Microtubules have been shown to depolymerize in vitro into a high-molecular-weight aggregate and a 6S component. Irradiation of these components separately has indicated that the 6S component is preferentially damaged. This was shown by a reduction in the extent of polymerization following reconstitution of the irradiated and control fractions. The irradiation had no apparent effect on either protein kinase activity or on <tex-math>$[{}^{14}{\rm C}]\text{-colchicine}$</tex-math> binding ability. In addition, irradiation of depolymerized microtubules at various protein concentrations has demonstrated that the 6S tubulin is also preferentially damaged in the presence of the high-molecular-weight component.

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