Emission spectrum of 2-AB-taxol in the presence and absence of the colchicine-tubulin complex. The colchicine-tubulin complex was formed by incubating tubulin with a 10-fold excess of colchicine for 30 minutes at 37 degC prior to removal of unbound ligand by rapid gel filtration. Dotted curve: 2-AB-taxol 2 (M) in PME buffer. Solid curve: 2 M 2-AB-taxol and 5 M tubulin-colchicine complex in PME buffer were incubated for 20 min at 25 degC prior to acquisition of the spectrum. Dashed curve: 2 M 2-AB-taxol and 5 M tubulin-colchicine complex in PME buffer were incubated for 20 min at 37 degC prior to addition of 10 M taxol. The excitation wavelength was 320 nm. Note the decrease in intensity of the dashed curve relative to the solid curve. Electron micrographs of the solutions used for the solid and dashed curves showed no high order tubulin structures.
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