Tm. Dezwaan et al., KINESIN-RELATED KIP3 OF SACCHAROMYCES-CEREVISIAE IS REQUIRED FOR A DISTINCT STEP IN NUCLEAR MIGRATION, The Journal of cell biology, 138(5), 1997, pp. 1023-1040
Spindle orientation and nuclear migration are crucial events in cell g
rowth 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 preparatio
n for mitosis. The position of the nucleus in the cell and the orienta
tion of the mitotic spindle was examined by microscopy of fixed cells
and by time-lapse microscopy of individual live cells. Mutations in KI
P3 and in the dynein heavy chain gene defined two distinct phases of n
uclear 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. T
he oscillatory motions sometimes brought the nucleus in close proximit
y to the bud neck, possibly accounting for the viability of a kip3 nul
l mutant. The kip3 null mutant exhibits normal translocation of the nu
cleus through the neck and normal spindle pole separation kinetics dur
ing anaphase. Simultaneous loss of KIP3 and kinesin-related KAR3 funct
ion, or of KIP3 and dynein function, is lethal but does not block any
additional detectable movement. This suggests that the lethality is du
e to the combination of sequential and possibly overlapping defects. E
pitope-tagged Kip3p localizes to astral and central spindle microtubul
es and is also present throughout the cytoplasm and nucleus.