Most kinesins use energy from ATP hydrolysis to move and generate force along microtubules; however, exceptions exist. Rather than moving along microtubules, kinesin-13 proteins (kinesin-13s) actively depolymerize microtubules, which is important for chromosome segregation during mitosis. What is the structural basis of this unusual behaviour by kinesin-13s? And what is the mechanism that is used to depolymerize microtubules? Tan et al. now provide some answers.

They found that kinesin-13s form rings and spirals around microtubules...

The authors used electron microscopy to investigate the structure of the complex that is formed between microtubules and Drosophila melanogaster or hamster kinesin-13s. They found that kinesin-13s form rings and spirals around microtubules — rings only formed in the presence of both microtubules and kinesin-13s, and were favoured specifically by the ATP-bound forms of these kinesins.

To gain further insights into the structure of the ring, Tan et al. created a three-dimensional reconstruction of the spirals that are formed on microtubules with 15 protofilaments. The molecular model showed that contacts along a tubulin protofilament in the outside ring must stabilize the spiral and that the innermost part of the ring is a kinesin motor domain that interacts with the microtubule wall. In addition, interactions between two kinesin motor domains bridged the inner and outer ring regions, which implies that the interactions between the kinesin molecules are part of the mechanism that leads to ring and spiral formation.

So, what could be the function of these rings? In vivo analysis showed that the D. melanogaster kinesin-13 KLP59C specifically accumulates at the depolymerizing microtubule end and slides along the tubule lattice as the depolymerizing end advances. Based on their findings, the authors proposed that kinesin-13s in higher eukaryotes have a dual function during mitosis — they control microtubule depolymerization and they form a loose sleeve, or ring, that can slide along the microtubule lattice and keeps kinesin-13s associated with the microtubule ends.

Three-dimensional reconstruction of kinesin-13 spirals. Image courtesy of H. Sosa, Albert Einstein College of Medicine, Bronx, New York, USA.