Regulatory aspects of the colchicine interactions with tubulin

Limited proteolysis of tubulin with subtilisin results in the cleavage of both tubulin subunits yielding S-tubulin heterodimer and 4 kDa peptide fragments containing the carboxyl-terminal domains of alpha- and beta-polypeptide chains. S-tubulin binds colchicine and the characterization of the binding of colchicine to S-tubulin molecules showed a decreased rate of decay of colchicine binding activity as compared to that of undigested tubulin. However, S-tubulin exhibited a lower colchicine binding constant than tubulin. Peptide fragments resulting from the controlled tryptic proteolysis of both pure tubulin and S-tubulin were purified by filtration chromatography and presented a strong colchicine binding activity with association constants of 4.5 X 10(6) and 2.7 X 10(6) M-1, respectively. Furthermore, these studies support our initial findings on the localization of the tubulin site for colchicine (Serrano L, Avila J, Maccioni RB: J Biol Chem 259:6607-6611, 1984) and define the colchicine binding domain in a domain of alpha-subunit from the point of limited tryptic cleavage to the site of subtilisin controlled proteolysis of that tubulin subunit. On the basis of these alterations in the interaction of colchicine upon removal of the C-terminal moiety of tubulin and since no change in the number of binding sites was found after subtilisin digestion, we suggest that the carboxyl-terminal region of tubulin subunits modulates the binding of colchicine.