Microtubule polymerase; required for anaphase elongation of the mitotic spindle, for spindle orientation and metaphase chromosome alignment; required for plus-end directed kinetochore movement along metaphase microtubules enabling chromosome biorientation and congression; stabilizes or destabilizes kinetochore-microtubule attachment depending on kinetochore tension; dissociates from the kinetochore Ndc80 complex in response to Cdc5p-mediated phosphorylation; metazoan CKAP5 /XMAP215 family member