| Microtubules (MTs) are required for nuclear movements and chromosome segregation. MTs are polymers of tubulin protein with a defined polarity---the minus end is associated with the microtubule organizing center (MTOC) whereas the plus end probes the cellular environment. Additional proteins associated with the plus end act as adaptors between the MT and binding sites that facilitate nuclear and chromosome movements. The budding yeast Saccharomyces cerevisiae was used as a model system to examine two questions: (1) During mitotic spindle elongation, how does the cell ensure each daughter inherits a single MTOC? To investigate this, Mitotic Exit Network (MEN) proteins were analyzed in living cells. I find that, after anaphase onset, the MEN activator Tem1p accumulates persistently on the MTOC that enters the daughter cell. In contrast, the abundance of the MEN inhibitor Bub2p decreases during anaphase. This suggests that after anaphase onset, the presence of one MTOC in the daughter cell persistently signals mitotic exit. (2) By what mechanism do plus end binding proteins link oppositely oriented MT arrays? In the budding yeast mating pathway, the translocation of haploid nuclei toward each other is known as nuclear congression. I find nuclear congression is driven by MT plus end interactions. The plus end binding proteins Kar3p and Bik1p are required for efficient MT-MT interactions. Additionally, I find that during mitosis in vegetative cells, Kar3p and Bik1p are part of a family of plus end binding proteins that are required for anti-parallel MT interactions. Taken together, these data suggest a specific subset of plus end binding proteins function similarly during mating and mitosis on oppositely oriented MT arrays. |
