Stu2 acts as a microtubule destabilizer in metaphase budding yeast spindles

The microtubule-associated protein Stu2 (XMAP215) has the remarkable ability to act either as a polymerase or as a destabilizer of the microtubule plus end. In budding yeast, it is required for the dynamicity of spindle microtubules and also for kinetochore force generation. To understand how Stu2 contributes to these distinct activities, we analyzed the contributions of its functional domains to its localization and function. We find that Stu2 colocalizes with kinetochores using its TOG domains, which bind GTP-tubulin, a coiled-coil homodimerization domain, and a domain that interacts with plus-end interacting proteins. Stu2 localization is also promoted by phosphorylation at a putative CDK1 phosphorylation site located within its microtubule-binding basic patch. Surprisingly, however, we find that kinetochore force generation is uncorrelated with the amount of kinetochore-colocalized Stu2. These and other data imply that Stu2 colocalizes with kinetochores by recognizing growing microtubule plus ends within yeast kinetochores. We propose that Stu2 destabilizes these plus ends to indirectly contribute to the "catch-bond" activity of the kinetochores.