Figure 9.

Model for mitotic spindle position and movement. First, the nucleus (○) moves to the nascent bud site. This movement requires the kinesin Kip3p and presumably attachment of astral microtubules (straight black lines) to the cortex of the bud (⋄). After forming a short mitotic spindle, the nucleus remains at the neck and the spindle is kept generally aligned along the mother–bud axis. Maintenance of nuclear position and spindle alignment also require Kip3p and presumably attachment of astral microtubules to the cortex of the bud. The cortical attachment site for astral microtubules during Kip3p-dependent movements depends, in part, on filamentous actin, the formin Bni1p, Bud6p, and Kar9p. After initiating spindle elongation (anaphase), the mitotic spindle moves into the neck concurrent with astral microtubules sliding along the cortex. Both actin-dependent (⋄) and actin-independent (angled lines) attachment sites participate in moving the spindle into the neck. Once in the neck, the elongating spindle exhibits dynein-dependent oscillations. The cortical attachment site for astral microtubules during dynein-dependent movements is not yet known.