The carboxy terminus of the alpha subunit of tubulin regulates its interaction with colchicine.

Controlled proteolysis of goat brain tubulin by subtilisin was carried out to investigate regulatory aspects of the binding of colchicine to tubulin. Tubulin S, obtained by the cleavage of the carboxyl termini of both the alpha- and beta-subunits of tubulin by subtilisin, exhibited the following differences compared to native tubulin: (a) Reaction with colchicine, which has an optimum pH of 6.8, becomes independent of pH (in the range 5.7-8.0). (b) The colchicine-binding site, which is labile at 37 degrees C (t1/2 = 4-5 h), becomes highly stable (t1/2 greater than 12 h). (c) The affinity for colchicine is lowered. (d) This lowering of affinity arises from a faster dissociation (higher off rate) of the complex. The above characteristics of tubulin S were not shown by a partially digested hybrid in which the C-terminus of the beta-subunit alone was cleaved. The hybrid behaved very much like the undigested native protein. These results strongly suggest that the regulatory switch for colchicine-tubulin interaction is located in a small region (about 15 residues) of the C-terminus of the alpha-subunit of tubulin. Possibilities of the C-termini being involved in nonbonded contacts with the main body of tubulin are also noticed from the change in conformation between tubulin and tubulin S.