Properties of Rat Brain Tubulin

Abstract

Rat brain tubulin is composed of two electrophoretically distinguishable subunits. The state of the protein sulfhydryl groups affects the electrophoretic mobility of these subunits on sodium dodecyl sulfate or sodium dodecyl sulfate-urea polyacrylamide gels, so it is not possible to determine whether the subunits differ in size or charge by these techniques. The complete amino acid composition of rat brain tubulin is presented and the ultraviolet absorption spectra of tubulin in guanidine HCl and in sodium dodecyl sulfate are shown. All the half-cystine residues of tubulin are present as free sulfhydryl groups and are titrated with 4,4'-dithiopyridine with a half-time of about 2 min. The NH₂-terminal residue of both chains of rat brain tubulin is methionine. Tryptic maps of tubulin (equimolar mixture of the α and β chains) suggest that the two chains are very similar, but interpretation of the maps is complicated by the fact that the tryptic digestion is only 80% complete and that there is a significant amount of acid insoluble material. Electrophoresis of both tryptic and peptic digests of in vivo ³²P-tubulin indicate that there is a single ³²P-labeled peptide. The ³²P-labeled tryptic peptide was purified from adult in vivo ³²P-tubulin; it is approximately 50 to 52 amino acids long and contains 20% of the aspartic and glutamic residues and 2% of the basic residues of the entire β chain.