Radioimmune assay of tubulin applied to oligomers, synaptic membranes, and plants

A radioimmune assay for microtubule protein, tubulin, is described, in which unknown amounts of native or denatured tubulin can be quantitated by the ability to compete with pure ¹²⁵I]tubulin for rabbit antibodies produced against purified bovine brain tubulin. The assay is used to demonstrate that crude extracts of mouse brain contain negligible amounts of 30–36S tubulin oligomers under conditions where purified tubulin forms substantial amounts of such structures. Also, the particulate fraction of osmotically shocked and sonicated brain synaptosomes contains negligible tubulin antigenic activity. By contrast, soluble extracts of soybean, especially rapidly dividing regions of the plant, were found to contain significant amounts of cross-reacting material, providing further evidence for the conservative evolutionary nature of this ubiquitous and important protein.