Abstract
Plants synthesize small amounts of carbohydrate-binding proteins on exposure to stress. For example, on exposure to drought, high salt, wounding and by treatment with some plant hormones or by pathogen attack. In contrast to the ‘classical’ plant lectins that are mostly located in the vacuolar compartment, this new class of inducible lectins is present in the cytoplasm and in the nucleus. Taking into account that any physiological role of plant lectins most likely relies on their specific carbohydrate-binding activity and specificity, the discovery of these stress-related lectins provides strong evidence for the importance of protein-carbohydrate-interactions in plant cells. Hitherto, six families of such nucleocytoplasmic lectins have been identified in plants. This review will focus on the nucleocytoplasmic lectins with one or more Euonymus lectin (EUL) domain(s). The carbohydrate-binding specificity of EUL proteins from a monocot, a dicot and a lower plant has been compared. Furthermore, modeling of the different EUL domains revealed a similar ß-trefoil fold consisting of three bundles of ß-sheet organized around a pseudo three-fold symmetry axis. Despite the sequence similarity and the conserved amino acids in the binding site, glycan array analyses showed that the EUL domain has a promiscuous carbohydrate-binding site capable of accommodating high mannose N-glycans, blood group B related structures and galactosylated epitopes.
Highlights
Lectins are carbohydrate-binding proteins that recognize and bind well-defined simple sugars or more complex carbohydrates in a reversible way
In an attempt to unravel which amino acids are required for the carbohydrate-binding activity of the Euonymus lectin (EUL) domain, three-dimensional models were made for the EUL domains in europaeus agglutinin (EEA), ArathEULS3, OrysaEULS2 and PhypaEULS3
This evidence shows that the EUL domain can be considered a universal lectin domain within the plant kingdom
Summary
Lectins are carbohydrate-binding proteins that recognize and bind well-defined simple sugars or more complex carbohydrates in a reversible way. N- and O-glycans the idea was gradually developed that most of these lectins represent a special class of aspecific defense proteins that help the plant to cope with attacks from phytophagous invertebrates and/or herbivorous animals This concept is further supported by the high expression levels of most lectins (generally 0.1–10% of the total protein), their accumulation in a developmentally regulated manner and the toxicity of multiple lectins for insects and fungi [3]. During the past 10 years, evidence has accumulated that some plant species synthesize well-defined carbohydrate-binding proteins upon exposure to stress situations such as drought, high salt, wounding, by treatment with some plant hormones or pathogen attack [5,6,7] These lectins are present in low but physiologically relevant concentrations and are exclusively expressed in the cytoplasm and/or nucleus of the plant cell, and are called nucleocytoplasmic lectins [8]. An overview of the occurrence, carbohydrate-binding properties and three-dimensional conformation will be presented and discussed in view of the putative physiological role of these so-called EUL-related lectins in the plant
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