Binding interactions between barley thaumatinâlike proteins and (1,3)âβâDâglucans: Kinetics, specificity, structural analysis and biological implications
Osmond, Ronald I. W. | Hrmova, Maria | Fontaine, Fabien | Imberty, Anne | Fincher, Geoffrey B.
The specificity and kinetics of the interaction between the pathogenesisârelated group of thaumatinâlike proteins (PR5) in higher plants and (1,3)âβâdâglucans have been investigated. Two thaumatinâlike proteins with 60% aminoâacid sequence identity were purified from extracts of germinated barley grain, and were designated HvPR5b and HvPR5c. Purified HvPR5c interacted with insoluble (1,3)âβâdâglucans, but not with cellulose, pustulan, xylan, chitin or a yeast mannoprotein. Tight binding was observed with unbranched and unsubstituted (1,3)âβâdâglucans, and weaker binding was seen if (1,6)âβâlinked branch points or βâglucosyl substituents were present in the substrate. The HvPR5b protein interacted weakly with insoluble (1,3)âβâdâglucans and did not bind to any of the other polysaccharides tested. This indicated that only specific barley PR5 isoforms interact tightly with (1,3)âβâdâglucans. The complete primary structures of HvPR5b and HvPR5c were determined and used to construct molecular models of HvPR5b and HvPR5c, based on known threeâdimensional structures of related thaumatinâlike proteins. The models were examined for features that may be associated with (1,3)âβâdâglucan binding, and a potential (1,3)âβâdâglucanâbinding region was located on the surface of HvPR5c. No obvious structural features that would prevent binding of (1,3)âβâdâglucan to HvPR5b were identified, but several of the amino acids in HvPR5c that are likely to interact with (1,3)âβâdâglucans are not present in HvPR5b.
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