Binding interactions between barley thaumatin-like proteins and (1,3)-beta-D-glucans. Kinetics, specificity, structural analysis and biological implications.

The specificity and kinetics of the interaction between the pathogenesis-related group of thaumatin-like proteins (PR5) in higher plants and (1,3)-beta-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)-beta-D-glucans, but not with cellulose, pustulan, xylan, chitin or a yeast mannoprotein. Tight binding was observed with unbranched and unsubstituted (1,3)-beta-D-glucans, and weaker binding was seen if (1,6)-beta-linked branch points or beta-glucosyl substituents were present in the substrate. The HvPR5b protein interacted weakly with insoluble (1,3)-beta-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)-beta-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)-beta-D-glucan binding, and a potential (1,3)-beta-D-glucan-binding region was located on the surface of HvPR5c. No obvious structural features that would prevent binding of (1,3)-beta-D-glucan to HvPR5b were identified, but several of the amino acids in HvPR5c that are likely to interact with (1,3)-beta-D-glucans are not present in HvPR5b.