The chemical characterization of calf brain microtubule protein subunits

Calf brain microtubule protein (tubulin) was characterized chemically. Amino acid analysis and hydrogen ion titration in 5 M guanidine hydrochloride yielded a chemical composition similar to that of tubulin from other sources. The NH₂ terminal residue was identified as methionine. When the protein was denatured and reduced in 8 M urea, two distinct protein bands were observed in gel electrophoresis in the presence of urea, suggesting the existence of two nonidentical subunits. The molecular weight of the tubulin subunits was determined by a variety of techniques, including sedimentation equilibrium and light scattering in 6 M guanidine hydrochloride, sodium dodecyl sulfate gel electrophoresis, chromatography on an agarose column in the presence of 6 M guanidine hydrochloride, and viscometry of S carboxymethylated, reduced protein in 6 M guanidine hydrochloride. All methods yielded an average molecular weight value of (54,000 ± 1,000), whether the measurements were done with or without reducing agents, indicating that all disulfide bonds present are intrachain.