Microtubules grow and shorten at intrinsically variable rates.

Microtubules were assembled from pure tubulin with axonemal pieces serving as nuclei. They were observed by video-enhanced differential-interference-contrast light microscopy. Their lengths were measured from videotaped images at frequent intervals (0.13-5 s). Error analysis indicated that the uncertainty in measuring a single length was quite small; the 95% confidence limit approximated the microscope's limit of resolution. Rates of growth and shortening of the dynamically unstable microtubules, obtained from the length-versus-time data, were found to be highly variable. The variability was far too large to be attributed to known random error of measurement and must be a property of the microtubules. Further experiments were aimed at finding its structural cause. The variability of rates exhibited by a single microtubule was as great as that of the whole population. The locations at which a growing microtubule changed its rate of growth were not related to the locations at which rates changed during its subsequent shortening. The cause of the variability must therefore be both small relative to the size of a microtubule and transient relative to its lifetime. Fluctuations in configuration of the microtubule's cap appear to be the likeliest source.