Probable microtubule plus-end tracking protein that promotes the stabilization of dynamic microtubules. Required for the formation of mitotic and meiotic spindles. Specifically promotes the polymerization of kinetochore-bound microtubules. Also required for cytoplasmic streaming. Essential for embryonic development. {ECO:0000269|PubMed:12885567; ECO:0000269|PubMed:15854912}.