How kinetochore proteins form a dynamic interface with microtubules is largely unknown. In budding yeast, the 10-protein Dam1 complex is an Aurora kinase target that plays essential roles maintaining the integrity of the mitotic spindle and regulating interactions with the kinetochore. Here, we investigated the biochemical properties of purified Dam1 complex. The complex oligomerized into rings around microtubules. Ring formation was facilitated by microtubules but could occur in their absence. Mutant alleles led to partially assembled complexes or reduced microtubule binding. The interaction between rings and microtubules is mediated by the C termini of both Dam1 and alphabeta-tubulin. Ring formation promotes microtubule assembly, stabilizes against disassembly, and promotes bundling. A GTP-tubulin lattice is the preferred binding partner for the complex, and Dam1 rings can exhibit lateral mobility on microtubules. These observations suggest a mechanism by which the kinetochore can recognize and stay attached to the plus ends of microtubules.
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