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* Department of Physiology, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania 19104-6085; and Spindle orientation and nuclear migration
are crucial events in cell growth and differentiation of
many eukaryotes. Here we show that KIP3, the sixth
and final kinesin-related gene in Saccharomyces cerevisiae, is required for migration of the nucleus to the bud site in preparation for mitosis. The position of the nucleus in the cell and the orientation of the mitotic spindle was examined by microscopy of fixed cells and by
time-lapse microscopy of individual live cells. Mutations in KIP3 and in the dynein heavy chain gene defined two distinct phases of nuclear migration: a
KIP3-dependent movement of the nucleus toward the
incipient bud site and a dynein-dependent translocation
of the nucleus through the bud neck during anaphase. Loss of KIP3 function disrupts the unidirectional
movement of the nucleus toward the bud and mitotic
spindle orientation, causing large oscillations in nuclear
position. The oscillatory motions sometimes brought
the nucleus in close proximity to the bud neck, possibly
accounting for the viability of a kip3 null mutant. The kip3 null mutant exhibits normal translocation of the
nucleus through the neck and normal spindle pole separation kinetics during anaphase. Simultaneous loss of
KIP3 and kinesin-related KAR3 function, or of KIP3
and dynein function, is lethal but does not block any additional detectable movement. This suggests that the
lethality is due to the combination of sequential and
possibly overlapping defects. Epitope-tagged Kip3p localizes to astral and central spindle microtubules and is
also present throughout the cytoplasm and nucleus.
Departments of Pediatric Oncology, The Dana-Farber Cancer Institute, and Pediatric Hematology, The Children's Hospital,
Harvard Medical School, Boston, Massachusetts 02115
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