Walk this (and that) way

In one model of spindle extension, a cross-linking motor walks outwards simultaneously on two antiparallel microtubules, thus shoving them apart. The Eg5 kinesin fits that bill, according to Lukas Kapitein, Erwin Peterman, Christoph Schmidt (Vrije Universiteit, Amsterdam, Netherlands), Tarun Kapoor (Rockefeller University, New York, NY), and colleagues. This homotetrameric, bipolar motor walks toward the plus ends of both microtubules that it cross-links.To visualize the hand-over-hand and foot-over-foot process, the group manipulated surface chemistry to ensure the formation of microtubule–motor–microtubule sandwiches. But 20-nm/s movement in two directions looks like 40-nm/s in one direction. The solution was to drag Eg5-linked microtubules into X-shaped conformations using antibody-coated beads and a laser trap. Motors were then seen to be translocating toward the plus ends of both linked microtubules.

This confirms a “mitotic muscle” model of spindle extension. The model...