Assembly of GMPCPP-Bound Tubulin into Helical Ribbons and Tubes and Effect of Colchicine

Microtubule assembly and disassembly is a complex structural process that doesnot proceed by simple addition and subtraction of individual subunits to and from ahelical polymer, as would be the case for actin and other helical assemblies. Thedynamic process of microtubule growth and shrinking involves short-lasting polymerforms that differ substantially from the microtubule itself and constitute crucial assemblyand disassembly intermediates. Structural characterization thus depends on thestabilization of these brief intermediates and their preservation as polymericassemblies. This paper gives experimental details on the polymerization of GMPCPPtubulininto low-temperature stable polymers that we propose to correspond to the earlystages in microtubule assembly and includes new data on the effect of colchicine onGMPCPP-tubulin polymerization. Finally, we include our thoughts on the possiblebiological meaning of tubulin polymerization versatility.