Calf brain tubulin was subjected to isoelectric focusing and tryptic peptide map analysis. Results from isoelectric focusing experiments showed a total number of 17 well-resolved protein peaks. The number of peaks and the mass distribution under each peak remained the same when the concentration of protein or ampholyte was altered. When the protein was subjected to two-dimensional isoelectric focusing, a diagonal pattern was observed, indicating that the multiple peaks observed are not a manifestation of tubulinampholyte interaction. Further investigation by isolating these individual subspecies and subjecting them to isoelectric focusing yielded single peaks corresponding to the original ones without generating the initial pattern of multiple peaks. Tryptic peptide maps showed that among the subspecies of the α subunit there are 26 spots that are common among them. There are, however, 7 ± 1 spots that are unique in each subspecies. Similar observations were obtained for the subspecies of the β subunit although there are only 2 ± 1 unique spots in each subspecies. These results suggest that tubulin subunits probably consist of polypeptides with both constant and variable regions in their sequences. Identical results were obtained for canine and rabbit brain tubulin, indicating that tubulin polymorphism is common among brain tissues. Tubulin isolated by either the polymerization-depolymerization or the modified Weisenberg procedures yielded identical results. These results show that the same subspecies of tubulin are extracted by both isolation procedures.
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