Fungal β-Tubulin, Expressed as a Fusion Protein, Binds Benzimidazole and Phenylcarbamate Fungicides

Benzimidazoles are important antitubulin agents used in veterinary medicine and plant disease control. Resistance is a practical problem correlated with single amino acid changes in β-tubulin and is often linked to greater sensitivity to phenylcarbamates. This negative cross-resistance creates opportunities for durable antiresistance strategies. Attempts to understand the molecular basis of benzimidazole resistance have been hampered by the inability to purify tubulin from filamentous fungi. We have overcome some of these problems by expressing β-tubulin as a fusion with a maltose binding protein. This fusion protein is soluble, and we confirm for the first time using a gel filtration assay that benzimidazoles indeed bind to β-tubulin. This binding is reduced by the mutation Glu(198)→Gly(198), which also confers resistance. Binding of phenylcarbamates is the complete opposite, reflecting their biological activity and the negative cross-resistance. This suggests that the fungicide binding sites fold correctly in the fusion protein.