Microtubule plus-end-interacting proteins (؉TIPs) promote the dynamic interactions between the plus ends (؉ends) of astral microtubules and cortical actin that are required for preanaphase spindle positioning. Paradoxically, ؉TIPs such as the EB1 orthologue Bim1 and Kar9 also associate with spindle pole bodies (SPBs), the centrosome equivalent in budding yeast. Here, we show that deletion of four C-terminal residues of the budding yeast ␥-tubulin Tub4 (tub4-⌬dsyl) perturbs Bim1 and Kar9 localization to SPBs and Kar9-dependant spindle positioning. Surprisingly, we find Kar9 localizes to microtubule ؉ends in tub4-⌬dsyl cells, but these microtubules fail to position the spindle when targeted to the bud. Using cofluorescence and coaffinity purification, we show Kar9 complexes in tub4-⌬dsyl cells contain reduced levels of Bim1. Astral microtubule dynamics is suppressed in tub4-⌬dsyl cells, but it are restored by deletion of Kar9. Moreover, Myo2-and F-actin-dependent dwelling of Kar9 in the bud is observed in tub4-⌬dsyl cells, suggesting defective Kar9 complexes tether microtubule ؉ends to the cortex. Overproduction of Bim1, but not Kar9, restores Kar9-dependent spindle positioning in the tub4-⌬dsyl mutant, reduces cortical dwelling, and promotes Bim1-Kar9 interactions. We propose that SPBs, via the tail of Tub4, promote the assembly of functional ؉TIP complexes before their deployment to microtubule ؉ends.

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The kinetochore, a protein complex that links chromosomes to microtubules (MTs), is required to prevent spindle expansion during S phase in budding yeast, but the mechanism of how the kinetochore maintains integrity of the bipolar spindle before mitosis is not well understood. Here, we demonstrate that a mutation of Spc24, a component of the conserved Ndc80 kinetochore complex, causes lethality when cells are exposed to the DNA replication inhibitor hydroxyurea (HU) due to premature spindle expansion and segregation of incompletely replicated DNA. Overexpression of Stu1, a CLASP-related MT-associated protein or a truncated form of the XMAP215 orthologue Stu2 rescues spc24-9 HU lethality and prevents spindle expansion. Truncated Stu2 likely acts in a dominant-negative manner, because overexpression of full-length STU2 does not rescue spc24-9 HU lethality, and spindle expansion in spc24-9 HU-treated cells requires active Stu2. Stu1 and Stu2 localize to the kinetochore early in the cell cycle and Stu2 kinetochore localization depends on Spc24. We propose that mislocalization of Stu2 results in premature spindle expansion in S phase stalled spc24-9 mutants. Identifying factors that restrain spindle expansion upon inhibition of DNA replication is likely applicable to the mechanism by which spindle elongation is regulated during a normal cell cycle. INTRODUCTIONPreserving the integrity of the genome is a fundamental requirement for eukaryotic cell viability. DNA replication must be completed before segregation of the chromosomes to prevent the transmission of partially replicated chromosomes to daughter cells. In the budding yeast Saccharomyces cerevisiae, cells undergo a closed mitosis and the microtubule (MT) organizing centers, or spindle pole bodies (SPBs), are imbedded in the nuclear envelope. SPB duplication begins at the end of anaphase and spindle formation begins during S phase when duplicated SPBs separate from each other (Adams and Kilmartin, 1999;Jaspersen and Winey, 2004). Because chromosomes remain attached to kinetochore MTs throughout the cell cycle, spindle expansion must be restrained until all 16 chromosomes have duplicated and kinetochores on sister chromatids have formed bipolar MT attachments. When DNA replication is stalled by hydroxyurea (HU) treatment, cells arrest with a large bud, an undivided nucleus positioned at the mother-bud neck and a short bipolar spindle (Allen et al., 1994). Maintaining a short spindle is crucial for cell survival during HU-induced arrest, which activates the DNA replication checkpoint effectors Mec1 and Rad53 (Kolodner et al., 2002). When mec1 and rad53 mutants are treated with HU, the DNA replication checkpoint is not activated, and, as a result, replication forks are not stabilized, the spindle expands, and unequal division of incompletely replicated nuclear material occurs-all of these events contribute to cell lethality (Allen et al., 1994;Weinert et al., 1994;Lopes et al., 2001).In human cells, stalled replication forks activate the ataxia telangiectasia mutat...