Are cytoplasmic microtubules heteropolymers?

Colchicine-binding protein, considered to be microtubule protein, was purified from chick embryo brain by column chromatography in one step on DEAE-Sephadex. The active colchicine-binding unit is a dimer, MW 115,000 +/- 5000, which is composed of two nonidentical monomeric units. The two subunits are separable by urea-acrylamide gel electrophoresis after they have been reduced and acetylated. Sodium dodecyl sulfate-acrylamide gel electrophoresis indicates that the subunits both have molecular weights of 55,000 +/- 2000. The amino-acid compositions of the two subunits showed statistically significant differences in six amino-acid residues. These results indicate that colchicine-sensitive cytoplasmic microtubules are heteropolymers.