title: Photoaffinity labeling of tubulin with (2-nitro-4-azidophenyl)deacetylcolchicine: direct evidence for two colchicine binding sites.
abstract: A new photoaffinity analogue of colchicine, (2-nitro-4-azidophenyl)deacetylcolchicine (NAPDAC), bound to two classes of sites on bovine renal tubulin and photolabeled both the alpha- and beta-subunits. The apparent Ki for the photoaffinity analogue was 1.40 +/- 0.17 microM (mean +/- SD, n = 3) as measured by competition with [3H] colchicine. Values of the apparent KdS for the two sites, as measured by the direct binding of the [3H]NAPDAC to tubulin, were 0.48 +/- 0.11 microM and 11.6 +/- 3.5 microM (mean +/- SD, n = 6), and the corresponding stoichiometries of binding of the two sites were 0.25 +/- 0.06 and 1.3 +/- 0.4 mol/mol of tubulin (mean +/- SD, n = 6). NAPDAC was a potent inhibitor of microtubule formation as detected by electron microscopy. When tubulin was photolabeled with NAPDAC at 25 degrees C, 15 +/- 3 mol % (mean +/- SD, n = 6) of the [3H]NAPDAC was covalently bound to the alpha-subunit, and 67 +/- 9 mol % (mean +/- SD, n = 6) was covalently bound to the beta-subunit. Since NAPDAC is a mixture of two interconvertible diastereomers, the photoincorporation of each was also examined. One diastereomer photolabeled both alpha- and beta-tubulin; however, the other did not significantly photolabel either subunit. Tubulin photolabeled with NAPDAC (1:1 mole ratio) exhibited a 23% decrease in colchicine binding. Preblocking and prephotolysis experiments with colchicine, NAPDAC, or ANPAH-CLC [Williams et al. (1985) J. Biol. Chem. 260, 13794-13802] provided evidence for conformational changes in tubulin upon colchicine binding. Peptide maps of [3H]NAPDAC-labeled alpha- and beta-tubulin, using Staphylococcus aureus V8 protease, demonstrated the presence of NAPDAC in one peptide of the alpha-subunit and in five peptides of the beta-subunit as detected by autoradiography. NAPDAC provides the first direct evidence for two colchicine binding sites on tubulin.
authors.affiliation: Department of Biochemistry, University of Texas Health Science Center San Antonio 78284-7760.
authors.forename: L J
authors.initials: LJ
authors.lastname: Floyd
authors.forename: L D
authors.initials: LD
authors.lastname: Barnes
authors.forename: R F
authors.initials: RF
authors.lastname: Williams
mesh_terms: D000345:Affinity Labels; D000818:Animals; D001386:Azides; D001665:Binding Sites; D002417:Cattle; D003078:Colchicine / Q000031:analogs & derivatives* / Q000378:metabolism*; D004591:Electrophoresis, Polyacrylamide Gel; D007668:Kidney / Q000378:metabolism*; D008024:Ligands; D015394:Molecular Structure; D013696:Temperature; D014404:Tubulin / Q000378:metabolism*
chemical_list: D000345:Affinity Labels; D001386:Azides; D008024:Ligands; D014404:Tubulin; C062327:(2-nitro-4-azidophenyl)deacetylcolchicine; D003078:Colchicine