Polymerization of the tubulin-colchicine complex and guanosine 5'-triphosphate hydrolysis.

The tubulin-colchicine (1:1) complex was shown to be able to polymerize in vitro under the buffer conditions of microtubule assembly from pure native tubulin. The physical characteristics of this peculiar polymer have been investigated under a variety of conditions and compared with those of microtubules. Polymerization consisted of the nucleation followed by the growth process, was characterized by a critical concentration, and exhibited divalent ion, temperature, and pH dependences very similar to those of microtubules, Guanosine 5'-triphosphate (GTP) or 5' -guanylyl methylene-diphosphate (GMPPCP) was required for polymerization, and guanosine 5'-diphosphate (GDP) was a potent inhibitor. GTP hydrolysis was totally disconnected from the polymerization process and occurred as well under nonpolymerizing conditions. The results are discussed in view of the different types of protein-protein interactions exhibited by tubulin and of the possible relationship between the conformation of the GTP site and the interaction areas.