In the cell the dynamics of microtubules is regulated by their interaction with different factors. Of special interest is the coupling of microtubules to the kinetochore, a process where microtubule dynamics reaches its "climax".

In collaboration with Georjana Barnes and David Drubin we are studying how the yeast kinetochore complex Dam1 binds and tracks microtubule plus ends. The complex self-assembles into rings and helices around the microtubule wall (Westermann et al., Mol Cell 2005). The binding is enhanced for GMPCPP microtubules, supporting a preferential binding to growing microtubule ends. The interaction is via the charged C-terminal of tubulin and allows for ring sliding along microtubules, which can be coupled to microtubule disassembly to allow for the movement of Dam1 as it remains attached to a depolymerizing end (Westermann et al., Nature 2006). This explains how this ring structure is able to track the depolymerizing ends of microtubules (as during anaphase), without requiring energy!

Defining the architecture of the Dam1 complex and its microtubule-driven self-assembly is essential to understanding the mechanisms by which rings couple processive movement to microtubule disassembly and thus contribute to the end-on attachment of chromosomes to the mitotic spindle. It is also crucial to determining how the assembly of the ring is regulated and how the ring attaches to other components of the kinetochore. We have used EM-based single-particle and helical analyses to obtain initial structures of the Dam1 complex before and after its oligomerization around microtubules. This work has allowed us to define the architecture of the Dam1 complex and the self-assembly mechanism (Wang et al., 2007, Nat Struct Mol Biol. 14, 721-6). Ring oligomerization seems to be facilitated by a conformational change upon binding to microtubules, suggesting that the Dam1 ring is not preformed, but self-assembles around kinetochore microtubules. The C terminus of the Dam1p protein, where most of the Aurora kinase Ipl1 phosphorylation sites reside, is in a strategic location to affect oligomerization and interactions with the microtubule. One of Ipl1's roles might be to fine-tune the coupling of the microtubule interaction with the conformational change required for oligomerization, with phosphorylation resulting in ring breakdown.