Walk this (and that) way

Text by William A. Wells [email protected] Walk this (and that) way n 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 has been around for a long time, and has had candidate mediators in budding yeast, but until now had not been directly visualized. Kapoor says that, for him and his collaborators, “this is just a starting point.” They hope to add opposing, minus-end–directed motors and more complex microtubule arrays to the in vitro assays, with the eventual aim of reconstructing a functioning spindle from known components.