Submitted to: Journal of Toxicology Toxins Reviews
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/25/2009
Publication Date: 5/14/2009
Citation: Chen, Z.-Y., Brown, R.L., Cary, J.W., Damann, K.E., Cleveland, T.E. 2009. Characterization of an Aspergillus flavus alkaline protease and its role in the infection of maize kernels. Journal of Toxicology Toxins Reviews. 28(2-3):187-197. Interpretive Summary: The fungus named Aspergillus flavus produces a poison called aflatoxin when it infects corn kernels. Aflatoxin prevents the corn from being used commercially. The best strategy for controlling this problem is to develop corn that is resistant to aflatoxin contamination. In order to implement this strategy, it is important to identify and understand fungal traits important to the maize kernel infection process. Towards this aim, we isolated and identified a fungal protein called alkaline protease that was present in Aspergillus flavus infected maize embryo tissue. This protein was shown to be a major protein activity in some culture media but not in others. The activity of this protease was inhibited by a common protease inhibitor and when this occurred, aflatoxin accumulation in culture was significantly reduced. This suggests that alkaline protease may play a role in the infection of maize kernels and subsequent aflatoxin accumulation. This knowledge could lead to the development of a strategy of developing aflatoxin-resistant corn lines through enhancement of kernel ability to inhibit fungal alkaline protease activity. This could then lead to future savings of millions of dollars to growers, as a result of the elimination of aflatoxin contamination of corn.
Technical Abstract: A 33 kDa protein present in Aspergillus flavus infected maize embryo tissue 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 cultural 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 µ' (200 µg/ml) maize 14 kDa trypsin inhibitor. Further study demonstrated that reduction of this ALK by PMSF also significantly reduced the level of aflatoxin accumulation in A. flavus culture, suggesting ALP may also play a role in infection of maize kernels and subsequent aflatoxin accumulation.