The specificity and kinetics of the interaction between the pathogenesis-re
lated group of thaumatin-like proteins (PR5) in higher plants and (1,3)-bet
a -D-glucans have been investigated. Two thaumatin-like proteins with 60% a
minoacid sequence identity were purified from extracts of germinated barley
grain, and were designated HvPR5b and HvPR5c. Purified HvPR5c interacted w
ith insoluble (1,3)-beta -D-glucans, but not with cellulose, pustulan, xyla
n, chitin or a yeast mannoprotein. Tight binding was observed with unbranch
ed and unsubstituted (1,3)-beta -D-glucans, and weaker binding was seen if
(1,6)-beta -linked branch points or beta -glucosyl substituents were presen
t 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 tes
ted. 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-l
ike 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-bin
ding region was located on the surface of HvPR5c. No obvious structural fea
tures that would prevent binding of (1,3)-beta -D-glucan to HvPR5b were ide
ntified, but several of the amino acids in HvPR5c that are likely to intera
ct with (1,3)-beta -D-glucans are not present in HvPR5b.