Submitted to: Mycopathologia
Publication Type: Peer reviewed journal
Publication Acceptance Date: 3/24/2011
Publication Date: 9/14/2011
Citation: Mellon, J.E., Cotty, P.J., Callicott, K.A., Abbas, H.K. 2011. Identification of a major xylanase from A. flavus as a 14-kD protein. Mycopathologia. 172:299-305. Interpretive Summary: Aflatoxin is a very potent carcinogen and toxin that is produced by the fungus, Aspergillus flavus. When this fungus infects target crops (corn, cotton, peanuts, tree nuts), the developing seed can become contaminated with this toxin, rendering the product unusable for additional agricultural uses. Xyloglucans and xylans are plant polysaccharides that are important structural components of plant cell walls. Cottonseed contains high concentrations of xylans and is particularly susceptible to aflatoxin contamination. The fungus is capable of producing an enzyme, xylanase, that is crucial to breaking down xylans. This hydrolase activity helps the fungus breach host cell walls and gain access to potential nutrient sources in cottonseed that include free sugars (raffinose), lipids and storage proteins. In order to better understand the role of this enzyme in fungal virulence, a xylanase activity from a field isolate of A. flavus was purified and characterized. This A. flavus hydrolase is a low molecular weight, heat stable protein that is consistent with a highly mobile (diffusible), plant cell wall degrading factor. This research will benefit cotton breeders, producers and pathologists, and will aid in the formulation of methods to prevent aflatoxin contamination of cottonseed.
Technical Abstract: Aspergillus flavus K49 secreted at least two xylanase activities when grown on a medium containing larch (wood) xylan as a sole carbon source. Enzyme activity was assayed using an agar medium containing Remazol Brilliant Blue R conjugated oat spelt xylan as substrate. Crude enzyme preparations were inhibited by Hg**+2, with an ED50 of 17.5 mM and maximum inhibition of 83% at 50 mM. A concentrated sample of A. flavus K49 exlanase preparation was subjected to gel filtration chromatography on a P-30 column. A small protein peak coinciding with the major peak of xylanase activity was separated from the other secreted fungal proteins. An additional peak of xylanase activity was observed in fractions with several fungal proteins. Analysis by denaturing SDS-PAGE of fractions containing the smaller molecular weight protein revealed a major and minor protein band in the vicinity of 20 kD. Analysis of these same fractions by acidic native PAGE revealed a single band, suggesting the presence of a homogeneous protein preparation. Data suggests this xylanolytic activity is associated with a low molecular weight protein, and is consistent with a highly mobile (deffusible), resistant plant wall hemicellulose degrading factor.