Identity and polymerization-stimulatory activity of the nontubulin proteins associated with microtubules

Microtubule protein purified by an in vitro assembly procedure contains several accessory proteins which copurify with tubulin and are required for efficient tubulin assembly. When purified microtubule protein was critically examined by sodium dodecyl sulfate gel electrophoresis, two principal high-molecular-weight bands and 33 additional nontubulin components were identified in addition to tubulin. These proteins were isolated and characterized with respect to their binding affinity for microtubules, their relative stimulatory activity and the specificity of the stimulatory effect with the following results: (1) although 35 nontubulin components were identified after two cycles of purification, only a few proteins, including the high-molecular-weight components, bound to microtubules with high affinity and maintained a constant stoichiometry to tubulin through six cycles of purification; (2) from analysis of an activity profile of the fractionated components, 60% of the stimulatory activity in our preparations of microtubule protein was attributed to the high-molecular-weight material and the remaining 40% was attributed to a spectrum of other nontubulin proteins; and (3) the stimulatory effect was not restricted to the HMW or other accessory proteins but was also exhibited by cationic substances and by glycerol, suggesting that many factors that bind to tubulin also stimulate microtubule polymerization. Since the kind and amount of nontubulin protein detected by other workers appear to depend on the methods employed for their purification, details are also presented for the ion-exchange procedures we have developed for preparing nontubulin proteins from porcine brain microtubules.