Antagonistic microtubule-sliding motors position mitotic centrosomes in Drosophila early embryos

The positioning of centrosomes, or microtubule-organizing centres, within c ells plays a critical part in animal development. Here we show that, in Dro sophila embryos undergoing mitosis, the positioning of centrosomes within b ipolar spindles and between daughter nuclei is determined by a balance of o pposing forces generated by a bipolar kinesin motor, KLP61F, that is direct ed to microtubule plus ends, and a carboxy-terminal kinesin motor, Ncd, tha t is directed towards microtubule minus ends. This activity maintains the s pacing between separated centrosomes during prometaphase and metaphase, and repositions centrosomes and daughter nuclei during late anaphase and telop hase. surprisingly, we do not observe a function for KLP61F in the initial separation of centrosomes during prophase. Our data indicate that KLP61F an d Ncd may function by crosslinking and sliding antiparallel spindle microtu bules in relation to one another, allowing KLP61F to push centrosomes apart and Ncd to pull them together.