Interaction of Ant-ATP with Tubulin: Evidence for ATP Competition for the GTP E-Site on Tubulin

The interaction of the ribose-moditied ATP analogue 3′-O-anthraniloyl adenosine 5′-triphosphate (Ant-ATP) with tubulin has been studied using steady-state fluorescence techniques. This analogue inhibits the polymerization of tubulin induced by ATP or GTP. When this analogue binds to tubulin, an increase in the fluorescence intensity of the analogue and a blue shift of about 10 nm in the emission maximum have been observed. It has been found that Ant-ATP binds to tubulin at a single binding site in the hydrophobic region with a dissociation constant of 0.5-1.0 μM. It has been possible to restore the fluorescence emission intensity of the tubulin-bound analogue to that of the free ligand with a concomitant shift in the wavelength of emission maximum to that of the free analogue by displacing the analogue with ATP or GTP. These results can be interpreted to suggest that ATP and GTP compete for the Ant-ATP binding site and that ATP binds at the GTP exchangeable site (E-site).