Structure of the tubulin dimer.

Microtubules are formed from a 110,000-dalton dimeric subunit called tubulin. Two forms of 55,000-dalton monomer, alpha and beta, are found in all microtubule preparations. The dimers could thus theoretically be either heterodimers (alphabeta) or homodimers (alphaalpha and betabeta). This problem was investigated by stigated by chemical cross-linking using several bifunctional reagents, of which one, dimethyl-3,3-(tetrame thylenedioxy) dipropionimidate dihydrochloride (DTDI), was able to make intradimer bonds in tubulin. When soluble chick brain tubulin was cross-linked with DTDI and analyzed by electrophoresis in an acrylamide gel system capable of resolving alphaalpha, alphabeta, and betabeta, 60 to 90% of the cross-linked dimer was alphabeta. If tubulin was incubated at 24 degrees prior to cross-linking with DTDI the total yield of cross-linked dimer increased with time, indicating that tubulin was forming loose aggregates. The relative amounts of cross-linked dimer alphaalpha and betabeta also increase with time, indicating that soluble tubulin is largely alphabeta, and suggesting that cross-linked alphaalpha and betabeta arise from nonspecific aggregation during tubulin purification. The aggregation observed by cross-linking with DTDI was strongly influenced by colchicine and Vinca alkaloids in a pattern similar to the effects of these drugs on tubulin polymerization.