Recognition of corn defense chitinases by fungal polyglycine hydrolases

Authors: Naumann, Todd; BAKOTA, ERICA - Former ARS Employee; Price, Neil

Submitted to: Journal of Biological Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/3/2017
Publication Date: 4/6/2017
Citation: Naumann, T.A., Bakota, E.L., Price, N.P.J. 2017. Recognition of corn defense chitinases by fungal polyglycine hydrolases. Protein Science. 26(6):1214-1223.

Interpretive Summary: ChitA is an antifungal plant protein that is present in the fluid-filled space between plant cells. Pathogenic fungi invade this space and grow, resulting in disease. Some fungal pathogens of corn secrete proteins called polyglycine hydrolases (PGHs) that degrade ChitA. In this research scientists at the National Center for Agricultural Utilization Research, Peoria, IL, studied how PGHs are able to recognize and degrade ChitA. During the research they discovered a way to disrupt PGH degradation of ChitA using small molecules called peptides. This work was done in test tubes but it suggests that peptides could be used to protect ChitA from PGHs in plants. Additionally, this research helps corn breeders by identifying specific positions in the corn genome where genetic diversity may be linked to fungal resistance. Increased fungal resistance in commercial corn hybrids will improve food safety by reducing the amount of fungal mycotoxins in harvested grains and improve yields for farmers.

Technical Abstract: Polyglycine hydrolases (PGH)s are secreted fungal endoproteases that cleave peptide bonds in the polyglycine interdomain linker of ChitA chitinase, an antifungal protein from domesticated corn (Zea mays ssp. mays). These target-specific endoproteases are unusual because they do not cut a specific peptide bond but select one of many Gly-Gly bonds within the polyglycine region. Some Gly-Gly bonds are cleaved frequently while others are never cleaved. Moreover, we have previously shown that PGHs from different fungal pathogens prefer to cleave different Gly-Gly peptide bonds. It is not understood how PGHs selectively cleave the ChitA linker, especially because its polyglycine structure lacks peptide sidechains. To gain insights into this process we synthesized several peptide analogs of ChitA to evaluate them as potential substrates and inhibitors of Es-cmp, a PGH from the plant pathogenic fungus Epicoccum sorghi. Our results showed that part of the PGH recognition site for substrate chitinases is adjacent to the polyglycine linker on the carboxy side. More specifically, four amino acid residues were implicated, each spaced four residues apart on an alpha helix. Moreover, analogous peptides with selective Gly->sarcosine (N-methylglycine) mutations or a specific Ser->Thr mutation retained inhibitor activity but were no longer cleaved by PGH. Additonally, our findings suggest that peptide analogs of ChitA that inhibit PGH activity could be used to strengthen plant defenses.