Heterogeneity of Tubulin Subunits

Tubulin, the subunit protein of microtubules, is a dimer that sediments at 6 S and has a molecular weight of 110,000. Using high resolution polyacrylamide gel electrophoresis, we have demonstrated the presence of two peptide chains, of molecular weight 56,000 and 53,000, in tubulin purified from brain. Two peptide chains of similar molecular weight were identified in each of the A- and B-tubulins isolated from flagella of sea urchin sperm. In all cases, the protein concentrations in the bands were equal. Each of the subunits ran as a single band when eluted from the gel and electrophoresed again in the same type of gel. Chromatography of purified brain tubulin on DEAE-Sephadex columns gave only a single peak containing both subunits in equal amounts. Cyanogen bromide peptides were prepared from each of the bands after elution from polyacrylamide gel. While certain of the peptides appear to be common to both subunits, substantial differences exist between them. The tubulin dimer is composed of two nonidentical subunits.