The regulated orientation of cell divisions in response to positional cues is critical for morphogenesis and cell-fate specification during development in many organisms ( Drubin and Nelson, 1996 Gonczy and Hyman, 1996). Cytoplasmic dynein is attenuated by these spindle polarity determinants and kinesin until anaphase onset, when dynein directs spindle elongation to distal points in the mother and bud. These observations support a model in which spindle polarity determinants ( BUD6, BNI1, KAR9) and cytoplasmic kinesin ( KIP3) provide directional cues for spindle orientation to the bud while restraining the spindle to the neck. In addition, dynein-mediated forces on astral microtubules are sufficient to segregate a 2N chromosome set through the neck in the absence of spindle elongation, but cytoplasmic kinesins are not. In the absence of KAR9,dynein-dependent spindle oscillations are evident before anaphase onset, as are postanaphase dynein-dependent pulling forces that exceed the velocity of wild-type spindle elongation threefold. The cytoplasmic motor protein, dynein, is not required for nuclear movement to the neck rather, it has been postulated to contribute to spindle elongation through the neck. As a consequence, there is a loss of asymmetry in spindle pole body segregation into the bud. Failure of “pulling” is evident in cells lacking Bud6p, Bni1p, Kar9p, or the kinesin homolog, Kip3p. In contrast, microtubules growing toward the neck and within the bud promote nuclear movement in the same direction of microtubule growth, thus “pulling” the nucleus toward the bud neck. The nucleus moves in the opposite direction of astral microtubule growth in the mother cell, apparently being “pushed” by microtubule contacts at the cortex.
Astral microtubule dynamics are critical to the mechanism that ensures nuclear migration to the bud neck.
In the budding yeast Saccharomyces cerevisiae, movement of the mitotic spindle to a predetermined cleavage plane at the bud neck is essential for partitioning chromosomes into the mother and daughter cells.
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