Effect of Cibacron blue on tubulin assembly in the absence and presence of microtubule-associated proteins.

Cibacron blue was found to inhibit assembly and increase the critical concentration of microtubule proteins. In the presence of 4 mol Cibacron blue/mol tubulin, assembly was completely inhibited and pre-formed microtubules disassembled. Addition of 8% (v/v) dimethylsulfoxide to Cibacron blue-inhibited samples induced assembly of normal microtubules in addition to sheets of protofilaments. Disassembly was induced upon addition of 1 mM colchicine or 2 mM Ca2+. No obvious difference was seen in the protein composition of these microtubules compared with controls. GTP exchange was not affected by the presence of Cibacron blue nor was GTP able to counteract its effect. This indicates that the exchangeable GTP site is not involved. The extent of assembly of phosphocellulose purified tubulin in the presence of 8% (v/v) dimethylsulfoxide was only slightly less in the presence of Cibacron blue, although the assembly rate was decreased. These results suggest that Cibacron blue might alter the binding of one or more of the associated proteins stimulating assembly.