page 1135 from Levesque and Compton. Their antibody experiments target a human kinesin motor called Kid that sticks to chromosome arms and is thought to mediate chromosome movement out along spindle microtubules. This polar ejection force has been proposed as a mechanism for pushing chromosomes away from spindle poles and thus aligning chromosomes on the metaphase plate.
The importance of Kid and the polar ejection force has been shown in fly germ cells mutant for the Nod kinesin, and in frog extracts, where depletion of Kid resulted in widespread wandering of chromosomes away from in vitro assembled metaphase plates. Levesque and Compton set out to see if Kid was equally important in a mitotic rather than meiotic system.
After injection of Kid antibodies into cells with monopolar spindles, chromosomes clustered tightly around the pole, whereas after control injections chromosomes kept a polite distance from the pole because of polar ejection forces. When the authors inhibited Kid in cells with bipolar spindles, most chromosomes moved to the metaphase plate correctly. The chromosome arms trailed toward the poles, but nevertheless, the chromosomes moved in the right direction, and only 17.5% of cells had chromosome disorganization sufficient to cause a noticeable cell cycle delay. Delayed cells tended to have one or more sister chromatid pairs hung up very close to one pole.
The relatively small numbers of problems suggests that mitotic kinetochores can usually be captured even by microtubules from a very distant pole, and once captured are smart enough to sense the midpoint of the spindle without the help of polar ejection forces. The kinetochore microtubules, like a rope in a tug-of-war, may be subject to forces that are proportional to the length of the microtubule, so that the opposing forces will cancel out when a chromosome reaches the metaphase plate. ▪