Xorbiting the spindle

Xorbiting the spindle he Xorbit microtubule-binding protein, say Hannak and Heald (page 19), keeps the spindle from getting away from its passenger DNA. Xorbit is a spindle-localized microtubule plus-end binding protein whose loss causes defects in spindle formation and chromosome alignment. To understand its specific function, the authors used video microscopy to compare spindle assembly in real time in frog egg extracts with and without Xorbit. The videos revealed a stabilizing effect of Xorbit on microtubules at the DNA. In extracts lacking centrosomes (in which the DNA is responsible for spindle assembly), spindle microtubule polymerization required Xorbit; no spindles formed in its absence. Microtubules organized by centrosomes, however, were not affected by Xorbit. Perhaps Xorbit is only activated by something on or near the chromatin, such as RanGTP. Xorbit also keeps a check on the necessary instability that occurs during anaphase, when microtubules depolymerize. When the authors depleted Xorbit at anaphase, the entire T