Abstract
Wheat germ agglutinin (WGA) from embryos of the monocotyledonous plant
Triticum vulgaris (Graminaceae) is a carbohydrate binding protein characterized by high specificity to
N-acetyl-
d-glucosamine and
N-acetyl-
d-neuraminic acid. In this study we show that parallel to its carbohydrate binding activities, WGA binds with several orders of magnitude higher affinity adenine, adenine-related cytokinins: kinetin, zeatin and isopentenyl-adenine as well as abscisic and gibberellic acids (
K
d 0.43–0.65 μM). Its interactions with these ligands cause conformational rearrangements in the protein molecules and significant enhancement of the protein tryptophan fluorescence (up to 60%) allowing characterization of the protein–hormone complexes. Dimeric WGA molecules possess two different classes of binding sites for the fluorescent hydrophobic probe 2-(
p-toluidinyl) naphthalene sulfonic acid (TNS) as suggested by the sigmoid shape of the fluorescence titration curve and the value of the Hill coefficient (
n
H 1.6±0.3). The plant hormones displace part of the bound TNS probe and share the higher affinity TNS binding sites. These results characterize WGA as a hormone-binding protein.