The B-ring substituent at C-7 of colchicine and the α-C-terminus of tubulin communicate through the "tail-body" interaction

Abstract

The carboxy terminals of αβ -tubulins are flexible regions rich in acidic amino acid residues that play an inhibitory role in the polymerization of tubulin to microtubules. We have shown that the binding of colchicine and its B-ring analogs (with C-7 substituents) to tubulin are pH sensitive and have high activation energies. Under identical conditions, the binding of analogs without C-7 substituents is pH independent and has lower activation energy. β -C-terminus-truncated tubulin (αβ _s) shows similar pH sensitivity and activation energy to native tubulin (αβ ). Removal of the C-termini of both subunits of tubulin (α_sβ_s) or the binding of a basic peptide P2 to the negatively charged α -C-terminus of tubulin causes a colchicine-tubulin interaction independent of pH with a low activation energy. Tubulin dimer structure shows that the C-terminal α -tail is too far from the colchicine binding site to interact directly with the bound colchicine. Therefore, it is likely that the interaction of the α -C-terminus with the main body of tubulin indirectly affects the colchicine-tubulin interaction via conformational changes in the main body. We therefore conclude that in the presence of tail-body interaction, a B-ring substituent makes contact with the α -tubulin and induces significant conformational changes in α -tubulin.