Human CLASP1 Is an Outer Kinetochore Component that Regulates Spindle Microtubule Dynamics

One of the most intriguing aspects of mitosis is the ability of kinetochores to hold onto plus ends of microtubules that are actively gaining or losing tubulin subunits. Here, we show that CLASP1, a microtubule-associated protein, localizes preferentially near the plus ends of growing spindle microtubules and is also a component of a kinetochore region that we term the outer corona. A truncated form of CLASP1 lacking the kinetochore binding domain behaves as a dominant negative, leading to the formation of radial arrays of microtubule bundles that are highly resistant to depolymerization. Microinjection of CLASP1-specific antibodies suppresses microtubule dynamics at kinetochores and throughout the spindle, resulting in the formation of monopolar asters with chromosomes buried in the interior. Incubation with microtubule-stabilizing drugs rescues the kinetochore association with microtubule plus ends at the periphery of the asters. Our data suggest that CLASP1 is required at kinetochores for attached microtubules to exhibit normal dynamic behavior.