Binding interactions between barley thaumatin‐like proteins and (1,3)‐β‐D‐glucans

The specificity and kinetics of the interaction between the pathogenesis‐related group of thaumatin‐like proteins (PR5) in higher plants and (1,3)‐β‐ d ‐glucans have been investigated. Two thaumatin‐like proteins with 60% amino‐acid sequence identity were purified from extracts of germinated barley grain, and were designated HvPR5b and HvPR5c. Purified HvPR5c interacted with insoluble (1,3)‐β‐ d ‐glucans, but not with cellulose, pustulan, xylan, chitin or a yeast mannoprotein. Tight binding was observed with unbranched and unsubstituted (1,3)‐β‐ d ‐glucans, and weaker binding was seen if (1,6)‐β‐linked branch points or β‐glucosyl substituents were present in the substrate. The HvPR5b protein interacted weakly with insoluble (1,3)‐β‐ d ‐glucans and did not bind to any of the other polysaccharides tested. This indicated that only specific barley PR5 isoforms interact tightly with (1,3)‐β‐ d ‐glucans. The complete primary structures of HvPR5b and HvPR5c were determined and used to construct molecular models of HvPR5b and HvPR5c, based on known three‐dimensional structures of related thaumatin‐like proteins. The models were examined for features that may be associated with (1,3)‐β‐ d ‐glucan binding, and a potential (1,3)‐β‐ d ‐glucan‐binding region was located on the surface of HvPR5c. No obvious structural features that would prevent binding of (1,3)‐β‐ d ‐glucan to HvPR5b were identified, but several of the amino acids in HvPR5c that are likely to interact with (1,3)‐β‐ d ‐glucans are not present in HvPR5b.