In this report we describe the different forms of motile behavior of individual native microtubules from squid giant axons. The three major types of motile behavior of native microtubules are gliding, fishtailing and circling. Gliding, the type of movement observed most often, is the straight translocation of an unbent microtubule segment. Gliding velocities observed in the population ranged from 0.2 to 0.7 microns/s with an average velocity of 0.45 microns/s. The direction of gliding was random with respect to the surface suggesting that physical features of the surface did not influence the direction of gliding. Microtubules are able to glide over objects on the surface and over each other without changing velocity or direction. These observations prove that gliding can continue under conditions where direct contact of the microtubule with the glass surface is not possible along its entire length. When a frontal segment of a microtubule becomes slowed down or attached to the surface, the microtubule begins to fishtail, a process whereby bends form in the frontal part and propagate rearward. The shapes of a fishtailing microtubule resemble that of a beating flagellum. Microtubules with focal attachment near the tip do not propagate bending waves but assume a spiral or circular shape and rotate horizontally (circling). The frontal end of these microtubules stays or rotates in place as pushing forces from the rear turn the microtubule in a circular pattern. An analysis of these data shows that all forms of motion can be explained by pushing forces due to kinesin acting along the length of the microtubule. In an attempt to transport the kinesin-covered cover glass as if it were a big organelle, microtubules translocate themselves in the opposite direction. We estimated the minimum density of force generating enzymes on the surfaces of our preparations as well as that required to maintain active gliding of microtubules. We concluded that the heads of the surface-bound kinesin molecules must display extreme rotatory freedom in order to explain the observed smoothness and straightness of microtubule motion. Few, but usually at least two molecules of kinesin have to work simultaneously to generate the forms of motility observed.
- Axonal Transport,
- Electron microscopy,
Available at: http://works.bepress.com/george_langford/22/