Dynamic positioning of mitotic spindles in yeast: Role of microtubule motors and cortical determinants

In the budding yeast Saccharomyces cerevisiae, movement of the mitotic spin dle to a predetermined cleavage plane at the bud neck is essential for part itioning chromosomes into the mother and daughter cells. Astral microtubule dynamics are critical to the mechanism that ensures nuclear migration to t he bud neck. The nucleus moves in the opposite direction of astral microtub ule growth in the mother cell, apparently being "pushed" by microtubule con tacts at the cortex. In contrast, microtubules growing toward the neck and within the bud promote nuclear movement in the same direction of microtubul e growth, thus "pulling" the nucleus toward the bud neck. Failure of "pulli ng" is evident in cells lacking Bud6p, Bni1p, Kar9p, or the kinesin homolog , Kip3p. ns a consequence, there is a loss of asymmetry in spindle pole bod y segregation into the bud. The cytoplasmic motor protein, dynein, is not r equired for nuclear movement to the neck; rather, it has been postulated to contribute to spindle elongation through the neck. Ln the absence of KAR9, dynein-dependent spindle oscillations are evident before anaphase onset, a s are post-anaphase dynein-dependent pulling forces that exceed the velocit y of wild-type spindle elongation threefold. In addition, dynein-mediated f orces on astral microtubules are sufficient to segregate a 2N chromosome se t through the neck in the absence of spindle elongation, but cytoplasmic ki nesins are not. These observations support a model in which spindle polarit y determinants (BUD6, BNI1, KAR9) and cytoplasmic kinesin (KIP3) provide di rectional cues for spindle orientation to the bud while restraining the spi ndle to the neck. Cytoplasmic dynein is attenuated by these spindle polarit y determinants and kinesin until anaphase onset, when dynein directs spindl e elongation to distal points in the mother and bud.