Characterization of an Aspergillus flavus alkaline protease and its role in the infection of maize kernels

A 33 kDa protein present in Aspergillus flavus-infected maize embryo was identified as a fungal alkaline protease (ALP). This protein became one of the major extracellular proteins of A. flavus in potato dextrose broth medium culture filtrate after 3 days, but was expressed at low levels or undetectable in A+M medium containing gelatin or starch, respectively. The activity of purified ALP was significantly inhibited in the presence of 0.5 mM phenylmethylsulfonyl fluoride (PMSF), or 1.43 mu M (200 mu g/ml) maize 14 kDa trypsin inhibitor. Further study demonstrated that reduction of this ALP by PMSF also significantly reduced the level of aflatoxin accumulation in A. flavus culture, suggesting an important role for ALP in the infection of maize kernels and subsequent aflatoxin accumulation.