title: The binding of terbium ions to tubulin induces ring formation.
abstract: The intrinsic fluorescence excitation and emission spectra of chicken brain tubulin showed the characteristic tryptophan fluorescence. The emission spectrum of Tb3+ in the presence of tubulin and GTP excited at 295 nm, showed four peaks, with the maxima at 490, 545, and 586 nm and a minor peak around 620 nm. Titration of tubulin with Tb3+ was followed by the increment in luminescence at 545 nm and showed a sigmoidal curve where the initial lag interval and the maximal luminescence intensity depended on tubulin concentration. The presence of Mg2+, Co2+, and Zn2+ diminished both the sigmoidicity of the curve and the maximal luminescence intensity. Titration of tubulin with Tb3+ also produced a sigmoidal increase in turbidity, which was shifted to the left with respect to the luminescence curve. The dependence of turbidity on the wavelength of the Tb(3+)-induced polymers revealed that the large structures formed were not microtubules. Electron microscopy of the aggregates induced by Tb3+ showed mainly a lattice of double rings with side-by-side contacts. These results indicate that Tb3+ induces principally double ring formation and that these rings (33 +/- 2 nm external diameter) aggregate in large-ordered arrays. The luminescence of Tb3+ seems to be induced mainly by the aggregation of rings.
authors.affiliation: Departamento de Biología, Facultad de Ciencias, Universidad de Chile, Santiago.
authors.forename: O
authors.initials: O
authors.lastname: Monasterio
authors.forename: M
authors.initials: M
authors.lastname: Acoria
authors.forename: M A
authors.initials: MA
authors.lastname: Díaz
authors.forename: R
authors.initials: R
authors.lastname: Lagos
mesh_terms: D000818:Animals; D001921:Brain / Q000378:metabolism; D002645:Chickens; D002712:Chlorides / Q000494:pharmacology; D003035:Cobalt / Q000494:pharmacology; D007700:Kinetics; D046911:Macromolecular Substances; D008274:Magnesium / Q000494:pharmacology; D008345:Manganese / Q000494:pharmacology; D017895:Manganese Compounds; D008854:Microscopy, Electron; D011485:Protein Binding; D011487:Protein Conformation; D013050:Spectrometry, Fluorescence; D013725:Terbium / Q000737:chemistry / Q000378:metabolism*; D014404:Tubulin / Q000737:chemistry / Q000378:metabolism* / Q000648:ultrastructure; D015032:Zinc / Q000494:pharmacology; D017967:Zinc Compounds
chemical_list: D002712:Chlorides; D046911:Macromolecular Substances; D017895:Manganese Compounds; D014404:Tubulin; D017967:Zinc Compounds; D013725:Terbium; D003035:Cobalt; D008345:Manganese; C016837:zinc chloride; C018021:cobaltous chloride; D008274:Magnesium; D015032:Zinc; C025340:manganese chloride