SEPARATION OF TUBULIN SUBUNITS UNDER NONDENATURING CONDITIONS

The dissociation and separation of the tubulin alpha- and beta-subunit s have been achieved by binding alpha-subunits to an immunoadsorbent g el and selectively inducing release of free beta-subunits. The immunoa dsorbent gel was prepared by coupling the monoclonal antibody YL1/2 to Sepharose 4B which specifically recognizes the C-terminal end of tyro sinated alpha-subunits. Extensive tubulin subunit dissociation and sep aration occurred in Tris buffer at neutral pH but was greatly enhanced at basic pHs (8.0-8.5). The binding of colchicine to heterodimeric tu bulin resulted in a marked protection against dissociation. The dissoc iation of tubulin subunits was accompanied by loss of colchicine bindi ng capacity, and ability to polymerize into microtubules. As shown by circular dichroism, loss of functional properties was not due to exten sive denaturation of tubulin, as tubulin retained most of its secondar y structure. Neither of the separated alpha- or (-)beta-subunits was a ble to bind colchicine, but functional tubulin that was able to bind c olchicine could be reconstituted from the dissociated subunits by chan ging the buffer to a neutral mixture of Tris and Pipes. The yield of r econstitution, as estimated from kinetic measurements of colchicine bi nding capacity, amounted to about 25%. Such a yield can probably be im proved with minor changes in experimental conditions. The quantitative dissociation of tubulin into separated ''native'' alpha- and beta-sub units should provide a powerful tool for further studies on the proper ties of the individual tubulin subunits and the structure-function rel ationships of the tubulins.