Purification of bovine adrenocortical and brain tubulin. A comparative study.
Microtubules from the cow adrenal cortex and brain were purified by three cycles of the temperature-dependent polymerization-depolymerization procedure. Whereas tubulin comprised approximately 8--10% of soluble brain protein, it comprised only 0.5-1.0% of the soluble adrenocortical protein. The partially purified tubulin from both sources gave similar results in the following studies: (1) [3H]colchicine binding examined by Scatchard analysis revealed an apparent Ka of 1 . 10(6) M-1 and a colchicine/tubulin molar binding ratio of 0.4-0.6; (2) tyrosylation studies using a specific tubulin-tyrosine ligase (which adds a tyrosine residue to the C-terminal glutamate or glutamine of the alpha-chain) in conjunction with carboxypeptidase A (which recovers the tyrosine) and (3) amino acid analysis. Examination of protein bands, in addition to the tubulin doublet of 55 000 molecular weight, on sodium dodecyl sulfate polyacrylamide gel electrophoresis revealed a difference between the two tubulin preparations. The adrenocortical preparation had protein bands corresponding to apparent molecular weight of 36 000, 60 000, and 68 000. In contrast the brain preparation had only proteins of molecular weights greater than 200 000 (these bands were absent in all adrenal preparations). It would thus appear that if proteins which copurify with tubulin through repeated cycles of polymerization-depolymerization play a role in either microtubule formation or function there is a distinct difference between neural and non-neural tissue.