title: Limited proteolysis of tubulin and the localization of the binding site for colchicine.
abstract: Limited proteolysis of porcine brain tubulin with trypsin resulted in a gradual loss of its colchicine binding activity as well as its ability of assembly into microtubules. The analysis of the tryptic degradation products showed a preferential proteolysis of alpha-tubulin subunit. This enzymatic proteolysis cleaved tubulin in one major site producing fragments of 36,000 and 16,000 daltons, the smaller polypeptides containing the carboxyl-terminal residue as shown by 14C tyrosination . However, proteolysis after incubation with 1 X 10(-3) M colchicine resulted in formation of the indicated fragments plus a 41,000-dalton fragment and smaller size peptides indicating the drug induces a second cleavage site closer to the carboxyl-terminal alpha-tubulin. Preincubation of tubulin with [3H]colchicine followed by proteolysis and separation of the fragments by Sephadex G-75 chromatography showed radioactive colchicine associated with the 16,000-dalton fragment and to the smaller size peptides resulting from digestion in the presence of the drug. The data indicate a localization of the colchicine-binding site in the 16,000-dalton segment containing the COOH-terminal region of alpha-tubulin subunit.
mesh_terms: D000818:Animals; D001665:Binding Sites; D001921:Brain / Q000378:metabolism*; D003078:Colchicine / Q000378:metabolism*; D004591:Electrophoresis, Polyacrylamide Gel; D007700:Kinetics; D046911:Macromolecular Substances; D011485:Protein Binding; D013552:Swine; D014357:Trypsin / Q000378:metabolism; D014404:Tubulin / Q000378:metabolism*
chemical_list: D046911:Macromolecular Substances; D014404:Tubulin; D014357:Trypsin; D003078:Colchicine