Microtubule stabilizer reveals requirement of Ca 2+ -dependent conformational changes of microtubules for rapid coiling of haptonema in haptophyte algae.

A haptonema is an elongated microtubule-based motile organelle uniquely present in haptophytes. The most notable and rapid movement of a haptonema is "coiling", which occurs within a few milliseconds following mechanical stimulation in an unknown motor-independent mechanism. Here, we analyzed the coiling process in detail by high-speed filming and showed that haptonema coiling was initiated by left-handed twisting of the haptonema, followed by writhing to form a helix from the distal tip. On recovery from a mechanical stimulus, the helix slowly uncoiled from the proximal region. Electron microscopy showed that the seven microtubules in a haptonema were arranged mostly in parallel but that one of the microtubules often wound around the others in the extended state. A microtubule stabilizer, paclitaxel, inhibited coiling and induced right-handed twisting of the haptonema in the absence of Ca2+ , suggesting changes in the microtubule surface lattice. Addition of Ca2+ caused bend propagation toward the proximal region. These results indicate that switching microtubule conformation, possibly with the aid of Ca2+ -binding microtubule-associated proteins is responsible for rapid haptonematal coiling.