Dynamics of microtubules visualized by darkfield microscopy: treadmilling and dynamic instability.

Individual microtubules undergoing treadmilling in vitro were visualized by darkfield light microscopy, and the relationship between treadmilling and dynamic instability was studied as a function of microtubule-associated proteins (MAPs). In order to demonstrate treadmilling directly by real-time observation, we constructed three-block microtubules, the center-block of which was decorated with Tetrahymena dynein. The decorated block can easily be distinguished from undecorated blocks in the darkfield microscope because the decorated one appears much thicker. At steady-state conditions, the length of an undecorated block at one end increased and that at another end decreased, while the decorated center-block did not change in its length. The results from these direct observations show that calf brain 3X-microtubules exhibit a treadmilling flux of 0.9 micron/h. Using a similar microscopy technique, we previously demonstrated that phosphocellulose PC-microtubules existed in either the growing or the shortening phase and alternated quite frequently at steady-state conditions (dynamic instability). How does treadmilling relate to dynamic instability? An image recording of individual 3X-microtubules containing MAPs revealed that the microtubules undergo treadmilling and do not exhibit any dynamic instability. This evidence shows that MAPs suppress the dynamic instability of microtubules. That is, treadmilling can take place in the steady state only after microtubules have been stabilized by MAPs.