CLIP-170 facilitates the formation of kinetochore-microtubule attachments

CLIP-170 is a microtubule 'plus end tracking' protein involved in several microtubule-dependent processes in interphase. At the onset of mitosis, CLIP-170 localizes to kinetochores, but at metaphase, it is no longer detectable at kinetochores. Although RNA interference (RNAi) experiments have suggested an essential role for CLIP-170 during mitosis, the molecular function of CLIP-170 in mitosis has not yet been revealed. Here, we used a combination of high-resolution microscopy and RNAi-mediated depletion to study the function of CLIP-170 in mitosis. We found that CLIP-170 dynamically localizes to the outer most part of unattached kinetochores and to the ends of growing microtubules. In addition, we provide evidence that a pool of CLIP-170 is transported along kinetochore microtubules by the dynein/dynactin complex. Interference with CLIP-170 expression results in defective chromosome congression and diminished kinetochore microtubule attachments, but does not detectibly affect microtubule dynamics or kinetochore - microtubule stability. Taken together, our results indicate that CLIP-170 facilitates the formation of kinetochore - microtubule attachments, possibly through direct capture of microtubules at the kinetochore.