form
observed previously in mitotic cells. Genetic and two-hybrid analyses suggest that Plo1p and Mid1p act in a
common pathway distinct from that involving Pom1p.
Plo1p localizes to the spindle pole bodies and spindles of mitotic cells and also to the medial ring at the time of
its formation. Taken together, the data indicate that
Plo1p plays a role in the positioning of division sites by
regulating Mid1p. Given its previously known functions
in mitosis and the timing of cytokinesis, Plo1p is thus
implicated as a key molecule in the spatial and temporal coordination of cytokinesis with mitosis.
Department of Physiology and § Department of Molecular Genetics and Microbiology, University of Massachusetts Medical Center, Worcester, Massachusetts
01605; Howard Hughes Medical Institute and Department of Cell Biology, Vanderbilt University School of Medicine, Nashville,
Tennessee 37232
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