Stable Preanaphase Spindle Positioning Requires Bud6p and an Apparent Interaction between the Spindle Pole Bodies and the Neck

Faithful partitioning of genetic material during cell division requires accurate spatial and temporal positioning of nuclei within dividing cells. In Saccharomyces cerevisiae, nuclear positioning is regulated by an elegant interplay between components of the actin and microtubule cytoskeletons. Regulators of this process include Bud6p (also referred to as the actin-interacting protein Aip3p) and Kar9p, which function to promote contacts between cytoplasmic microtubule ends and actin-delimited cortical attachment points. Here, we present the previously undetected association of Bud6p with the cytoplasmic face of yeast spindle pole bodies, the functional equivalent of metazoan centrosomes. Cells lacking Bud6p show exaggerated movements of the nucleus between mother and daughter cells and display reduced amounts of time a given spindle pole body spends in close association with the neck region of budding cells. Furthermore, overexpression of BUD6 greatly enhances interactions between the spindle pole body and mother-bud neck in a spindle alignment-defective dynactin mutant. These results suggest that association of either spindle pole body with neck components, rather than simply entry of a spindle pole body into the daughter cell, provides a positive signal for the progression of mitosis. We propose that Bud6p, through its localization at both spindle pole bodies and at the mother-bud neck, supports this positive signal and provides a regulatory mechanism to prevent excessive oscillations of preanaphase nuclei, thus reducing the likelihood of mitotic delays and nuclear missegregation.