Kar3 (green) and kinetochores (red dot at tip of arrow) slide along a microtubule together.

Kinetochores heading for the spindle poles early in mitosis have two travel options, as Tanaka et al. show on page 269. They can slide on microtubules or get tugged along as the fibers shrink.

Before it can parcel out its chromosomes, a mitotic cell has to position them between the spindle poles. The process starts when microtubules from one pole attach to the kinetochore on the centromere of a chromosome. The kinetochore then moves toward that pole. Two years ago, the researchers showed that, during this early stage, kinetochores attach to the side of the microtubules and slide along. The protein Kar3 was essential for the motion. But that wasn't the whole story.

Tanaka et al. probed the initial kinetochore–microtubule interaction by temporarily inactivating one yeast centromere, forcing the microtubules to disengage. Reactivating the centromere allowed the scientists to follow what happens after one microtubule catches on. They discovered that cells with Kar3 rely on sliding to transport chromosomes nearly 70% of the time. Further work demonstrated that Kar3 is the main power source for this form of transportation.

But what happens the other 30% of the time? The researchers found that kinetochores link to the ends of microtubules, traveling toward the poles as the structures shorten. They determined that this end-on pulling requires a protein complex called Dam1, which previous studies indicated encircles the microtubule like a ring. Collapse of microtubules might drive end-on pulling through Dam1. As the structures depolymerize, they fray and bend backward, pushing the Dam1 ring along.

End-on pulling capitalizes on energy from microtubule relaxation, whereas sliding requires ATP. Cells rely on sliding because it delivers kinetochores to their destination sooner, the researchers speculate. Although end-on pulling produces higher speeds, it also wastes time because it only works after the microtubule plus end grabs a kinetochore. The researchers suspect that Dam1 drives the end-on pulling that separates the chromosomes later in mitosis.