Submitted to: Fungal Genetics and Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/30/2020
Publication Date: 5/6/2020
Citation: Naumann, T.A., Naldrett, M.J., Price, N.P.J. 2020. Kilbournase, a protease-associated domain subtilase secreted by the fungal corn pathogen Stenocarpella maydis. Fungal Genetics and Biology. 141:103399. https://doi.org/10.1016/j.fgb.2020.103399.
Interpretive Summary: If crops are more resistant to fungal disease, then harvested grains will contain fewer mycotoxins. To improve plant defenses scientists are studying how mycotoxin producing fungi attack them. In the current research scientists at the National Center for Agricultural Utilization Research, Peoria, IL, report the identification of kilbournase, a protein that attacks corn defenses. It is produced by the fungus Stenocarpella maydis, a common ear rot pathogen that produces mycotoxins and causes diplodiosis, a fatal disease of sheep and cattle. Kilbournase, named after the town of Kilbourne, Illinois, is the first protein implicated in S. maydis ear rot of corn, commonly called diplodia ear rot. This discovery will aid the development of novel methods to improve diplodia ear rot resistance, ultimately reducing mycotoxin contamination of harvested grains.
Technical Abstract: Subtilases are a large family of serine proteases that occur throughout biology. A small subset contain protease-associated (PA) domains that are structurally separate from but encoded within the active site. In bacteria, subtilase PA domains function to recruit specific protein substrates. Here we demonstrate that a protease secreted by the fungal corn pathogen Stenocarpella maydis, which truncates corn ChitA chitinase, is a PA domain subtilase. Protease was purified from S. maydis cultures and tryptic peptides were analyzed by LC-MS/MS. Ions were mapped to two predicted PA domain subtilases. Yeast strains were engineered to express each protease. One failed to produce recombinant protein while the other secreted protease that truncated ChitA. This protease, that we named kilbournase, was purified and characterized. It cleaved multiple peptide bonds in the amino-terminal chitin binding domain of ChitA while leaving the catalytic domain intact. Kilbournase was more active on the ChitA-B73 alloform compared to ChitA-LH82 and did not cleave AtChitIV3, a homolog from Arabidopsis thaliana, indicating a high level of specificity. Truncation of corn ChitA by kilbournase resembles truncation of human C5a by Streptococcus pyogenes ScpA, arguing that PA domain proteases in bacteria and fungi may commonly target specific host proteins.