For their even distribution into daughter cells, chromosomes must first be attached to the opposite spindle poles. For the most part, a chromosome is first monooriented—that is, it is hitched by microtubule bundles (K-fibers), connecting its kinetochore to a single pole. That chromosome is then pulled toward its attached pole, where microtubules from the opposite pole are rare. Eventually, however, the chromosome leaves this pole and congresses to the metaphase plate.
This congression is generally assumed to be the result of biorientation, but the new results indicate that monooriented chromosomes can also make their way to the metaphase plate. EM images showed that kinetochores of congressing monooriented chromosomes interacted laterally with K-fibers of already bioriented chromosomes.
Monooriented chromosomes, says Khodjakov, “can borrow the K-fibers of other chromosomes and use them as rails to go to the middle of the spindle. At the middle, the chances of acquiring microtubules from the other pole are much higher.”
This opportunistic behavior suggests that spindle formation is cooperative—the first chromosome to biorient makes the process easier for its followers. “Late in the process,” says Khodjakov, “there are plenty of rails” for the few remaining monooriented chromosomes.
The only known microtubule motor at kinetochores with the appropriate directionality to walk to the metaphase plate is CENP-E. Upon depleting mitotic cells of this protein, the authors found that several monooriented chromosomes lingered near a pole rather than congressing to the center. The ability to pull monooriented chromosomes along neighboring tracks is the first true motor function suggested for CENP-E, which is better known for recruiting spindle checkpoint proteins to the kinetochore.