Differential toxicity of antifungal protein AFP against mutants of Fusarium oxysporum.

Antifungal protein (AFP) from Aspergillus giganteus was assayed for toxicity against the Fusarium oxysporum wild-type strain and mutants in genes involved in cell signaling (DeltapacC, pacCc Deltafmk1) or cell-wall biogenesis (DeltachsV, Deltachs7, Deltagas1). The mutants were classified into two groups according to their sensitivity to AFP: DeltapacC, Deltagas1 and Deltachs7, which were significantly more resistant to AFP than the wild-type, and pacCC, Deltafmk1 and DeltachsV, which were more sensitive. Western blot analysis revealed increased binding of AFP to the three resistant mutants, DeltapacC, Deltagas1 and Deltachs7, but also to DeltachsV, indicating that differential binding may not be a key determinant for sensitivity. Addition of Ca2+ or K+ dramatically reduced antifungal activity and binding of AFP, suggesting that these cations compete for the same targets as AFP at the surface of the fungal cell.