Mechanism for astral microtubule capture by cortical Bud6p priming spindle polarity in S. cerevisiae

Budding yeast is a unique model to dissect spindle orientation in a cell dividing asymmetrically. In yeast, this process begins with the capture of pole-derived astral microtubules (MTs) by the polarity determinant Bud6p at the cortex of the bud in G(1). Bud6p couples MT growth and shrinkage with spindle pole movement relative to the contact site. This activity resides in N-terminal sequences away from a domain linked to actin organization. Kip3p (kinesin-8), a MT depolymerase, may be implicated, but other molecular details are essentially unknown.

We show that Bud6p and Kip3p play antagonistic roles in controlling the length of MTs contacting the bud. The stabilizing role of Bud6p required the plus-end-tracking protein Bim1p (yeast EB1). Bim1p bound Bud6p N terminus, an interaction that proved essential for cortical capture of MTs in vivo. Moreover, Bud6p influenced Kip3p dynamic distribution through its effect on MT stability during cortical contacts via Bim1p. Coupling between Kip3p-driven depolymerization and shrinkage at the cell cortex required Bud6p, Bim1p, and dynein, a minus-end-directed motor helping tether the receding plus ends to the cell cortex. Validating these findings, live imaging of the interplay between dynein and Kip3p demonstrated that both motors decorated single astral MTs with dynein persisting at the plus end in association with the site of cortical contact during shrinkage at the cell cortex.

Astral MT shrinkage linked to Bud6p involves its direct interaction with Bim1p and the concerted action of two MT motors-Kip3p and dynein.