Griseofulvin: Interaction with normal and subtilisin-treated tubulin

Griseofulvin, an antifungal drug, possesses antimitotic activity by inhibiting microtubule assembly. The interaction of griseofulvin with tubulin is unique in that it increases the sulfhydryl titer of tubulin without affecting the hydrophobic areas. Because the C-terminal regions of both α- and β-tubulin influence the conformational and the drug-binding properties of tubulin, we decided to study the interaction of griseofulvin with αβ tubulin and tubulin treated with subtilisin to selectively remove the C-terminal regions from both the α and β-subunits (α_sβ_s). We found that the apparent Κd of griseofulvin for α_sβ_s tubulin (83 μM) was virtually unaltered compared to its Κd for αβ tubulin (91 μM) as determined by the tryptophan fluorescence quenching assay. The increment of the sulfhydryl titer (0.47 mol/mol) by griseofulvin was not affected by removing the C-termini from the α- and β-subunits. Moreover, the lack of effect of griseofulvin on the hydrophobic areas was not changed after the cleavage of the C-termini of the α- and βsubunits. These data therefore strongly suggest that griseofulvin binds at a certain region of tubulin distant from the C-terminal domains of the α- and β-subunits and that the C-terminal domains of both subunits do not have any conformational influence over the binding site of griseofulvin. Determining the binding site of griseofulvin will help in understanding its role in regulating microtubule assembly and dynamics. Drug Dev.