Colchicine binding by the "isolated" beta-monomer of tubulin

At mole ratios of lactoperoxidase to tubulin monomers of 3-4, bovine lactoperoxidase forms 1:1 adducts with both alpha- and beta-tubulin from rat brain, thereby separating the tubulin heterodimer into its monomers. This mixture binds colchicine normally, and we show here by direct photoaffinity labeling that the bulk of the [3H]colchicine becomes attached to beta-tubulin under these conditions. When the alpha-tubulin has been displaced by lactoperoxidase, the ratio of label in beta-tubulin to alpha-tubulin is increased. The amount of label in alpha-tubulin decreases with a corresponding appearance of label in lactoperoxidase. The rate of labeling of beta-tubulin remains slow. We conclude that alpha-tubulin is not necessary for colchicine binding and propose a model wherein the A and C rings of colchicine bind to beta-tubulin, while the B ring faces alpha-tubulin in the dimer.