Molecules in mammalian brain that interact with the colchicine site on tubulin.

Colchicine, a plant alkaloid, is a potent inhibitor of mitosis and other physiological processes that involve microtubules. These effects are mediated by the specific binding of colchicine to a high-affinity receptor site on tubulin, the major protein of microtubules. It seemed possible that the colchicine site on tubulin might also be the receptor for endogenous cellular molecules. We now report that mammalian brain does, in fact, contain a class of molecules that interact with the colchicine site on tubulin. Tubulin-agarose affinity chromatography has been used to isolate factors from soluble extracts of bovine brain that interact with tubulin. Certain of these factors inhibit the binding of [3H]colchicine to tubulin. Using the inhibition of colchicine binding as an assay, I have partially resolved at least two species. One is a protein and another may be a heat-stable peptide. In parallel to the action of colchicine, the smaller species inhibits the assembly of isolated microtubules. This class of molecules may include the endogenous ligands for which the colchicine site on tubulin is the receptor. They may have a significant role in the regulation of cellular microtubule function and assembly.