Interactions of the tau-tubulin-vinblastine complex with colchicine, podophyllotoxin, and N,N'-ethylenebis(iodoacetamide).

Microtubule assembly is inhibited by anti-mitotic drugs such as colchicine or podophyllotoxin and also by sulfhydryl-oxidizing reagents, but it is not known which tubulin-tubulin interactions are disrupted by these agents. We have studied the interactions of a complex of tubulin, vinblastine, and tau protein with these agents. This complex has the form of a spiral filament and may consist of tubulin dimers joined end to end as in a protofilament; presumably, therefore, the lateral interaction sites should be accessible in this structure but not in the intact microtubule. Unlike the microtubule, the complex binds to colchicine and podophyllotoxin with high affinity. Again, unlike intact microtubules, the complex reacts with N,N'-ethylene-bis(iodoacetamide) to generate an intra-chain cross-link in beta-tubulin. Tubulin molecules containing this cross-link are unable to polymerize, suggesting that formation of this cross-link involves sulfhydryl groups that are critical for assembly. These results are consistent with a model whereby colchicine-, podophyllotoxin-, and sulfhydryl-oxidizing agents inhibit microtubule assembly by preventing lateral interactions between tubulin molecules in adjacent protofilaments.