Dominant-negative Xorbit (green) results in short spindles and chromosome (blue) alignment defects.

The 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 spindle rapidly disassembled, leaving behind the unsegregated chromosomes. The spindle-stabilizing mechanism is not yet clear. Xorbit might directly protect plus ends or possibly counteract a depolymerizing activity.

Xorbit may also physically link the kinetochore to the spindle, given the chromosome misalignment defects seen in its absence. The link might be through CLIP-170, which the authors found interacts with Xorbit. They also noted an interaction between Xorbit and CENP-E, suggesting Xorbit might connect chromosomes to the motors that drive their movements.