Griseofulvin: a novel interaction with bovine brain tubulin

Griseofulvin is an anti-fungal drug whose mechanism of action is directed against microtubules. Although it inhibits the assembly of mammalian brain tubulin, its binding to tubulin has not been directly measured successfully. We have examined the interaction of griseofulvin with tubulin fluorometrically by measuring the quenching of tubulin tryptophan fluorescence by griseofulvin. From Scatchard analysis, we found that griseofulvin bound to tubulin at one class of binding site with an affinity constant of 1.2 +/- 0.19 x 10(4) M(-1), and the binding was largely reversible. Griseofulvin caused a major change in the conformation of tubulin in that it increased the sulfhydryl titer of tubulin approximately 2-fold. The drug affected both the alpha and beta subunits of tubulin equally. Interestingly, griseofulvin did not increase the sulfhydryl titer of the tubulin-colchicine complex although the binding site of griseofulvin was distinctly different from that of colchicine. The change of conformation of tubulin upon interaction with griseofulvin did not affect the exposure of hydrophobic areas on tubulin as shown by binding of bis-5,5'-[8(N-phenyl)aminonapthalene-1-sulfonic acid] (BisANS). Even in combination with colchicine, griseofulvin had very little effect on BisANS binding to tubulin. Thus, griseofulvin appears to interact with tubulin in a manner that is very different from that of many other tubulin ligands.