The antimitotic compound methyl benzimidazol-2-yl carbamate (MBC) formed a complex in vitro with a protein present in mycelial extracts of fungi. The binding protein of Aspergillus nidulans showed a set of properties which is unique for tubulin. Binding occurred rapidly at 4 degrees C and was competitively inhibited by oncodazole and colchicine. Other inhibitors of microtubule function such as podophyllotoxin, vinblastine sulfate, melatonin, and griseofulvin did not interfere with binding of MBC. Electrophoretic analysis of partially purified preparations of the binding protein revealed the presence of proteins with similar mobilities as mammalian tubulin monomers. Hence it is concluded that the binding protein is identical with fungal tubulin. The effect of MBC on mycelial growth of mutant strains of A. nidulans was positively correlated with the affinity of the binding sites for this compound. The apparent binding constant for MBC and tubulin from a wild type was estimated at 4.5 X 10(5), from a resistant strain at 3.7 X 10(4), and from a strain with increased sensitivity to MBC at 1.6 X 10(6) liters/mol. Mutants showing resistance and increased sensitivity to MBC are candidates to have alterations in tubulin structure. Affinity of tubulin for MBC is probably a common mechanism of resistance to this compound in fungi. Low affinity of tubulin for MBC is probably a common mechanism of resistance binding constant of 2.5 X 10(3) liters/mol.

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