When cells divide, each chromosome is duplicated and one copy is passed to each daughter cell. The details of how these duplicated chromosomes are separated are still being puzzled out, but elsewhere in this issue, Georjana Barnes and colleagues (Nature 440, 565–569; 2006) add a large piece to the jigsaw.

During cell division, an array of tiny filaments called microtubules spreads from the poles of the cell to the central plane, where the duplicated chromosomes congregate. This microtubule structure — the ‘mitotic spindle’ — is responsible for dragging the two copies of the chromosome apart into the two regions that will form the daughter cells. Microtubules are linear polymers of the protein tubulin of diameter 24 nm that grow or shrink as tubulin monomers are added to or lost from their ends. During chromosome segregation, the microtubules depolymerize, and thus shorten, from the end nearest the central plane of the cell. So, as the spindle shrinks, the attached chromosomes are carried towards the pole.

The chromosomes stick to the depolymerizing ends of the spindle microtubules through specialized protein complexes known as kinetochores. But if these ends are continually dissolving as the microtubules pull back, how do the chromosomes stay attached? The answer lies with a subcomplex in the kinetochore, called the Dam1 complex, which assembles into rings and paired helices that encircle the ends of depolymerizing microtubules (pictured).

Credit: G. BARNES ET AL.

Barnes et al. used real-time imaging to watch the Dam1 complex on dynamic microtubules in vitro. They labelled the Dam1 complex and the microtubules with different coloured fluorophores. As the microtubules shortened during depolymerization, the Dam1 complex could clearly be seen progressing along the microtubules. This movement could be explained either by a ‘sliding’ mechanism, in which the Dam1 complex is continually associated with the microtubule lattice, or by a ‘turnover’ mechanism, with new subunits of the Dam1 complex rapidly incorporating into existing rings as old subunits leave, thus propelling the ring along the microtubule. The authors demonstrated that turnover of Dam1-complex subunits was not taking place and that the ring structure is sliding intact along the depolymerizing microtubule.

Finally, Barnes and colleagues showed that the Dam1 complex can function as a coupling device to move objects along the microtubules. These experiments relied on the strong bonds formed between streptavidin and biotin molecules. Streptavidin-coated microbeads were recruited to the microtubules using a biotin-labelled Dam1 complex. As the microtubules depolymerized, the Dam1 complex slid along the spindle away from the shrinking ends, bearing its cargo of beads. It will be interesting to find out whether chromosomes are transported in the same way.