A stretched out CENP-E molecule sports two molecular motors (arrows).

The first close-ups of a protein that's crucial for lining up the chromosomes during mitosis have been snapped by Kim et al. Their work might clarify how chromosomes retain connections to the microtubules that help move them around.

Microtubules attach to a mitotic chromosome at the kinetochore, a structure at the chromosome's midsection. Kinetochores have to keep a grip on microtubules that are lengthening and shortening, but scientists don't understand how they do it. Kim et al. came up with a possible explanation while investigating the role of CENP-E, an essential microtubule motor that clings to the kinetochore.

CENP-E is tricky to purify, but Kim et al. isolated enough of the protein to scrutinize under the electron microscope. The images show that the protein contains two motors at the end of a long, springy strand. When the researchers tested the protein's pulling ability, they found that it was 50 times slower than any other molecular motor. However, CENP-E was sticky, sometimes hanging onto microtubules for more than 20 minutes.

The protein thus has flexibility, reach, and tenacity. These attributes, the authors suggest, allow CENP-E to anchor to the kinetochore with one end, while the other, motor-carrying end gropes around and grabs microtubules. Once it's gotten a grip, its ability to slide keeps the chromosome attached to the shrinking or lengthening microtubule.

Kim, Y., et al.
J. Cell Biol.