Assessment of benzimidazole binding to individual recombinant tubulin isotypes from Haemonchus contortus

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Abstract
One α- and 2 β-tubulin isotypes (isotypes 1 and 2) from the parasitic nematode Haemonchus contortus were artificially expressed in E. coli and purified to obtain tubulin that was capable of polymerizing into microtubules. Binding of [14C] mebendazole (MBZ), a benzimidazole compound, to each individual unpolymerized isotype and to microtubules polymerized from recombinant α- and β-tubulin was assessed and Kd and Bmax values determined. Mebendazole bound to the individual tubulin isotypes with a stoichiometry of 1:1. Binding occurred with highest affinity to α-tubulin followed by β-tubulin isotype 2 and β-tubulin isotype 1 indicating that α-tubulin may play a role in benzimidazole binding to microtubules. Upon polymerization of α- and β-tubulin isotype 2 into microtubules the stoichiometry of binding increased to 2:1 (mebendazole:tubulin) while binding affinity remained the same. Mebendazole binding to α/β-isotype 1 microtubules remained unchanged following polymerization. The increase in the number of benzimidazole receptors on α/β-isotype 2 microtubules suggests the formation of a new benzimidazole receptor upon polymerization.

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