Shedding Light on Mysterious Microtubules

Abstract

We have known for decades that vertebrate kinetochores can nucleate microtubules. However, the role of such microtubules was unclear. Kitamura et al. investigated this issue by following a marked budding yeast centromere after its conditional reactivation to examine microtubule capture at unattached kinetochores. They found not only that yeast kinetochores can nucleate microtubules, but also that such microtubules facilitate attachment by decreasing the time required for spindle-pole-associated microtubules to make contact with unattached kinetochores. Historically, the fact that microtubules nucleated at kinetochores are opposite in polarity to those in mature spindle fibers was used to argue that they were not physiologically relevant for spindle assembly. Kitamura et al. have now shown that they, in fact, do play a role, and then rapidly depolymerize after spindle fiber attachments form. In sum, this paper provides a function for a previously mysterious microtubule population and outlines a surprising and dynamic mechanism through which kinetochore-originated microtubules assist spindle pole microtubules to efficiently locate unattached kinetochores. It also shows how this assistance mechanism is shut off to dim the locator beacon after correct attachments have been made. Understanding Kinetochore-Nucleated Microtubules Tomoyuki Tanaka, with the help of Etsushi Kitamura and Shinya Komoto, explains his group's key findings regarding the roles of kinetochore-generated microtubules in mitosis.