Figure 10.

A model for Bud6's role in directing the program of cytoplasmic microtubule interactions during spindle assembly. (A) After bud emergence, positional information at the bud tip directs microtubule interactions, thereby orienting duplicated SPBs facing the bud neck (a). As SPBs separate, the SPB_daughter retains contacts to the bud tip (b). In concert with spindle assembly, de novo cytoplasmic microtubules are directed to a new Bud6-dependent area of capture at the neck and prevented from undergoing capture in the bud (c). As spindle assembly proceeds, cytoplasmic microtubule interactions draw the SPB_mother away from the neck and cause spindle alignment (d and f). Neck interactions also contribute to spindle retention at the neck. Premature organization of cytoplasmic microtubules in cdc28-4 clb5 cells (Segal et al., 2000) favors de novo contacts with the bud tip before an area for cytoplasmic microtubule–bud neck interactions develops. Bud6 partition between the bud tip and neck is influenced by Bni1 and Bud3, respectively. (B) Alternative modes of spindle orientation in cortical cue mutants. A bud6 mutation impairs initial orientation of duplicated SPBs facing the bud neck and ensuing neck interactions. Orientation relies primarily on microtubule search and capture by the bud. A bni1 mutation directs interactions to the neck throughout spindle morphogenesis. In both mutants, Kar9 still contributes to orientation. On the other hand, a kar9 mutation reduces cytoplasmic microtubule capture in the bud while orientation via neck interactions can occur after spindle assembly (when Bud6 accumulates at the neck) or during anaphase.